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Author Topic: Relationships between spaced omnis and center mic  (Read 22301 times)

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Offline heathen

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Relationships between spaced omnis and center mic
« on: June 18, 2017, 05:02:35 PM »
I've read several old threads on here about spaced omnis, with some discussions as well about running a mic (or pair) in the center.  The threads have been very interesting, though sadly I'm unable to keep up with the more technical discussions.

My question concerns the relationship between the omnis and the center mic.  I gather that they should all be on the same horizontal plane (right?), and that it's generally okay for the center mic (in my case a cardioid) to be further forward than the omnis.  There must be a point where the center mic is too far forward, though, right?  How far is too far, though, and does it depend on the distance the omnis are spread? 
Mics: AT4050ST | AT4031 | AT853 (C/SC) | Line Audio CM3 | Sennheiser e614 | Sennheiser MKE2 | DPA 4061 Pre: CA9200 Decks: Zoom F8 | Roland R-05

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Re: Relationships between spaced omnis and center mic
« Reply #1 on: June 19, 2017, 07:17:20 PM »
Yes to most of your questions.  You're on the right track in your thinking.  The answer ranges between all mics in a single horizontal line, to the center mic being moved well forward of the omnis (with compensating delay applied to it).  Usually having the center just slightly but not overly far out in front (without any compensating delay) works well for both practical and acoustical reasons, which I'll try to outline in more detail later.

The good news is there is a lot of leeway without screwing things up.  Much more than there is with simple 2-channel stereo configurations.


Edit-
After posting a bunch of stuff explaining why later in the thread, here's a short synopses of my practical take away- 

For section recording using only a pair of omnis alone, I want an spacing of about 3'.  Less isn't as good in terms of imaging, ambiance, bass, and openness.  More spacing might work but risks a hole-in-the middle. 

If using a center mic I want an omnis spacing of 3' or more.  Less isn't as good in terms of imaging, ambiance, bass, and openness, and risks imaging and comb-filtering complications more than just a pair of omnis alone.  It could be advantageous for other reasons though.  More spacing between omnis can be beneficial and is less problematic because the center mic fills the hole in the middle, but I still wouldn't go crazy wide or the center is becomes separate and "spotlighted".

If the omnis can't be spaced far enough apart, it can help to move the center mic well far forward (like 6' or so) and delay it to compensate for that distance, but that's not often practical.  Alternatively one could move the center mic it straight up or straight down, without requiring any delay, but that might not be very practical either.  Either approach gets the three mics far enough apart to minimize problems of having them too close to each other.  You don't want the three mics too close together.

By using a coincident center pair in the center instead of a single mic, I optimize things and get the best of envelopment and imaging at the same time.  Like a single center mic, I ideally want an omni spacing of 3' or more for the same reasons.  Yet because I can then adjust the image width of the center coincident pair, even wider spacings between omnis no longer present a problem.  I can go double-wide or even more with the omnis without potential problems, giving me a wide diffuse ambience which stays out of the way of the sharp directional imaging stuff in the center provided by the coincident pair.  I also gain control over image blend between the center and sides of the playback image by varying the width from the coincident stereo pair afterwards.
« Last Edit: June 23, 2017, 07:18:03 PM by Gutbucket »
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Offline heathen

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Re: Relationships between spaced omnis and center mic
« Reply #2 on: June 20, 2017, 10:37:05 AM »
The answer ranges between all mics in a single horizontal line, to the center mic being moved well forward of the omnis (with compensating delay applied to it)

How do I figure out how much compensating delay to apply to the center mic? 
Mics: AT4050ST | AT4031 | AT853 (C/SC) | Line Audio CM3 | Sennheiser e614 | Sennheiser MKE2 | DPA 4061 Pre: CA9200 Decks: Zoom F8 | Roland R-05

Offline rocksuitcase

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Re: Relationships between spaced omnis and center mic
« Reply #3 on: June 20, 2017, 11:01:41 AM »
Heath,
in our experience the center mic can be 3-6 inches forward of the omnis with no delay needed. We haven't done a config where the centers are more than that so I can't answer the how far is far enough to require delay. Lee can answer that I'm sure.
edited to add two photos for you-
First one is set up at home with three mics, you can see the center is about 3-4 inches ahead of the sides. (it is also a 1-2 inches out of axis in the horizontal plane, which we've tried to have the correct adapters to eliminate that distance- that said, I do not think it has affected the sound quality)
Second is our set up at DeadnCo 2016. (pink dead rats not our rig!)
« Last Edit: June 20, 2017, 11:11:41 AM by rocksuitcase »
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Mics:         AKG460|CK61|CK1|CK3|CK8|Beyer M 201E|DPA 4060 SK
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Offline heathen

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Re: Relationships between spaced omnis and center mic
« Reply #4 on: June 20, 2017, 11:27:52 AM »
That's a cool setup rocksuitcase.  What are the outer mics?  Looks like maybe AKG bodies, but capsules that are at a right angle?  Pardon my ignorance.

I'd be curious to hear a sample of one of your recordings...are there any on LMA?
Mics: AT4050ST | AT4031 | AT853 (C/SC) | Line Audio CM3 | Sennheiser e614 | Sennheiser MKE2 | DPA 4061 Pre: CA9200 Decks: Zoom F8 | Roland R-05

Offline rocksuitcase

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Re: Relationships between spaced omnis and center mic
« Reply #5 on: June 20, 2017, 11:49:26 AM »
That's a cool setup rocksuitcase.  What are the outer mics?  Looks like maybe AKG bodies, but capsules that are at a right angle?  Pardon my ignorance.

I'd be curious to hear a sample of one of your recordings...are there any on LMA?
Outside are AKG c460 bodies with ck22 Omni caps (the flattest Omni AKG made) the angles are the A-60 "knuckle" adapters which offer the swivel angling. We use the swivel to gain about 10 cm on the outside spread.
The center in that pic is an old Beyer M201e, but we typically run AKG ck61 actives in center/rear.
Here is one:
https://archive.org/details/steepcanyon2016-07-14.24.ck22ck414.flac               (this one has AKG 414 cards in the center)

One more:
https://archive.org/details/erevival2016-07-16.ck22ck61
We ran the centers with both AKGck61 and AKGck8 facing fwd. In mixdown, I chose the ck61 only. But it is nice to have options. (photo is of the two fwd facing mics in this config)


« Last Edit: June 20, 2017, 01:12:59 PM by rocksuitcase »
music IS love

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Offline heathen

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Re: Relationships between spaced omnis and center mic
« Reply #6 on: June 21, 2017, 12:21:57 AM »
Here's a sample of a recording I made with my current setup over the weekend, I'd appreciate any feedback (especially as to whether there are any sonic issues from the center being relatively far forward from the sides...I didn't apply any delay in post): https://we.tl/HH4AeXxm70 (WeTransfer download link).

Details...

Source: Church Audio CA 14 omnis spaced ~9 inches, Church Audio Ugly Battery Box; AKG 460/ck61 (center, about 8 inches forward of the omnis); Tascam DR-70D (24/96 wav)

Location: Stand ~7 feet high, ~5 feet right of center, next to sound/light booth

The crowd was pretty loud and talkative, so you may want to skip to the middle of the track.  Also, there was a wall not far to my right, but it was relatively open all to the left (I didn't have a lot of say in where I was able to put up my stand).  I only had the omnis spread ~9 inches because if I went further that would put the right side that much closer to the wall, and potentially in the light guy's face.

In post I panned the omnis hard left and right, and gave them a tiny bit of gain.  I gave the left side 1 dB of gain more than the right because of the wall issue.
Mics: AT4050ST | AT4031 | AT853 (C/SC) | Line Audio CM3 | Sennheiser e614 | Sennheiser MKE2 | DPA 4061 Pre: CA9200 Decks: Zoom F8 | Roland R-05

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Re: Relationships between spaced omnis and center mic
« Reply #7 on: June 21, 2017, 01:20:32 PM »
The answer ranges between all mics in a single horizontal line, to the center mic being moved well forward of the omnis (with compensating delay applied to it)

How do I figure out how much compensating delay to apply to the center mic?

Speed of sound is ~1120 feet per second depending on altitude and temperature. So measure the distance....

There's an online calculator here: http://www.doctorproaudio.com/doctor/calculadores_en.htm

Follow links for an iOS app as well.
« Last Edit: June 21, 2017, 01:29:15 PM by noahbickart »
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Offline rocksuitcase

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Re: Relationships between spaced omnis and center mic
« Reply #8 on: June 21, 2017, 01:29:24 PM »
Here's a sample of a recording I made with my current setup over the weekend, I'd appreciate any feedback (especially as to whether there are any sonic issues from the center being relatively far forward from the sides...I didn't apply any delay in post): https://we.tl/HH4AeXxm70 (WeTransfer download link).

Details...

Source: Church Audio CA 14 omnis spaced ~9 inches, Church Audio Ugly Battery Box; AKG 460/ck61 (center, about 8 inches forward of the omnis); Tascam DR-70D (24/96 wav)

Location: Stand ~7 feet high, ~5 feet right of center, next to sound/light booth

The crowd was pretty loud and talkative, so you may want to skip to the middle of the track.  Also, there was a wall not far to my right, but it was relatively open all to the left (I didn't have a lot of say in where I was able to put up my stand).  I only had the omnis spread ~9 inches because if I went further that would put the right side that much closer to the wall, and potentially in the light guy's face.

In post I panned the omnis hard left and right, and gave them a tiny bit of gain.  I gave the left side 1 dB of gain more than the right because of the wall issue.
Heath, that link goes to a file titled A Live One 20140617 Theme from the Bottom        10:05 tracktime. is it possible this is not the file you intended to post for us?
music IS love

When you get confused, listen to the music play!

Mics:         AKG460|CK61|CK1|CK3|CK8|Beyer M 201E|DPA 4060 SK
Recorders:Marantz PMD661 OADE Concert mod; Tascam DR680 MKI x2; Sony PCM-M10

Offline heathen

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Re: Relationships between spaced omnis and center mic
« Reply #9 on: June 21, 2017, 01:32:16 PM »
Heath, that link goes to a file titled A Live One 20140617 Theme from the Bottom        10:05 tracktime. is it possible this is not the file you intended to post for us?

No, that's the one I intended to post.  Why?
Mics: AT4050ST | AT4031 | AT853 (C/SC) | Line Audio CM3 | Sennheiser e614 | Sennheiser MKE2 | DPA 4061 Pre: CA9200 Decks: Zoom F8 | Roland R-05

Offline heathen

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Re: Relationships between spaced omnis and center mic
« Reply #10 on: June 21, 2017, 01:35:38 PM »
Speed of sound is ~1120 feet per second depending on altitude and temperature. So measure the distance....

There's an online calculator here: http://www.doctorproaudio.com/doctor/calculadores_en.htm
So we're talking significantly less than a hundredth of a second...is that even going to be perceptible to human ears?
Mics: AT4050ST | AT4031 | AT853 (C/SC) | Line Audio CM3 | Sennheiser e614 | Sennheiser MKE2 | DPA 4061 Pre: CA9200 Decks: Zoom F8 | Roland R-05

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Re: Relationships between spaced omnis and center mic
« Reply #11 on: June 21, 2017, 01:37:13 PM »
Heath, that link goes to a file titled A Live One 20140617 Theme from the Bottom        10:05 tracktime. is it possible this is not the file you intended to post for us?

No, that's the one I intended to post.  Why?
because you said: a recording I made with my current setup over the weekend
and this sample is titled : A Live One 20140617         which appears to be from 2014.
If it is the sample you want us to listen to, I have heard it once and can say I do not hear any center channel timing issues.
music IS love

When you get confused, listen to the music play!

Mics:         AKG460|CK61|CK1|CK3|CK8|Beyer M 201E|DPA 4060 SK
Recorders:Marantz PMD661 OADE Concert mod; Tascam DR680 MKI x2; Sony PCM-M10

Offline heathen

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Re: Relationships between spaced omnis and center mic
« Reply #12 on: June 21, 2017, 01:38:23 PM »
Whoops that's just a typing error on my part in the file name.  Sorry about that, and thanks for giving it a listen!
Mics: AT4050ST | AT4031 | AT853 (C/SC) | Line Audio CM3 | Sennheiser e614 | Sennheiser MKE2 | DPA 4061 Pre: CA9200 Decks: Zoom F8 | Roland R-05

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Re: Relationships between spaced omnis and center mic
« Reply #13 on: June 21, 2017, 03:32:36 PM »
I think it sounds good too! I don't hear any kind of delay and overall it sounds really good, minus the chatter! That's why I LOVE running my mk41 supercards when they Chatty Cathy's are out ;D
Schoeps MK 4V & MK 41V ->
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Sound Devices MixPre-6 & MixPre-3

http://www.archive.org/bookmarks/diskobean
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Re: Relationships between spaced omnis and center mic
« Reply #14 on: June 22, 2017, 03:27:35 PM »
First, at the risk of a collective ::), allow me a quick philosophical muse on the approach to recording in order to better ground my contribution to the discussion.   I'll then get into specifics on technique.  Partly because all this is related, and partly because I think it helps in determining where to best focus one's efforts before getting mired in the details. It's easy to get swept up in specifics and loose the big-picture focus on what's actually important, especially when discussing alternative approaches to the norm.  So please forgive a brief full forest view before closer examination of tree bark.

There is a whole lot of stuff going on with stereo recording and reproduction.  Stereo is an illusion, a mental trick in which we willingly play a part as listeners.  Creating a convincing illusion is not a simple direct application of one optimal working method, but rather a juggling of multiple phenomena to produce an illusion which is sufficiently convincing for the listener.  That is to say there is no one correct way of doing things.  In that light, the art of recording concerns juggling various phenomena in pursuit of a more convening illusion.  Some of the phenomena involved are intuitive and others are not, and some are contradictory with each other.  In the end whatever works to make the illusion more believable for the listener is what makes an approach correct or not.  Realistically, we're usually leveraging a combination of approaches, juggling benefits and trade-offs.

Below is a list of aspects we have control over as recordists, ranked in order of what I feel is most important.  Others may disagree on the the order of this list, but I have good arguments for ranking it this way.  The most critical, fundamental stuff is at the top, the higher-order nice-to-have stuff at the bottom.  There are of course other more important and less important things, above all else the quality of the performance at the very start of the chain and a willing suspension of disbelief on the part of the listener at the very end of the chain, both of which are outside our sphere of influence-

signal integrity (sufficient elimination of noise, overload, clipping, dropouts and other problems)
frequency balance
direct/reverberant energy balance (intelligibility, sense of proximity)
loudness dynamics
sense of space (width and depth)
directional imaging
sense of envelopment and immersion


Ideally I want all those things optimized, but that's a lot to ask.  It's also not necessary.  Fortunately we can lean on our willing suspension of disbelief and enjoy less than perfect recordings that don't satisfy all those aspects.  The point I'm trying to make is this- when the stuff closer to the top of that list isn't quite right, it really impedes our listening enjoyment.  When the stuff closer to the bottom of the list isn't quite right (or isn't present at all), it doesn't matter as much. The higher-order aspects at the bottom of the list (which is the stuff we tend to get excited about in good recordings) really only become valuable when they are supported by a strong foundation consisting of stuff at the top working well.  The higher-order bottom of the list stuff doesn't have a good change at becoming convincing unless the lower order stuff has been rendered sufficiently transparent such that the listener is no longer consciously aware of them.
« Last Edit: June 22, 2017, 04:08:27 PM by Gutbucket »
musical volition > vibrations > voltages > numeric values > voltages > vibrations> virtual teleportation time-machine experience
Better recording made easy - >>Improved PAS table<< | Made excellent- >>click here to download the Oddball Microphone Technique illustrated PDF booklet<< (note: This is a 1st draft, now several years old and in need of revision!  Stay tuned)

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Re: Relationships between spaced omnis and center mic
« Reply #15 on: June 22, 2017, 04:43:18 PM »
There are several different reasons one may want to use a center mic (or center pair, or even more) between a pair of spaced omnis.  Here are three reasons which immediately come to mind-

To solidify or otherwise improve the center of the playback image.  Which is to say- To allow a wider spacing of the omni pair than one could otherwise get away with.
To gain some useful control over the balance between direct sound and the ambient sound (ambient sound being primarily room reverberance and audience reaction).
For playback over three front speakers instead of two.

The last won't apply to the majority here at TS, so the first two will be my focus.

I almost added "To more evenly cover a large, distributed sound-source such as a choir or orchestra, or a relatively close stage-lip recording of an ensemble with a wide on-stage setup", but realized that's really just a subset of the first one.
musical volition > vibrations > voltages > numeric values > voltages > vibrations> virtual teleportation time-machine experience
Better recording made easy - >>Improved PAS table<< | Made excellent- >>click here to download the Oddball Microphone Technique illustrated PDF booklet<< (note: This is a 1st draft, now several years old and in need of revision!  Stay tuned)

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Re: Relationships between spaced omnis and center mic
« Reply #16 on: June 22, 2017, 05:29:06 PM »
To back up just a bit, please note that the reasons mentioned above are quite different than those which would apply to what may seem a very similar question on the surface of things, but really isn't-

"What about adding a pair of omnis to the 2-channel mic setup I am familiar with and like using?"

The reasons for wanting to do that would be quite different.  This is partly why I started with a bit of philosophy of recording.  Adding omnis to a standard 2-channel setup changes the way the standard setup works in complex ways which aren't obvious.  Commonly, that gets ignored.  The understandable temptation is to stick with the known-good two channel setup and add additional mics in an attempt to make things better.  Unfortunately it's not that simple.  Well in some cases it can be - low pass the added omni(s) around where the 2-ch setup naturally rolls off and that keeps everything relatively simple.  It won't significantly change the way the 2-ch setup behaves in the range in which it is sensitive.

The main point is this- typically adding another mic channel (or two, or more) is going to require significant changes to the original setup in order to see a significant benefit.  And for things to work optimally the setup will definitely need to be changed.
« Last Edit: June 22, 2017, 05:30:39 PM by Gutbucket »
musical volition > vibrations > voltages > numeric values > voltages > vibrations> virtual teleportation time-machine experience
Better recording made easy - >>Improved PAS table<< | Made excellent- >>click here to download the Oddball Microphone Technique illustrated PDF booklet<< (note: This is a 1st draft, now several years old and in need of revision!  Stay tuned)

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Re: Relationships between spaced omnis and center mic
« Reply #17 on: June 22, 2017, 06:02:27 PM »
So let's go back to the two main reasons for using a center mic with a pair of spaced omnis-

1) To solidify or otherwise improve the center of the playback image.  Which is to say- To allow a wider spacing of the omni pair than one could otherwise get away with.
2) To gain some useful control over the balance between direct sound and the ambient sound (ambient sound being primarily room reverberance and audience reaction).

From my own experience, it's clear that unless setting up on stage or at the stagelip, most tapers usually can't achieve a very wide spacing between omnis for simple practical reasons.  It's hard to support a wide spread from a single stand, and it's just too much hassle to set up (and protect) multiple stands.  Sure there are exceptions: balcony clamping with wide splits, great sounding superwide outdoor two channel omni A-B section recordings and the like - yet the fact that those can work at all more than anything says much about the odd nature of PAs as a sound source and how unusual ways of recording them from a distance can work which otherwise would not for most anything else.

So for most tapers, adding a center mic will usually be less about filling a hole in the middle of the soundstage than about pulling in more direct sound and making the simple omni pair sound more present and upfront.  And that's reasonable for relatively narrow omnis spacings like most here will be using.  There is no hole-in-the-middle problem with relatively narrow omni spacings, rather the opposite problem becomes the issue- too much common information picked up by all three microphones.
musical volition > vibrations > voltages > numeric values > voltages > vibrations> virtual teleportation time-machine experience
Better recording made easy - >>Improved PAS table<< | Made excellent- >>click here to download the Oddball Microphone Technique illustrated PDF booklet<< (note: This is a 1st draft, now several years old and in need of revision!  Stay tuned)

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Re: Relationships between spaced omnis and center mic
« Reply #18 on: June 22, 2017, 07:08:59 PM »
One post on approaches to this from the mixing side before getting into mic arrangement specifics-

So we're probably using a directional mic in the center pointing directly forward mostly to achieve more focus on the direct sound, with less room and ambient pickup in that channel than were're getting from the omni pair on it's own (If the primary goal was filling a hole-in-the-middle between widely-spaced omnis, a 3rd omni would work best, blending in better, with timbre and spatial pickup qualities more similar to the other two omnis). 

So we make some trial recordings and play them back to asses them.  We pan the omnis hard left and hard right and pan directional center mic to the middle. We bring up just the omnis and balance them.  Then we slowly bring up the center mic.  The first thing one is likely to hear is likely a simple increase in level.  The second, a difference in timbre or overall frequency balance.  Does it sound better?  Maybe so, maybe not, but let's assume it does.  We have lots of control over those aspects and to my way of thinking we should use that control in a couple ways: First of all to keep ourselves honest - does it sound better because it's louder?, does it sound better because the frequency balance is changed?  The only way to know is to listen both ways and compare.  Adjust the level and EQ of the two omnis alone to sound as close as possible to what you heard when you brought up level of the center mic.  Is that just as good?  If so at least at this point into the investigation of usefulness there is no reason to record using the center mic, unless you are using it as an easy way to avoid having to EQ (nothing wrong with that).  The point is to constantly check changes and adjustments in an iterative way, to to make sure actual improvements are being made without the addition of  unnecessary complications, and to prove to ourselves that the way we are doing it can't be achieved in some other simpler way.

The second way of using that control is to fine tune everything so it all works together more optimally.  Maybe you find you can only use a tiny bit of center channel without things sounding weird (let's say because the omnis aren't really far enough apart for a center channel to work right without complications), but if you EQ the center channel in a certain way that mitigates the problem and allows you to bring up the center channel level to where it's contributing a lot more good stuff.  Again it's a balancing act, how much is the right amount, and of what?  Some of these changes require these kind of adjustments to work right.  EQ, compensation delay, etc.

Some folks will be averse to adjusting all this stuff.  That's fine, if it works for you without adjusting things like EQ and fine-tuning levels, great.  But a huge advantage in having more than two recorded channels is the greatly increased ability to tweak things which doesn't exist with just two channels directly feeding two speakers.  Once you've sort of figured out what works and what needs to be done, you can better asses if it is worthwhile or not, and when.

The level, EQ, and timing of the center may be adjusted (for any number of reasons) without throwing off the left/right balance.  Use that to your advantage.  Just double check to make sure that what you are doing is really an improvement, and that you can't achieve the same in a simpler way.
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Re: Relationships between spaced omnis and center mic
« Reply #19 on: June 22, 2017, 07:47:55 PM »
Okay now that that's out of the way, here's some straight dirt on geometry.

A few different aspects are in play.  First time of arrival for the direct sound component-
If all the mics are arranged along a single line (center mic in the same plane as the omnis), arrival of a wave-front originating from directly ahead will be coincident in time as it reaches all mics.  That's generally thought of as a good thing, because at the opposite extreme we certainly don't want a slap echo between the center and omnis, or comb filtering timbre effects at less dramatic timing differences.  But it may be advantageous to use a bit of precedent effect to advantage in drawing attention to the center without needing as much center level to do so.  A center mic position slightly forward means a wave-front from directly ahead pings the center mic a fraction of a second before the omnis- not enough to hear an echo, not enough to change timbre radically, but enough to pull the image to center more strongly via the precedence effect.  A slightly forward center mic can also make the imaginary line between the center mic and left omni perpendicular to the left PA (and vice versa for the right omni, center mic, and right PA) so that the wave-fronts arriving directly from each PA are synchronized in time at the center mic and mic on that side, yet slightly delayed on the opposite side.  Again, if that's a good or bad thing is totally a judgement call.

If the center is far enough out in front (why that might be the case I'll get to later) a compensating delay to it can bring that timing alignment back into sync, but that will only do so for sound arriving from that particular direction- in this case for sounds arriving from in front.  For sound arriving from the rear the compensating delay is added to the time of arrival difference between the omnis and center.  For sound arriving from the sides somewhat less so. That might be used to advantage- or not.

Besides direct-sound wave-front time of arrival aspects, the other main thing with regards to setup geometry is the phase-correlation between channels with regards to diffuse sound pickup.  That deals with how the reverberant/ambient sound which fills the room is handled and is a large part of why spaced omnis sound the way they do.  In a lot of ways, that's the invisible elephant in the room.  I'll try and get to that tomorrow.
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Re: Relationships between spaced omnis and center mic
« Reply #20 on: June 23, 2017, 10:38:19 AM »
So much great information here...I'll be thinking about this, and referring back to this thread, for a while.
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Re: Relationships between spaced omnis and center mic
« Reply #21 on: June 23, 2017, 11:46:46 AM »
Cool, glad your enjoying all this and I hope it helps.  My aim is to explain what's going so you and others can make educated decisions on what to do, rather than just following convention by rote.  To me that's more helpful than just saying "do this".  It's what I was looking for when I first came around here years ago.


Continuing with geometry, now getting to what I consider the important stuff about spaced omnis.

What is it about a simple pair of well-spaced omnis?  For now I'm just considering a 3' spaced pair, without any center mic.  They have a few traits which are often valued.  A few of those are simply due to them being pressure omnis: They have deep bass extension; they are resistant to wind and handling noise; the really small ones are not very particular about how you point them; they are generally more weather resistant; and they tend to sound natural.  Some of that natural sound quality is due to their generally flat non-resonant response and open polar pattern.  Of course, the open omni polar pattern is good when the room sounds good, but might not work as well if the room or audience is poor.

Yet some of their other advantageous traits are a mostly a product of the spaced setup geometry- specifically a wide-enough spacing.  Wide-enough spaced omnis sound big and open, lush, wide and ambient, and certainly some of their associated naturalness is due to that.  The wide-enough spacing produces stereo bass information down to a significantly low frequency.  Narrow spaced omnis don't sound as open, airy or as lush, and are essentially remain monophonic up to a higher frequency.  Why is that?  A lot of the open, airy lushness is to do with how they pick up the Diffuse-Field compared to the Direct-Sound.  I'll post about that next.  The stereo bass quality is due to the relationship between the mic spacing distance and the wavelength of the bass frequencies in question - the wider the mic spacing, the lower the cutoff frequency where the mic pair can detect sufficient phase differences between channels to convey low frequency stereo information.  Below that point , the microphone sensitivity retains pressure omni response flatness down to the lowest octave, yet as the frequency decreases and the wavelength grows large in comparison to the mic-spacing distance, the phase difference information picked up between the two mics decreases until the bass becomes mono-ized below a frequency that corresponds with the mic-spacing distance.  The wider the spacing, the lower the stereoized bass quality goes.  The narrower the spacing, the higher the frequency where the bass goes mono.  This low frequency difference information has a large effect on bass externalization and the  spatial qualities which convey a sense of "you are there".  A lot of that depends on having sufficient spacing.

That same relationship also effects a related aspect of the bass quality from spaced omnis- the mic's themselves each have a very flat bass response as pressure omnis, but the interaction of the two mics with each other based on the distance between them creates a partial reinforcement/cancellation curve.  You get a reinforcement peak where the spacing is equivalent to a one wavelength spacing and a partial cancellation dip at half-wavelength spacing, and that shifts with frequency as the spacing is changed.  This can be quite audible with lots of bass information as the spacing between mics is adjusted.

When I first started seriously recording outdoor concerts with just a pair of spaced omnis, I did a series of experiments listening using highly isolated headphones (wearing ear-muffs over isolating in-ear 'phones) while making spacing adjustments to the omnis.  Besides changes in imaging width and diffuse ambient pickup, I could clearly hear the changes in stereo-bass depth as the spacing was varied.  I expected to hear those things. What I had not expected was the relatively strong reinforcement/cancellation curve thing.  It was quite apparent as the spacing was shifted, I could 'tune' the sound of the low bass using the spacing.  Obviously that's not something one would do each time, but at least in that particular ampitheater I settled on an approximate 3' spacing being the best of all worlds with regards to low bass tuning, stereo bass effect, imaging width, and ambient/diffuse qualites, and that remained my standard omni spacing for years until revisiting it again when I started adding center mics.  That the basis upon which I suggest 3' as a good default target spacing for a pair of omnis, and the minimum spacing I recommend if using a center mic.  As always, there can be exceptions.  But I stand by that recommendation and its usually worked very well for me.

Okay that covers stereo bass qualities.  Next I'll touch on to stereo imaging, then diffuse-field correlation.  And finally the implications all these things have when adding center mics.
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Re: Relationships between spaced omnis and center mic
« Reply #22 on: June 23, 2017, 01:19:29 PM »
On imaging-
I dig good imaging. It's cool to hear sound sources clearly laid out across the stereo field, even though I consider it a higher-order playback aspect that's not absolutely necessary.  Other things are more important to me to get correct before good imaging.  And often I can't really hear sharp imaging when at a concert.  The sound is big and enveloping, and there is a definite sense of sound coming from the stage contrasted against the sound bouncing around out in the room and the audience sound all around me, but sharpness of image isn't especially apparent.  Mostly I'm relying on vision to accurately pick out where the sound sources are located.  Good imaging is more important in playback, where it serves to reinforce the illusion.

That said, if the direct/reveberant balance is good and once the EQ balance has been dialed in (two of the vitally important things!) I'm relatively happy with the imaging I get from just a spaced omni pair in my live music recordings.  And I do think about the imaging effects of the spacing between omnis along with the other things like bass response and achieving low diffuse field correlation (more on that next).

We talk a lot about Michael William's Stereo Zoom approach to microphone setup here at TS, which mostly deals with the imaging aspects of microphone setups and how to modify them.  Wiliams' empirically derived Stereo Zoom tables are mostly applicable to directional microphones, but it loosely applies to omnis as well.  I'll sometimes refer to the Stereo Zoom charts when thinking about what omni spacing I want to use. The basic relationship is well understood by most here- with regards to imaging, a wider microphone spacing has a narrower Stereo Recording Angle.  That is to say- the region of the stage from the center-line out to some angle to either side of that will be reproduced as points of sound between the two speakers (that's the Stereo Recording Angle or SRA), grows narrower as the mic spacing is made larger.  With narrow spacings, the SRA is very wide and everything in the room is reproduced between the two speakers.  With wider mic spacings more stuff is pushed outside of the region between the two speakers.  At the extreme, we get only the fully centered stuff appearing centrally between the speakers (including mono PA information, which is why super-wide omni splits work at big outdoor concerts without a massive hole) and most everything else is shoved over to one speaker or the other.  That's the proverbial spaced-omni hole-in-the-middle.

That narrowing of SRA with increased spacing can be somewhat confusing to comprehend, because by all other aspects except imaging pickup angle, most everything else sounds bigger and wider and more open as the spacing is increased.  I covered why that is for the bass region, and will get to that next for the mids and highs.  But first, more on imaging..
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Re: Relationships between spaced omnis and center mic
« Reply #23 on: June 23, 2017, 01:49:45 PM »
Here's a complication with imaging- when we add another mic or mic pair in the center, we are introducing additional imaging relationships on top of the one which already exists from just the pair of omnis.  The SRA imaging relationship between the omnis doesn't go away, and we now also have additional imaging relationships between the omni on each side and the center mic.  If we use a coincident stereo pair in the center we also have that imaging relationship thrown into the mix.  If we were to put a near-coincident pair in the center we'd introduce even more imaging complications- we'd have the imaging relationship between the two omnis, the relationship between the near-spaced pair, the relationship between each omni and the directional mic on the same side, and the relationship between each omni and the directional mic on the opposite side.  That's 6 different relationships between 4 different microphone positions.  Things get complicated fast, and unless we're careful about minimizing conflicts we can easily end up with a mess of things making for not just confusing imaging but also numerous comb-filtering complications.  This is partly why I don't recommend using a near-spaced pair in the center.  A coincident pair in the center keeps the imaging relationship count at 4, just one more than using a single center mic, and the comb-filtering relationships at 3, the same as with a single center mic.

The easy way to avoid these problems is to space the omnis further.  Make the omni spacing wider so that the SRA imaging relationship between each omni and the center mic works on it's own.  Michael Williams extended the Stereo Zoom a couple decades back to multichannel array recording with more than two channels.  In doing so, he describes techniques used to seamlessly join the SRA between the left and center mic with the SRA between the center and right mic.  That's intended for 3 channel L/C/R playback, most commonly for 5 or 7 channel surround recording, but I find it also applies to 3 channels mixed down to 2-channel stereo as well.  If interested, I can link to his paper where he describes how moving one mic forward of the other shifts the SRA sideways.  That in combination with mic angle and spacing is what allows for seamless SRAs all the way around a surround array.  It has a direct correlation with how far forward the center mic might be positioned in front of the left/right pair.  All of his published stuff so far deals with microphones using identical pickup patterns all the way around the array, so one needs to sort of read between the lines to get the implications for a directional mic between omnis, but the general concept applies.  Supposedly he's working toward arrays using different mic patterns, I'd like to see that.
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Re: Relationships between spaced omnis and center mic
« Reply #24 on: June 23, 2017, 06:05:09 PM »
Okay, here's a big one, one of the main reasons sufficiently spaced omnis sound sweet and why I use them whenever I can in the pursuit of recordings which aim to transport the listener back to the time and place of the recording-  they produce recordings with low diffuse-field correlation (DFC), which is highly desirable for the ambient/reverberant content in a recording.  Correlation is one measure of signal 'sameness' between channels. Basically, high correlation means the signals are very similar to each other and will image somewhere within the playback window in a somewhat monophonic-like way.  That's good for the direct sounds which we want to reproduce with sharp imaging between the speakers.  It's bad for reproducing the diffuse sound filling the recording space in a directionless way.  Low correlation means the information is reproduced diffusely, sounding wide, open, airy and much more natural.

Spaced omnis are excellent at diffuse-field ambience upon reproduction because of their wide spacing.  Near spaced omnis aren't.  Similarly near spaced or coinciding directional mics are not very good at at this unless the mics are significantly more directional than cardioid and pointed far enough apart, due to the omni component of the cardioid pattern combined with the limited or non-existent spacing.  Blumlien crossed bidirectionals has excellent low diffuse field correlation, and widely-angled hypercards are pretty good too.  Coincident cardioids are terrible at this and partly why many don't care much for X/Y cards at a distance.  Spacing cardioids apart a bit helps some, but most near-spaced configs are still to close together to achieve significantly low DFC. Back in 2008 I posted a thread here at TS investigating why recordings made with spaced omnis and coincident crossed bi-directional mics (Blumlein) in many ways sound quite similar to me, even though they are total polar opposites in terms of mic spacing, polar pattern, and imaging.  http://taperssection.com/index.php?topic=110861.0 Now I have a much clearer understanding as to why.

With Blumlein coincident crossed 8's the mics are coincident, yet have no omni component, and have minimal shared in-phase pickup pattern overlap over the entire sphere, even directly above and below, except for the narrow quadrant directly in front and behind where the sound is correlated (in front is the only place we we want it to be correlated).  With omnis, somewhat similarly the direct sound arriving at the microphone pair from the median plane produces a correlated signal by arriving at both microphones simultaneously, but from all other directions the sound is increasingly uncorrelated as arrival moves off the median plain, corresponding to the source's increasing off-center angle, the distance between microphones, and the wavelength of the frequency in question.  The diffuse sound in the room - that is, the directionless sound which effectively arrives from all directions more or less equally, is picked up with very low correlation overall as long as the spacing between mics is sufficient.  The greater the mic spacing, the lower the frequency down to which the signals will have a randomized phase response producing low diffuse field correlation.

Okay, that gets pretty technical.  But it's a huge deal (the elephant in the room if you will) and why I had quotes about the importance of low DFC from acousticians, audio researchers, and famous recording engineers in my TS signature for so many years.  Achieving low DFC is the magic sauce which makes the very highest-order stuff from in my list of what is important possible - that is achieving a sense of envelopment and immersion.
« Last Edit: June 23, 2017, 06:09:41 PM by Gutbucket »
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Re: Relationships between spaced omnis and center mic
« Reply #25 on: June 23, 2017, 06:56:10 PM »
So following the above reasoning, I now ideally have quite widely spaced omnis which achieve low DFC, and since they are widely spaced (lets say 5'+ or so) they not only do that better, they also provide good stereo bass to a lower frequency, and their wide spacing produces an imaging hole-in-the-middle which sort of clears that area for the sharp imaging contribution of a coincident pair in the center.  Also the wider A-B omni spacing forces any nearby localized audience chatter to the outer edges of the playback image because those close-proximity noise sources tend to localize more strongly at one mic or the other rather than in both simply due to the difference in relative proximity to those nearby sources.  That doesn't make them go away, but it does make them far less distracting and helps to get them out of the way of the music.  Everything falls nicely into place, without conflicts.  This is why I ideally suggest turning the single center mic into a M/S pair or a PAS X/Y pair between more widely-spaced omnis as an ideal solution when and where one can get away with doing so.

Rocksuitcase- the optional rear-facing directional mic for gaining control over additional ambient content which you and KindMS have been using, spaced slightly rearward from the center mic, keeps DFC between channels low because it is as distant from the omnis as is practical, and is spaced apart from the forward facing center mic and pointing the opposite direction.  Yet at the same time it is not far enough away from the plane in which all the mics are arranged to produce a significant delay to the direct sound component from the PA or stage. 


Okay, enough for today.  Since most of what I've posted consists of arguments for wider omni spacings than most can easily achieve, next week I'll post more about about what one might do when unfortunately the omnis can't be spread wide enough.
 Thanks for the soapbox, I hope all this doesn't come across as pedantic.  I very much enjoy sharing what I've learned and am really thrilled to find some of these ideas being taken up by the larger taping community over the past couple years.  I never really expected others to try much of this stuff- it's just plain weird compared to traditional 2-microphone recording setups at first glance, even though the conceptual basis is sound.  But it's fun and provides powerful tools unavailable to traditional recording, which apply uniquely to the oddball application of recording live music from a distance.  We're really the only folks doing that in the recording world, and unique solutions can provide unique and welcome advantages.

Apologies for commandeering the thread thus far..
TL:DR!
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Re: Relationships between spaced omnis and center mic
« Reply #26 on: July 07, 2017, 04:12:05 PM »
Back for more-

Following what I outlined above, when adding a center microphone or pair of mics between a pair of omnidirectional microphones,  the omnis ideally need to be spaced far enough apart - farther than they normally would be if used as a pair on their own. So, what to do when you can't or otherwise space a pair of omnis widely enough? Use some method to generate additional level difference between the two omnis:

1) Angle the omnis further apart.  This works at high frequencies only, down to a frequency determined by the size of the microphone capsule housing.  Since both the limited omni spacing and the introduction of a third microphone or pair in the center do much to eliminate any "hole-in-the-middle" problem, go ahead and maximize the limited effect by pointing the omnis directly to either side, 180-degrees apart from each other rather than directly ahead or partly angled outwards.  This is easy to do and effective, even if the frequency range in which it is effective is quite limited, making it the first and most obvious thing to do.  In addition, the center mic or pair is handling forward detail and presence, so don't worry about pointing the omnis so far away from the "main source of sound up front". Microphone attachments like spheres, disks, and angled plates for boundary mounted microphones can lower that frequency somewhat into the upper-midrange/low-treble region, significantly enough that their use is usually audible, but don't typically lower the frequency at which the microphones become directional a whole lot further so as to strongly effect stereo imaging.  They mostly affect frequency balance and apparent spatial spectral balance, which may or may not be desirable for other reasons. 

2) Place a baffle of some sort between the omnis.  This is the dummy-head/Jecklin-disk thing. The baffle shadows the microphones from sounds arriving from the opposite side, increasing level differences.  Like microphone attachments this only works above a certain frequency point, which is determined by the size of the baffle.  Jorg Jecklin experimented with a three microphone version of his system using a somewhat wider omnis spacing and a bigger diameter disk with a notch cut into it to fit a forward facing center cardioid.  Problem is that baffles which are large enough to be optimal are unwieldy and not conducive to audience taping.  But if you can't get enough spacing between your omnis, have a Jecklin disk or something similar which will work as a baffle on hand, and can rig it without problems, a baffle can help significantly.

3) Use of a Blumlein Shuffler circuit to filter the output of the omni pair.  This is a circuit which converts low frequency phase differences to level differences.  Alan Blumlein used this with a pair of close spaced baffled omnidirectional microphones for his initial stereo experiments in the early 1930s before quality bi-directional microphones became available to his EMI development team, and it's use is described in his original patent.   Jecklin's later ideas about using a baffle between omnis is a direct decedent of this technique, and he also experimented with Blumlein Shuffling.  I've never tried it, but am quite intrigued by it as it could potentially be a very valuable tool for audience taping using omnis which can't otherwise be spaced far enough apart, and may also apply to recordings made using near-spaced directional mics, allowing them to sound more spatially enveloping at low frequencies than they otherwise do.   In terms of the direct sound (not the ambient sound), by choosing a particular spacing distance between mics we are selecting a particular frequency range in which we want to optimize stereo imaging, sacrificing imaging performance at the other end of the range.  Wide spacing is more optimized for low frequency stereo imaging.  Narrow spacing is more optimized for higher frequency range stereo qualities.  One way of looking at it is that Blumlein Shuffling is a way of correcting for insufficient spacing at low frequencies.  This is related but not quite the same as converting from Left/Right to Mid/Side stereo and boosting the low frequency Side information with a corresponding cut to the low frequency Mid information- a technique I do use frequently.  For more information on this search out Blumlein's original work and Micheal Gerzon's extension of it 40 years later.  Also here is an outfit building hardware boxes which do several types of Shuffling including the original Blumlein variety intended for narrow spaced omnis- http://www.phaedrus-audio.com/shuphler.htm.  Partway down that page are explanations of the different shuffling circuits and what they are intended for.  I've not heard them, but that site posts listening examples here- http://www.phaedrus-audio.com/intro_to_shuphlers.htm.   Lest costly and probably more applicable to processing our recordings is the "in-the-box" version of those same filters, which are available for Apple platforms here- http://pspatialaudio.com/index.htm. With a further description of Blumlein difference techique here- http://pspatialaudio.com/blumlein_delta.htm.  In addition to being more affordable (I presume) and eliminating the need for a dedicated hardware box incorporating analog circuitry, the digital filters take advantage of non-causal time-symmetric filtering which supposedly correct for some inherent deficits of analog filtering which can only be realized via digital processing.  Unfortunately it doesn't seem to be available for Windows platforms.  If anyone has experience with Blumlein Shuffling or use of other shuffling filters other than simply boosting the LF of the Side channel, please let me know.
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Re: Relationships between spaced omnis and center mic
« Reply #27 on: July 07, 2017, 04:53:59 PM »
Wide spacing is more optimized for low frequency stereo imaging.  Narrow spacing is more optimized for higher frequency range stereo qualities. 
Does this only apply to the Blumlein Shuffler you discussed, or to any spaced stereo pair of omnis?  Also, if forced to choose, wouldn't one generally prefer to have the stereo imaging in the higher frequencies, since we can't really hear direction of the really low frequencies?
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Re: Relationships between spaced omnis and center mic
« Reply #28 on: July 07, 2017, 07:02:08 PM »
^ It applies to some problems inherent with stereo reproduction in general.   Blumlein recognized that early on with regards to stereo recording using omnis.  EMI recognized it when stereo came of age in the late 50's and built somewhat different shuffler circuits into their first stereo mixing desks.  The  Landmark 1957 article by Clark, Dutton and Vanderlyn which marked the start of the stereo era is an outstanding reference on the basis of two channel stereo- how it works, hearing, recording and reproduction.  Pretty amazing that it more or less comprehensively covers many of the things we still discuss over half a century later.  I rank it up there with the original Blumlein patent for both historical and academic value as well as breadth of coverage.  The discussion in the appendix is worth checking out as well.  You can find it here- http://www.phaedrus-audio.com/stereosonic.pdf

The basic issue is that low frequency and high frequency information image differently in stereo for numerous reasons.  If you were to listen while panning a monophonic low-passed source and a monophonic high-passed source to what sounds like the same mid-left or mid-right position, and then look down at the pan pots, the two would be set differently.  Usually you'll find that you'll pan the low-passed source further to the side than the high-passed source to achieve the same apparent position.  That's actually an argument for using microphone configurations or mixer panning laws which slightly over-emphasize low frequency width, rather than an argument for less stereo width in the bass than the highs.  Much of that imaging problem correction stuff ended up ignored and forgotten by the time stereo really took hold, partly due to the tools available to do so at the time were not as advanced as now (the analog vs digital filtering thing) and apparently caused coloration problems which were more problematic than the imaging problems they were designed to correct.  And that's the correct value judgement to my way of thinking. 

But remember that this is about a lot more than just imaging.  And although stereo imaging isn't my top priority, I still want as good imaging as I can get.  If you forced me to choose a single frequency range for good imaging, I'd pick the 700Hz to 3kHz range where our hearing is most attuned to imaging aspects.  If forced to choose between monophonic bass up to 500Hz or monophonic highs above 5kHz I'd have to do a listening test to confirm, but I suspect I'd choose stereo bass and monophonic highs.  Good stereo bass information is a big part of conveying the "you are there" enveloping experience of live music.  But I don't want to have to choose, I want good imaging across as wide a frequency range as possible.  And these multi-microphone techniques are ways I can achieve that.  Engineering is all about juggling and optimizing tradeoffs to maximize what you want while minimizing what you don't.
« Last Edit: July 07, 2017, 07:21:35 PM by Gutbucket »
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Re: Relationships between spaced omnis and center mic
« Reply #29 on: July 07, 2017, 07:04:32 PM »
Okay, one more post and I'll call it quits as I'm sure most of this is way deeper than most here care to go.  But happy to discuss if anyone wants to do so. 

My previous post covers things you can do if you can't space a pair of omnis as far as you'd like, and relates to either straight 2-omni recording or using an additional microphone or pair in the center between omnis.  But what about a situation where you have the omnis spaced exactly as far apart as you want them for recording using the omni pair alone, but then want to add a center microphone to that? 

As mentioned previously, in most cases we want to greatly increase the omni spacing when introducing a third mic in the center- often doubling the spacing in general simplified terms.  There are several reasons for that- reducing  combfiltering from three mics in close proximity, improving the imaging across three mics instead of two, etc.  But there is another way of getting the spacing we need without moving the omnis.  We can space the center mic sufficiently far away from the omni pair.  In other words, we can move the center mic away from the omnis instead of increasing the spacing between the Left and Right omnis, in order to get the omnis far enough away from the center mic.

One way to do that is to mount the center mic directly between the omnis as before, but put it much higher or much lower than the omnis.   Imagine a tall 15' mic stand with the forward facing center mic at the very top and the spaced omnis on a bar clamped to the same stand something like 7' or 8' up.  Together, the three microphone locations form an upward pointing triangle.  That arrangement keeps the timing relationship more or less unchanged for direct sounds arriving from the horizontal plane.  Sound from directly in front reaches all three mics with about the same timing relationships as if all three were in a single line.  But the diffuse reverberant sound arriving from all directions has a significant time of arrival difference between the center mic and omnis because the arrival time difference serves to decorrelate the signal between the center and omnis, reducing comb-filtering issues and providing some of that wide-spaced omni openness and ambient sheen.  I recently suggested this as one option which avoids having to do any post filtering in Edtyre's thread- Suggestions for a center channel omni to run into my MixPre3. Basically it's the same idea applied there, even though we were mostly talking about adding a single omni to a standard near-spaced directional mic configuration rather than a center directional mic to a spaced omni pair.  Alternately one can simply mount all three mics relatively close together and low pass the omni channel so that it only provides low bass below where the cardioids begin to roll off, extending the low bass response without comb filtering and imaging problems since it will not interact with the near-spaced cardioids at higher frequencies.

Another way to physically space the center mic away from the omnis is to leave all the mics in the horizontal plane, but move the center microphone far enough forward of the omni pair to sufficiently decorrelate it from them.  To keep the imaging correct we need to introduce a compensating delay the center mic so that it's signal will be time-aligned with the omnis for sounds arriving from in front (approximately 1.1ms for each foot of distance).  Sounds from other directions will arrive with different timing relationships between the three mics.  Sound from in back will arrive with a delay equal to that of twice the forward spacing of the center mic (corresponding to it's distance in front + the delay compensating for that same distance for sound arriving from the opposite direction).  The obvious problem with this is the setup- you probably need a separate mic stand for the forward center mic.

That can actually be done in such a way that the SRA imaging between each omni and the center mic is seamless and can be made to match that of the two omnis alone, eliminating the inherent imaging conflicts which otherwise would occur between three mics all pointed in the same direction.  Michael Williams extends the Stereo Zoom (2channel) to Mulitchannel Microphone Array Design (3 to 7 channels) to special multi-channel arrays which can be used to record both simultaneously which he dubs "Magic Arrays".  His AES papers are available at his website- http://www.mmad.info/.  Below is a photo of an 8-channel Magic Array from Jerry Bruck's Posthorn site.  Obviously that's a specialty multi-channel recording array, but if you were to eliminate all but the 3 forward-most microphones, notice that the center mic pair are more or less in a NOS or DIN configuration with the center microphone moved forward by a significant distance.

Edit to insert this note of clarification on the photo below- These may appear to be wide-Left/Right-spaced omnis plus six other mics but are not.   A 3-mic main output like what we are discussing as produced from this array would consist of just the forward most extended mic and the two angled mics closest to it.  In the photo those microphones are the three closest to the top of the frame (center mic top-most + the Left/Right pair immediately beneath it).  Please ignore the other five microphones for the sake of this discussion.  The diagrams below are more clear on this- the center microphone being well forward of the left/right pair (closer to the top of the diagram). With respect to the diagrams, simply ignore the two rear facing (bottom most) microphones.



Below are William's setup examples for a few different 5-channel Magic Array using omnis, cardioids, and supercarioids.  Each array can be used to provide mono (center forward facing mic only), 2-channel stereo (90degrees/24.5cm forward facing pair only), quad-stereo (all four central mics in an IRT-cross-like configuration), or 5-channel audio at the same time.  Although not intended this way I also find that of the MMAD arrays I've used, they all seem to fold down to 2-channel stereo really well. I extracted the setup diagram images below from this paper on his site- Magic Arrays - Multichannel Microphone Array Design applied to Multi-format Compatibility 

As you look at these examples, simply ignore the two rear-facing microphones and you have a 3-microphone array.  The first example below is then a spaced omni pair arranged to achieve a 90 degree SRA, with a completely optional center omni added to it which in imaging terms at least isn't going to mess up the main 2-channel omni pair whether you choose to use much of the center mic or not.  Substitute a cardioid for the center omni and you have very taper optimized spaced omni pair with a directional center mic.  Notice that in the examples using cardioids and supercardioids, the left/right pairs are pretty much in basic microphone configurations everyone here is familiar with- close to NOS and ORTF for the cardioids, and 110 degree X/Y for the supercards.

This sort of brings my thread-jack full circle, with reasoning for why one might want to put a center mic far forward of a pair of omnis- It allows you to use standard stereo setups for the left/right pair.  Another way of thinking about a far forward center mic is as a sort of as a spot mic, or like matrixing a SBD feed with your AUD pair.  In booth of those cases you often need to delay the spot or SBD to align it with the AUD source, and after doing that everything meshes nicely without comb filtering conflicts because the spot or SBD are placed far enough away from the main AUD pair.
« Last Edit: July 13, 2017, 12:54:09 PM by Gutbucket »
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Better recording made easy - >>Improved PAS table<< | Made excellent- >>click here to download the Oddball Microphone Technique illustrated PDF booklet<< (note: This is a 1st draft, now several years old and in need of revision!  Stay tuned)

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Re: Relationships between spaced omnis and center mic
« Reply #30 on: October 26, 2021, 11:57:35 AM »
Bumping to ask if anyone knows of any examples of this technique I can critically listen to. I'm most interested in the wider omnis + XY PAS configuration.

I'm also interested in any post-processing tips. If I try this, I would most likely just clap three times at the beginning and end of the show and use those spikes to line the sources up (M10 & A10). The blend of sources is obviously a matter of taste, but did you apply a low shelf cut on the cardioid source, etc?

Thanks!

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Re: Relationships between spaced omnis and center mic
« Reply #31 on: October 26, 2021, 05:34:52 PM »
I've been meaning to put up samples of the individual pairs and mix combinations for years.. and will try to get to that.  I'm too involved with work stuff currently.

You'll find mixed examples of spaced omnis + X/Y in the kickdown and at archive.

Use a 4+ channel recorder if you can.  If you need to record to two stereo recorders instead of one multichannel recorder, the clap thing can help you line up the two recordings.  Clap from directly in front, that way the peaks will line up for all four mic channels.  If you clap off to one side or the other, once you zoom in to see the transient, the closer omni will be seen to peak 1st, then the X/Y pair simultaneously, then the far omni.

I'd clap again at the end, so you can determine how much drift occurred over the course of the recording..  if audibly significant, you can use the two transient peaks to stretch one source to fit the other.

No need to do any processing other than lining up and adjusting levels to start.  Give each pair a listen on its own, and correct the level balance of each pair individually if necessary before playing around with mixing them.  Then unmute one pair and bring up the level of the other slowly while listening.  Play around with various level combinations.  Try it the other way too, starting with the other pair first then slowly bringing up the level of the second.  That may be all you need to do.

You can go further and try all kinds of things (the combination of two pairs provides many degrees of freedom), but its usually best to start with simply getting each pair sounding well balanced and correct on its own first, then play with the mix level between the two while listening.
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Better recording made easy - >>Improved PAS table<< | Made excellent- >>click here to download the Oddball Microphone Technique illustrated PDF booklet<< (note: This is a 1st draft, now several years old and in need of revision!  Stay tuned)

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Re: Relationships between spaced omnis and center mic
« Reply #32 on: October 26, 2021, 07:32:14 PM »
Quote from: Gutbucket
I've been meaning to put up samples of the individual pairs and mix combinations for years.. and will try to get to that.  I'm too involved with work stuff currently.
I completely understand.
Quote from: Gutbucket
You'll find mixed examples of spaced omnis + X/Y in the kickdown and at archive.
I've searched the LMA with some success, although I'm mostly finding very large omni spacing alone; no center cards. I'll search for something in kickdown.
Quote from: Gutbucket
Use a 4+ channel recorder if you can.  If you need to record to two stereo recorders instead of one multichannel recorder, the clap thing can help you line up the two recordings.  Clap from directly in front, that way the peaks will line up for all four mic channels.  If you clap off to one side or the other, once you zoom in to see the transient, the closer omni will be seen to peak 1st, then the X/Y pair simultaneously, then the far omni.
This was what I had intended to do, but I'm glad that you shared your experience. I'm not yet ready to invest in a 4 channel recorder.
Quote from: Gutbucket
No need to do any processing other than lining up and adjusting levels to start.
Sounds good, thanks!

If I come up with something good or interesting to share, I'll post it here.

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Re: Relationships between spaced omnis and center mic
« Reply #33 on: October 26, 2021, 08:38:01 PM »
Bumping to ask if anyone knows of any examples of this technique I can critically listen to. I'm most interested in the wider omnis + XY PAS configuration.

I'm also interested in any post-processing tips. If I try this, I would most likely just clap three times at the beginning and end of the show and use those spikes to line the sources up (M10 & A10). The blend of sources is obviously a matter of taste, but did you apply a low shelf cut on the cardioid source, etc?

Thanks!
I know we have done this exact combo, but cannot locate it with a quick search.
Here are some which come close: click the links and see the mic configs
https://archive.org/details/delmccoury2016-07-15.24.ck22ck8ck61   

https://archive.org/details/steepcanyon2016-07-14.24.ck22ck414.flac

https://archive.org/details/woodbro2019-01-24.AKGc426AKGck61DPA4061

https://archive.org/details/woodbros2019-01-24.dpaakg-24    ***This one is wide omnis XY PAS ***    (I'm trying..)  :P

https://archive.org/details/ttb2019-07-14.akgomt-24

https://archive.org/details/ttb2017-07-03.akgck2261-24             

Most posts have photos of the mic array either as the front photo OR in the .jogs section. I will think and ask kindms if we can nail down some more pure OMT4 XY PAS|Wide omnis for you.
I have more that are not on LMA, such as Gov't Mule and Phish.  Maybe WT them to you if you are interested. PM me of so
 
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Re: Relationships between spaced omnis and center mic
« Reply #34 on: October 26, 2021, 09:35:11 PM »
Much appreciated! If you locate some more with XY PAS, please let me know.

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Re: Relationships between spaced omnis and center mic
« Reply #35 on: October 27, 2021, 12:40:01 AM »
https://archive.org/details/sbb2018-09-21.AKGc426AKGck22
AKG c426 (hypers, 65deg) >V3 >PMD 661 & AKG ck22 (4ft Split) >V2 >Tascam DR680

Specifically PAS in the middle but with hypers
------------------------------------------------
https://archive.org/details/twiddle2017-09-03.akgck22beyerm201-24
Source: 75 feet from stacks Rt corner SBD tent
ch1|2 AKG 460|ck22 naiant active spread 60 cm
ch3|4 Beyer M201e hypercardiod X-Y 60'
> Tascam DR680|SD
Specifically PAS in the middle but with Beyer hypers

« Last Edit: October 27, 2021, 12:44:32 AM by rocksuitcase »
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Re: Relationships between spaced omnis and center mic
« Reply #36 on: October 27, 2021, 10:22:16 AM »
Thanks again! After listening, my initial impressions are:
  • I prefer the outdoor examples (not surprising since wide omnis excel in that environment to my ears), and the center mic(s) really do solve the 'hole in the middle' problem that I hear on some of the super wide examples I've listened to.
  • The 0/180 examples really excel at the 'you are there' feel with a natural crowd/ambience presence.
  • The one wide omni + XY example with no other mic configs (Twiddle) sounds excellent! I'm intrigued and will most likely give this a go. Do you recall about how far away you were for that one?
My motivation for trying this is that, while I'm generally very happy with my cardioid pulls in terms of stereo imaging, the resulting low frequency energy is naturally lacking. I have a K.I.S.S. and minimalist philosophy, so I tend to resist adding complexity to my setup.

My initial conclusion: to capture more low frequency energy I can either get closer to the PA with my cardioids to (maximize the proximity effect) or try this hybrid approach outdoors from the section.

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Re: Relationships between spaced omnis and center mic
« Reply #37 on: October 29, 2021, 11:15:37 AM »
I'm a latecomer to this thread and I haven't read the preceding posts, which may doom what I have to say to irrelevance, but: A third, center microphone in a two-channel stereo recording is useful when there is something particular for it to pick up--something that is available mainly to it, and not (or not clearly enough) to the other two microphones. Decca and RCA classical records used three main microphones (and three recording channels on 1/2" tape) for many years, with or without acoustic baffles to increase the separation of sound pickup among the three microphones at mid and high frequencies. (Decca went through far more variations than the public seems to know about; there's no one cookbook formula for a "Decca Tree" that deserves any special prominence over all others.)

But if the center mike is only going to pick up another "spatial sample" of the same general sound that the left and right microphones are immersed in, then it has a rather different kind of effect, which is to scramble the phase relationships in the recording and produce "comb-filter" effects--which can be interesting and useful, or harmful, or meh. To a great extent it's a game of chance. Sometimes it helps if you filter out the bass from the center microphone, and sometimes not.

Just make sure that you record the mid mike on its own track, because when you get home and review the recording, if you're honest with yourself, half the time or more you will find that the input from the center mike changes the sound but doesn't really improve it. There's something about listening to the output of two microphones with two speakers and two ears that is more engaging and less tiring than listening to multiple microphones working out their petty conflicts. Try not to be biased by the expense and trouble that you've gone to in setting three mikes up rather than two, if you can.

--best regards

P.S. added later: I see that one issue is the attempt to get more bass when the main microphones are cardioids. Proximity effect can't be used for this in most cases. The situation can definitely be helped by taking the low-frequency output from one or a (spaced) pair of pressure transducers. But mono bass is inherently boring, so the spaced pair is enormously preferable in terms of the spaciousness of the result, and four tracks rather than three are then called for. As a crossover frequency I suggest something quite low such as 50 Hz.
« Last Edit: October 29, 2021, 11:25:33 AM by DSatz »
music > microphones > a recorder of some sort

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Re: Relationships between spaced omnis and center mic
« Reply #38 on: October 29, 2021, 11:25:49 AM »

  • The one wide omni + XY example with no other mic configs (Twiddle) sounds excellent! I'm intrigued and will most likely give this a go. Do you recall about how far away you were for that one?

Source: 75 feet from stacks Rt corner SBD tent
Here is another XY center with the AKG c426 with AKG omnis split about 4 feet, and the 426 in XY hypercardioid 65' (PAS)
https://archive.org/details/sbb2018-09-21.AKGc426AKGck22
« Last Edit: October 29, 2021, 11:27:35 AM by rocksuitcase »
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Re: Relationships between spaced omnis and center mic
« Reply #39 on: October 29, 2021, 11:34:00 AM »
Thanks for the insights, DSatz. I stated my motivation to experiment with this technique in my last post. I'm certainly not looking for more post-processing work, as that is not task that I particularly enjoy.

Based on the examples I've listened to, this technique seems like it would be most useful in an outdoor setting to capture the PA from the soundboard. The widely spaced omnis alone in this setting capture the low frequencies I'm desiring, but at the expense of a hole in the middle in terms of stereo imaging. If I do try it, I will follow your advice and record the center to its own track.

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Re: Relationships between spaced omnis and center mic
« Reply #40 on: October 29, 2021, 11:41:14 AM »

  • The one wide omni + XY example with no other mic configs (Twiddle) sounds excellent! I'm intrigued and will most likely give this a go. Do you recall about how far away you were for that one?

Source: 75 feet from stacks Rt corner SBD tent
Here is another XY center with the AKG c426 with AKG omnis split about 4 feet, and the 426 in XY hypercardioid 65' (PAS)
https://archive.org/details/sbb2018-09-21.AKGc426AKGck22

Thanks again! You had shared that one previously, and I found it to be a bit distant sounding (I find that to be often true of my own recordings from the section) without the same high-frequency energy as the Twiddle recording. You're a bit closer than for Twiddle, but perhaps different PA and volume level? I think gutbucket mentioned earlier that it's best to point the XY capsules at the stacks. Do you recall if that was the case for this one?

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Re: Relationships between spaced omnis and center mic
« Reply #41 on: October 29, 2021, 01:22:15 PM »

  • The one wide omni + XY example with no other mic configs (Twiddle) sounds excellent! I'm intrigued and will most likely give this a go. Do you recall about how far away you were for that one?

Source: 75 feet from stacks Rt corner SBD tent
Here is another XY center with the AKG c426 with AKG omnis split about 4 feet, and the 426 in XY hypercardioid 65' (PAS)
https://archive.org/details/sbb2018-09-21.AKGc426AKGck22

Thanks again! You had shared that one previously, and I found it to be a bit distant sounding (I find that to be often true of my own recordings from the section) without the same high-frequency energy as the Twiddle recording. You're a bit closer than for Twiddle, but perhaps different PA and volume level? I think gutbucket mentioned earlier that it's best to point the XY capsules at the stacks. Do you recall if that was the case for this one?
65' would be PAS at that venue. Those recommendations are mainly to try to pick up more directional signal from the PA into the center mics than using the traditi0onal 90 or 110 degree XY. The PA's at each venue are certainly different. The Ives is not a class A system IMO.
https://archive.org/details/breakfast2018-03-24.akgck3_ck22
https://archive.org/details/jgb2018-03-24.akgck3_ck22

Here are two sets where we had the run of the place and so spaced the omnis about 15 feet apart, then put an XY 60' (PAS) Hyper center.
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Re: Relationships between spaced omnis and center mic
« Reply #42 on: October 29, 2021, 03:03:31 PM »
65' would be PAS at that venue. Those recommendations are mainly to try to pick up more directional signal from the PA into the center mics than using the traditi0onal 90 or 110 degree XY. The PA's at each venue are certainly different. The Ives is not a class A system IMO.
https://archive.org/details/breakfast2018-03-24.akgck3_ck22
https://archive.org/details/jgb2018-03-24.akgck3_ck22

Here are two sets where we had the run of the place and so spaced the omnis about 15 feet apart, then put an XY 60' (PAS) Hyper center.
Awesome, thanks again. I'm not familiar with the venue. Is it indoors or out?

Same venue on the same day, with the same setup...yet they sound different to me. The JGB has more high frequency energy than The Breakfast; it sounds really good! I would imagine that such a wide spread would have a major hole in the middle by itself. The more I listen to different examples, the more I'm starting to understand why gutbucket recommends wider spreads for this technique.

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Re: Relationships between spaced omnis and center mic
« Reply #43 on: October 29, 2021, 03:33:55 PM »
65' would be PAS at that venue. Those recommendations are mainly to try to pick up more directional signal from the PA into the center mics than using the traditi0onal 90 or 110 degree XY. The PA's at each venue are certainly different. The Ives is not a class A system IMO.
https://archive.org/details/breakfast2018-03-24.akgck3_ck22
https://archive.org/details/jgb2018-03-24.akgck3_ck22

Here are two sets where we had the run of the place and so spaced the omnis about 15 feet apart, then put an XY 60' (PAS) Hyper center.
Awesome, thanks again. I'm not familiar with the venue. Is it indoors or out?

Same venue on the same day, with the same setup...yet they sound different to me. The JGB has more high frequency energy than The Breakfast; it sounds really good! I would imagine that such a wide spread would have a major hole in the middle by itself. The more I listen to different examples, the more I'm starting to understand why gutbucket recommends wider spreads for this technique.
indoors, it is an old Catskills Borchtbelt dinner theatre.
Interesting take and proves you have GOOD ears.
JGB brought their own soundman, and they spent a long time doing SIM in the room which certainly had not been done prior to the BKFST set.

Split past 10 feet is the perfect way to do OMT. In fact, I'd say, if one split the omnis out to the exact width of the PA stacks, THEN run the center, you would be optimal from a mixdown perspective.
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Re: Relationships between spaced omnis and center mic
« Reply #44 on: October 29, 2021, 05:24:47 PM »
I don't know what SIM stands for, but I'm guessing you mean that they used measurement mics and EQ to dial in the room?

What's interesting is that this is the first example of an indoor pull with spread omnis that I have liked. I tried to look up photos of the hall, but it's hard to tell the shape, etc. Being 50ft from the stage probably helped a lot in terms of capturing direct energy vs the room and crowd.

Anyway, it's an excellent recording so kudos to you! It's funny, because a good friend just recommended that I check the band out.

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Re: Relationships between spaced omnis and center mic
« Reply #45 on: November 02, 2021, 10:42:34 AM »
DSatz raises a good point about potential phase interaction complexities as multiple microphone channels are mixed together.  I've found using more than two microphones in an array to be beneficial for many taper situations and provides a lot of flexibility, but it does require thought and attention regarding how those microphones are arranged and used in order to minimize potential problems and maximize the potential benefits. In light of that, limiting a multichannel microphone arrangement to three or four channels is likely to represent the sweet spot for most tapers who want to play around with what a multiple microphone array can do, as it strikes something of a reasonable balance between simplicity and complexity in providing additional degrees of freedom without the complexity getting out of hand.

Phase interactions arise from the output of multiple microphones placed in close proximity but slightly different points in space being mixed together.  A pair of spaced omnis plus a center microphone represents three separate points in space, two of which are summed to each playback channel (Left omni and center microphone summed to the Left channel; Right omni and center microphone summed to the Right channel).  One reason I recommend a coincident X/Y or M/S pair in the middle for a four microphone array rather than a near-spaced pair arrangement such as ORTF, NOS, "DIN" or whatever near-spaced configuration a taper might prefer over a coincident-pair when when recording only two channels, is that the substitution of a coincident pair for a single microphone does not increase the total number of different microphone positions in space.  It's four channels, but still only three separate positions.   

Another way of thinking about it is that a coincident pair (ideally) produces no phase difference between it's two channels, so the introduction of a fourth microphone placed coincidentally with one already present in a spaced three microphone array will not increase phase interaction complexities over those that are already present with just a single microphone in that position.

Keep your ear open for phasiness with any spaced configuration.  Especially those using more than two physically spaced apart microphone positions that will be summed together.  But don't let the potential for that keep you from trying multi-microphone spaced array techniques if they are interesting to you.
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Re: Relationships between spaced omnis and center mic
« Reply #46 on: November 02, 2021, 11:13:06 AM »
I stumbled upon a recent example: http://phish.in/2021-10-23
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Source: DPA 4006a w/30mm APE spheres(split 7.5'@9')->Portico 5012 + DPA 4011c(xy - centered between 4006s/-9.5db)->SD788t(24bit/96kHz)->MBit(16bit/44.1kHz)->FLAC

Recorded and transferred by Scott Schneider
Even with the spheres in use, there is an element of distance to this recording and I'm still perceiving a hole in the middle (on headphones).

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Re: Relationships between spaced omnis and center mic
« Reply #47 on: November 02, 2021, 11:52:54 AM »
Some more in-depth dirt on phase interactions between channels-

Fundamentally, there are two basic strategies for reducing potentially problematic phase interactions between microphone channels, and they are essentially opposite approaches to each other.  One seeks to eliminate the phase differences between channels by placing both microphones in a single "coincident" point in space, making the phase relationship between those channels identical.  The other seeks to make the phase relationship between channels different enough that they will not interact problematically when summed, achieved by spacing the microphones far enough apart from each other that the phase relationship between them is essentially randomized. That's the basis of the 3-to-1 spacing rule for close microphones that will be summed together into a single channel (Note- the 3-to-1 rule is not intended to apply to stereo microphone configurations where the channels are not summed. Many sources erroneously state that it does apply to stereo mic'ing, but a stereo pair is not summed, and 3-to-1 is geometrically impossible to apply anyway).  A complication of the spacing  approach is that the phase relationship between spaced positions varies in a complex way.  It varies with spacing distance, by frequency, and with the angle of arrival of the wavefront, and all three aspects interact with each other.  For wavefronts that arrive from any angle other than very close to perpendicular / directly on-center, the phase relationships at the high end of the frequency range is essentially randomized down to an increasingly lower frequency as spacing is increased.  The greater the off-axis angle from which the wavefront arrives, the greater that difference becomes.  The difference varies with frequency as well - there is increasingly less phase shift as the frequency decreases. 

When two similar signals with different phase relationships are summed, at each multiple of one full frequency cycle or 360 degree of phase difference the signals will constructively interfere to the greatest degree, and at each half cycle or 180 degree difference the signals will destructively interfere and cancel to the greatest degree.  Below some frequency where the difference in path length to each microphone is close enough to the wavelength of the frequency in question, the phase difference between channels will be less than 360 degrees and no longer effectively random in a perceptual sense.  At low enough frequencies with significantly long wavelengths in comparison to the spacing between microphones, the phase difference will be less than a 1/4 wavelength and the two signals will essentially remain in phase, acting effectively like coincident placement below that frequency.  That may or may not help explain how spacing a pair of omnis more widely can create more low frequency difference between channels.

How does all this relate to why a pair of wide spaced omnis with a coincident pair in the middle might be a highly effective four microphone arrangement for taping from an audience position?  And how might it be applied to deciding how to arrange it?  Consider the different qualities of direct-arriving verses reverberant sound. Direct-arrival sound requires a more highly correlated, or identical phase relationship between channels to present a well defined stereo image that features directional clarity and well-defined imaging, while reverberant sound has a more highly-randomized phase relationship and requires the preservation of that between channels to sound open, airy and natural rather than closed-in, flat and monophonic.  A recording will translate better if we can arrange things so as to preserve or even enhance both of these relationships, however the two are at odds with each other.  When using a near-spaced pair we are finding something of a best middle-way compromise solution that may not be optimal for either but works well enough for both. 

When using a coincident pair in combination with a wide-spaced pair we can set things up so as to have each pair better optimized for what it can best contribute.  A coincident pair produces no phase difference between channels and does a great job in translating clear and distinct directional imaging of direct arrival sound. A wide omni pair randomizes phase across a significant portion of the frequency range, which does a great job of translating a big lush open reverberant sense of space, and at lower frequencies can provide useful non-random phase differences that makes for more enveloping, non-monophonic bass.  There is still a compromise to be made however, although now it is a different one - managing the phase interaction complexity between both pairs.  Using a coincident pair in the middle helps with that, as does spacing the omnis far enough apart that they won't produce objectionable phase interactions with the coincident center pair (yet not too far), and a somewhat wider spacing that can be used when a center microphone or pair is included also helps with portrayal of the reververant aspects of the recording.
« Last Edit: November 02, 2021, 03:56:54 PM by Gutbucket »
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Re: Relationships between spaced omnis and center mic
« Reply #48 on: November 02, 2021, 01:05:11 PM »
As mentioned above, although a wavefront arriving from somewhat off axis will produce phase differences across two spaced microphone positions, particularly so at higher frequencies, a wavefront arriving absolutely directly on-axis, perpendicular to the spacing axis will not produce a significant phase difference between channels.  This is an interesting phenomena.

This aspect can sometimes be apparent when listening to a pair of wide spaced omnis where a hole in the middle is evident.  Listening closely, one may perceive a narrow "island" of sound centered in the middle of the hole.  It's also why using a particularly wide spacing can sometimes work amazingly well for recording a PA amplified concert from a more distant section position without producing a gaping perceived hole, where a hole would otherwise seem assured.  In that case there actually is a hole in the middle with regards to the reactions of specific audience members that aren't directly centered, which are likely to be heard as being clumped well over toward one speaker or the other (even though diffuse non-specific general audience din, roar and applause should be portrayed in a more even way). But because much of the PA sound is monophonic, if each widely spaced omni is essentially equidistant to the PA stack on its side, pickup of that PA content with such a wide arrangement can produce relatively minor phase differences so the direct sound from the PA sounds well centered without a hole.  The cross-path delay from the PA stack on one side to the spaced microphone position on the other likely serves to create some short-delay source-widening effect that also helps the centered PA material fill the soundstage.

One interesting implication of this may be it's potential application to the positioning of the center microphone or pair. We've talked here or in other threads about either placing the three microphone positions directly inline with each other, or shifting the center position forward somewhat, and if so by how much?   It might be useful to shift the center position forward just enough so that the path from the left PA stack arrives perpendicular to the axis between the left omni and center pair, and vice versa with the PA stack and omni/center-pair axis on the other side.  That seeks to minimize phase difference between the wide omni and center position for pickup of the PA on each side.  The farther back the recording position, and the narrower the distance between PA stacks, the less far forward the center microphone position would be placed in front of the omnis.
« Last Edit: November 02, 2021, 03:59:16 PM by Gutbucket »
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Re: Relationships between spaced omnis and center mic
« Reply #49 on: November 02, 2021, 01:19:50 PM »

One interesting implication of this may be it's potential application to the positioning of the center microphone or pair. We've talked here or in other threads about either placing the three microphone positions directly inline with each other, or shifting the center position forward somewhat, and if so by how much?   It might be useful to shift the center position forward just enough so that the path from the left PA stack arrives perpendicular to the axis between the left omni and center pair, and vice versa with the PA stack and omni/center-pair axis on the other side.  That seeks to minimize phase difference between the wide omni and center position for pickup of the PA on each side.  The farther back the recording position, and the narrower the distance between PA stacks, the less far forward the center microphone position would be placed in front of the omnis.
I've tried this once without a scientific measurement between the three sides of the triangle.
This paragraph would suggest you are getting toward a Decca Tree type arrangement for the center (fwd) channel(s). Am I headed in the correct direction as to your intent?
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Re: Relationships between spaced omnis and center mic
« Reply #50 on: November 02, 2021, 02:21:43 PM »
Sort of.. in a way, yes.  It would imply a more triangular arrangement when recording from closer in (as the apparent angle to the PA stack on each side becomes wider) as the opposite extreme to a very distant position where the wavefront arrival is flat and all three microphone positions would be arranged in essentially a straight line.

A Decca tree is traditionally placed above the conductors head, almost inside the the orchestra with sections wrapping around to each side.  I guess it might be thought of in that sense as having three axes normal to its sides where that kind of thing occurs: one facing forward (and rearward into the hall), one facing toward the left rear corner and one facing toward the right rear corner of the orchestra.  But I don't think that particular aspect or this analogy really works or extends to the widely distributed sources of an orchestra.  This is more specifically about the oddity of left/right spaced PA stack amplification and why super wide omni spacings which shouldn't work actually do work in that situation, and how we might make the best of that when adding a center pair.

So yeah, it turns into something of a triangular arrangement similar to a Decca tree, but perhaps for different reasons than how a Decca tree is traditionally thought of and used.

What was your experience in making those recordings?  May be hard to judge without a good baseline comparison with a straight line arrangement from the same position.

I'll say this- In moving to a directional center mic or pair I tend to not want the center coincident pair position out too far forward because I feel like I want the impulses from a front arriving wavefront well aligned across all forward facing mic channels, and I don't want to have to deal with the hassle of delaying the center channels to achieve that.  Yet I used to frequently use four omnis in a spaced diamond configuration from an FOB position in the center of the audience relatively close to the stage, using anywhere from a meter or two of horizontal spacing and a half-meter to a meter or so of front/back spacing.. and I never noticed a problem with smeared transients doing that.  In that case the axis between the center omni placed forward of the L/R omnis and the omni over on one side was probably close to being perpendicular to on-axis to the PA on that side much of the time.  I didn't think of it this way back then, though.  I thought of it more in terms of just achieving more spacing between the center and side omnis than I could otherwise, by pushing the center position forward, and also as a modification of the traditional Decca tree arrangement with a fourth microphone spaced out to the rear forming a diamond arrangement.
« Last Edit: November 02, 2021, 04:05:23 PM by Gutbucket »
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Re: Relationships between spaced omnis and center mic
« Reply #51 on: November 24, 2021, 09:53:05 AM »
Here's an example I made:  https://archive.org/details/garajmahal2018-05-11.AT4031.CA14

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Garaj Mahal
Be On Key Psychedelic Ripple
Denver, CO
May 11, 2018

Source 1: Audio-Technica AT4031s (XY at ~100*) > Zoom F8 (24/48 wav)

Source 2: Church Audio CA14 omnis (spread ~5') > Church Audio CA9200 (+10 gain) > Zoom F8 (24/48 wav)

Location: First column closest to stage (~10' from stage), ~7' high

Post: Mixed sources 1&2 (source 2 at -7 dB compared to source 1), amplify, fades, track splits, and conversion to flac using Audacity; File tagging using Mp3tag; ffp created using Trader's Little Helper
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Re: Relationships between spaced omnis and center mic
« Reply #52 on: November 24, 2021, 02:46:35 PM »
Here's an example I made:  https://archive.org/details/garajmahal2018-05-11.AT4031.CA14

Thanks for the nice example; sounds fantastic! 10' from stage in a small, relatively quiet club helps ;^)

Did you choose 100* in order to PAS?

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Re: Relationships between spaced omnis and center mic
« Reply #53 on: November 24, 2021, 08:57:55 PM »
Yeah, I was aiming for PAS and 100* was my estimate of what that was.  The driving force was the PAS part not the numerical part, if that makes sense.

Funny you should call that place relatively quiet...it often was anything but.  I think attendance was somewhat sparse that night though IIRC (or at least sparse compared to what I thought the band deserves...if there was any justice in the world a band of that caliber wouldn't even be able to fit the audience they'd draw into that place, but I digress).

Also note there are other sources available, like Scott's on stage DPAs.  That could make for interesting comparison, even if it's not necessarily like-for-like.
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Re: Relationships between spaced omnis and center mic
« Reply #54 on: December 29, 2022, 12:34:50 PM »
Reworking my recording rig to install Movos in place of BAS windscreens in preparation for a recording at an amphitheater earlier this month, I spent some time checking the current spacing relationships between microphone positions, modeling it's various 3-position relationships in the online Schoeps Image Assistant simulations and checking those against what Image Assistant suggests as optimal for the taping positions I commonly encounter.

It's really interesting to play around with 3-position arrays in Image Assistant enough to get a better feel for what changes actually effect which aspects, by how much, and how changing multiple variables tends to cause different aspects to interact or not.  Of the aspects it models, IA implies that center microphone spacing forward of the L/R pair primarily effects imaging, and that center microphone pattern doesn't matter much.

Now imaging isn't everything, and is not even most important IMO (even with regards to IA where I'm careful to check diffuse-filed correlation in addition to image linking), but in general Image Assistant suggests a more forward spacing of the center mic position than most of us are using.  I found myself juggling multiple variables and looking at the resulting plots, and realized I may be able to derive a table for 3-microphone-position PAS similar to the one for improved 2-microphone PAS.  Similar to Improved 2-ch PAS, it would include choice of pattern for the L/R pair, and because IA suggests that choice of center mic pattern is not strongly correlated with the other variables, I suspect the resulting table may be equally applicable to a coincident pair in the center in place of a single microphone. 

I considered starting a new thread to discuss this moving forward, but this one seems the most appropriate place to do so.  I will likely post in the current OMT thread additionally, specifically on how this may apply to arrays of 4 or more channels.  If it works out, I'll update the Improved PAS thread to include tables for both two and three position PAS. Stay tuned..
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^ I've been playing around with this a bit more, working toward a PAS table for 3-position microphone configurations, consisting of 3 or 4 channels: 3 if using a single center mic, 4 if using a coincident center pair.  So far I find it generally advantageous to position the center microphone or coincident pair about 20cm (8") forward of the baseline between spaced omnis, which works for a relatively wide range of omni spacings.  Overly narrow spacing benefit from more forward center spacing.  More details to follow..
« Last Edit: January 11, 2023, 12:30:16 PM by Gutbucket »
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Re: Relationships between spaced omnis and center mic
« Reply #56 on: January 16, 2023, 11:47:53 AM »
Forgive me if this is off topic or too specific for the thread, but I’m testing things out for a show this coming week, various combinations of omnis and wide cards (all Line Audio). Single or coincident cards in the middle and omnis about 28” apart as a starting point. I’m loving what I’m hearing with the coincident pair but I’m wary that I might think that simply because it’s a bit louder. I’m recording a rock trio so a really strong, straightforward center could work really well but it could be interesting to have them all more spaced.

For my question, are there rooms where I don’t want to deploy omnis? It’s a very small, 50 capacity room. The band will be very close together. I’ll be at stage lip and close miking. What is concerning me is that I could run wide NOS with the cards and know I’ll have a nice spread with space in the middle to mix vocals later, and I could run omnis as a second option and just take the better of the two. But if omnis are inherently no good in such a small space, and I go all in with LCR (preferable, all things being equal), then I’ll be in trouble if it fails.
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Re: Relationships between spaced omnis and center mic
« Reply #57 on: January 16, 2023, 02:15:55 PM »
If you are liking what you are hearing from the 28" spaced omnis + center coincident pair configuration, but are concerned about using omnis in a small, reverberant room that has challenging acoustics, and have an additional pair of directional mics available, substitute the extra pair of directional mics angled +/-45 degrees in place of the omnis while otherwise keeping the configuration the same.  With that 28" spacing you can probably angle them as tightly as +/-30 degrees or so when necessary, say if taping from the audience and that gets them pointed more PAS.  But depending on the stage setup, when recording at stagelip +/-45 degrees is probably about right, as it is likely to be from a recording position that is otherwise not overly distant.

Your 28" spacing is close to the spacing I use for a +/-45 degree directional L/R pair positioned between the center coincident pair and wider-spaced omnis, which I record as channels 5&6.  I basically keep the center and near-spaced directional portion of the array unchanged and include the omnis if/when appropriate.

In your case, think of this as substituting a directional pair for the omnis in situations where omnis don't work well.  Combination with the coincident pair in the center allows for more flexibility of spacing and angle in the "near-spaced" directional pair than would otherwise be possible with a 2-channel near-spaced pair used on its own in isolation (such as NOS, DIN, ORTF or whatever).  A bit more or less spacing and a bit more or less angle can work fine.  Still, a good starting point for spacing a near-spaced pair intended to work in combination with a center coincident pair is twice as much spacing as would typically be used for 2-channel near-spaced pair on its own.  The 28" you specify is right in that ballpark. 

NOS's 12" spacing is optimized for use as a single near-spaced pair without the inclusion of a center mic or coincident pair, but isn't spaced enough to work especially well in combination with a center mic position. EDIT- (that statement is too strong) NOS's 12" spacing was originally intended and optimized for use as a single near-spaced pair without the inclusion of a center mic or coincident pair. It isn't spaced enough to work really optimally in combination with a center mic position, even though the inclusion of a center mic may very well serve to improve the recording over 2ch NOS alone.  Your call on whether 2-ch NOS or 3 or 4chs consisting of 28" wide L/C/R using a coincident center pair is the way to go. If you are able and its not a hassle to do so, record six channels including the omnis as well.

Also as mentioned, when including a center coincident pair, best to place it somewhat forward of the other pair or pairs if you can.
« Last Edit: February 10, 2023, 09:47:37 AM by Gutbucket »
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Re: Relationships between spaced omnis and center mic
« Reply #58 on: January 16, 2023, 08:30:20 PM »
Sadly I don’t have an extra pair for this purpose but that’s a great tip for the future. I actually misspoke and it’s a “wide ORTF” mount from SRS that I mentioned worked well with mixing in vocals, etc, to fill in the center, but I think your advice still applies. How does one determine that a room is too small for omnis? Would that depend heavily on SPL?
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Re: Relationships between spaced omnis and center mic
« Reply #59 on: January 17, 2023, 11:41:06 AM »
^ Generally a room that sounds overly reflective and boomy.  Sometimes hard to tell without making a recording and assessing it that way, essentially getting to know the room and the usable recording locations. In my experience its easier for me to tell which rooms are likely to be good, only because some that seem like they'd be terrible aren't so bad after all.  A lot depends on recording location, the PA, the sound guy..  It can depend somewhat on SPL, I've frequently recorded in rooms that worked decently for acoustic and lightly amplified stuff but turned into a mess with more amplification.  You can sort of hear the room "load up" and get "boggy and messy", particularly in the lower frequencies.

But I often get away with using omnis from a closer position even in a not so good room.  The less good the room, the closer they need to be.  At one extreme, on stage is usually okay almost anywhere.  At the other, from a distance, I'd want to be outside for omnis work really well.  Put another way, the imperative of a good recording location being the most important thing becomes even more important.  Omnis will often work nicely from the down-front audience sweet-spot where we'd prefer to record from but may not be able or willing to do so.  Positions where the level of clean direct sound is proportionately much greater than the indirect reverberant sound.
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Re: Relationships between spaced omnis and center mic
« Reply #60 on: January 17, 2023, 12:42:36 PM »
Interesting! It makes sense, I just lack the experience to intuit that myself. I’m going to be super close to the source - like between 3 and 6 feet. But what I’ll probably do, since I have access to soundcheck, is record sound check with omnis and analyze, pulling the omnis if necessary.
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Re: Relationships between spaced omnis and center mic
« Reply #61 on: January 17, 2023, 01:26:24 PM »
You can probably get away with omnis that close. Might be best to..
Quote
run wide NOS with the cards and know I’ll have a nice spread with space in the middle to mix vocals later, and I could run omnis as a second option and just take the better of the two.
  ..and combine them or not depending on how it sounds.  That's a relatively safe bet.

If really wanting to add a center microphone or coincident pair to the "wide ORTF", substituting supercardioids in place of cardioids to maximize the limited difference between channels that will occur due to the minimal spacing and angle difference between all three microphone positions is likely to help somewhat.  Beyond that, I suspect what would work best when mixing the resulting recording may not be equal level from all microphone positions, but just enough from the center position to help solidify the center of the wide ORTF without getting into interference problems with close to identical levels.Or conversely, if you were using a coincident center pair and considering that the primary source, just enough "wide ORTF" to help that coincident center gain sufficient openness and lushness. In that way you would be using just enough of the extra channel(s) to fix what is lacking in the main pair, more so than using all the microphones as a single array. You are more likely to encounter problems when mixing levels are about equal, although that might not be a problem.  What sounds right is right.

Last spring I made one of the best recordings I've ever done of an instrumental prog-ish rock trio with the band playing a minuscule stage at the back of a tiny kava bar, placing my mic array at the edge of the stage essentially extending over the pedal boards of the guitarist and bassist, the drummer back only 4'  or so.  I used all the mics in my rig, and the (wide) omnis worked great, although I had to retract one of them each time the band got on or off the stage.  I did position the rig the way I prefer when recording a trio with a drummer in the center, which is to setup slightly off-center from the kick drum (avoiding to much woomp), and to point the center mic (a mid/side pair) directly at the snare.
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Re: Relationships between spaced omnis and center mic
« Reply #62 on: January 17, 2023, 01:28:26 PM »
I've been wanting to dive a bit deeper on what aspects are effected by the spacing between mics, and your "wide ORTF + center" question made for a good jumping off point for me typing this earlier this morning.

Similar to the addition of some soundboard feed to any taper stereo pair recording, if inclusion of soundboard vocals helps fill the center when using the "wide ORTF" mount, an additional center mic may work by sort of playing the same role, but ideally the spacing should be made greater when introducing that center microphone position at the array itself.  That's because introducing a center microphone or pair to the microphone array raises additional aspects that the inclusion of direct soundboard output does not.  There is more happening than just reinforcement of some sources in the center by mixing in some soundboard:

There are at least three things effected by the near-spacing between microphone positions:
1) Constructive/destructive interference
2) Diffuse field correlation/decorrelation
3) Imaging aspects

These things apply to any number of microphones greater than one, but generally, the well-known two-channel near-spaced microphone configurations have already been optimized in terms of spacing (the spacing for DIN, NOS, ORTF, etc, was determined long ago), and is of somewhat less significance when the channels aren't going to be mixed together electronically, which includes panning microphone outputs to positions other than hard left/right. With three or more microphone positions these things become more significant when some of those channels are going to be mixed together, such as the center microphone being mixed into Left and Right output channels. This is closely related to mono-compatibility, only extended to more than two microphones.

Somewhat more spacing helps with all three of those things.

1) The constructive/destructive interference thing relates the spacing between micrphones to wavelength.  The phase difference between direct-arriving signals at more than one microphone shifts with different angles of arrival.  That will produce comb-filtering to some degree, which differs with source position. Comb-filtering can be a most audible thing, most obvious by far while actively changing the spacing or effective spacing (or the delay between channels), especially when the two channels are mixed together. At closer spacings it audibly effects mid or even somewhat higher frequencies where it tends to be especially noticeable.  Wider spacings shift the peaks and valleys downward in frequency. It is very obviously audible when you hear those peaks and valleys shifting around as the spacing is changed, and audible but not nearly as obvious when the spacing remains are static - the peaks and valleys are still there though, acting sort of like and EQ with a wavy curve.  In my experience, this is something a taper needs to home in on empirically, by trying different spacings and figuring out what sounds right to them in terms of tone and frequency balance.  It's certainly the most tweaky aspect, and a relatively small change in spacing will shift the combing significantly, either for the better or worse.  Overall its probably the most important aspect of the three, yet is most difficult to predict beforehand simply by measurement.  I know from experience that the spacings I use myself work, but I cannot offer a prediction for folks using significantly different spacings with regard to this.  The same thing goes on at lower frequencies with wider spaced omnis, except the spacing between comb peaks and valleys is greater.  If you've ever found that sometimes you seem to get less bass rather than more with a pair of spaced omnis, and the bass sounded right in the room, its is likely that the spacing between omnis is causing a destructive valley at the frequency where the bass seems weak, relating to some lateral or off-axis bass mode.  With a different spacing that weak frequency zone would become neutral or emphasized.

2)  Diffuse field correlation/decorrelation is related, but easier to predict as the relationship is more straight forward, essentially about the phase difference between channels being above or below a certain threshold.  Essentially, we want the direct arriving sound from the stage and PA to produce a signal relationship between channels that is mostly phase coherent with clear and predictable phase correlation.  This can occur with spaced microphone positions when the spacing is perpendicular to the wavefront arrival, minimizing the difference in distance from the source to either microphone (even though its these relatively small non-zero differences that contribute to comb-filtering).  At the same time, it helps if the indirect-arriving reverberant sound, dominated by the room and audience sound, has a mostly random phase relationship between channels, making it decorrelated.  That makes that stuff sound diffuse, open, airy, lush, eliminates comb-filtering and perceptually keeps the reverberant room and off-axis audience sound from interfering with the coherent direct sound from the stage and PA.  More spacing = more decorrelation for sources that are increasingly off-center with respect to the array.  Directional pattern and angle can also achieve this, by having the null of one pattern line up with the on-axis direction of the other, but that means pretty wide angles which tend to put the mics off-axis from from the PA, so in most taping situations spacing is the better option to achieve good low diffuse field correlation.

3) Imaging-  This is probably least important but easiest to talk about, and predict (along with correlation).  It is what the Stereo Zoom and virtualization app tools do well.  Some of those, the Schoeps Image Assistant in particular, also predict diffuse correlation, but it's hidden on a different graph than the imaging information which tends to be the primary focus of the apps.  Adding a microphone or coincident pair between an existing stereo pair of microphones will make the stereo recording pickup angle wider.  That may seem somewhat contradictory to it also solidifying the center, and it is to some extent, but its more about how wide the recording sounds, sort of "how much is pulled in" rather than how solid the center seems.  Another aspect of imaging is how accurate the apparent source positioning is - do sources on stage sound like they are well focused and placed in the same position as they were live during the performance?  This is more of a nice to have thing.  Only folks that were there at the live event and pay attention to this kind of thing will know if the imaging accuracy isn't accurate, otherwise it just need to sound good and plausible, and some recordings are more plausible than others.  The previous two aspects are more fundamental and effect enjoyment of a recording far more.  They can even be heard and appreciated with one ear or one speaker, while imaging requires two ears and a proper stereo triangle or headphones.
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Re: Relationships between spaced omnis and center mic
« Reply #63 on: January 17, 2023, 01:38:44 PM »
When recording in close proximity to the source there is also consideration of placement closer to some sources than others and the loudness differences between microphone positions based on that alone, which is not a significant thing at greater distances.  In those situations it can be nice to have an array that includes enough spacing such that all sources on stage are more evenly covered, making getting a good balance of all sources easier, rather than tending to spotlight whatever ends up closest to a central very compact recording position.
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Re: Relationships between spaced omnis and center mic
« Reply #64 on: January 18, 2023, 07:29:33 PM »
I disagree with this statement 100%.

snip <NOS's 12" spacing is optimized for use as a single near-spaced pair without the inclusion of a center mic or coincident pair, but isn't spaced enough to work especially well in combination with a center mic position >snip

NOS with directional mics can very much benefit from a center forward facing single directional mic. I've made a bunch of recordings using this arrangement and it's been great more often than not.
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Re: Relationships between spaced omnis and center mic
« Reply #65 on: January 18, 2023, 09:45:06 PM »
When recording in close proximity to the source there is also consideration of placement closer to some sources than others and the loudness differences between microphone positions based on that alone, which is not a significant thing at greater distances.  In those situations it can be nice to have an array that includes enough spacing such that all sources on stage are more evenly covered, making getting a good balance of all sources easier, rather than tending to spotlight whatever ends up closest to a central very compact recording position.

Been thinking about this. Often (edit: always, up to this point) when I run stage lip I don’t have any influence over where amps, monitors, etc, are placed and I just need to embrace the constraints. In this case, I kinda do have some say, but the limited space will probably play a big role on what kind of live mix is possible. Bass placement, to an extent, matters the least I suppose in the live mix, since it would be centered in a final mix. To this end I’ve also wondered whether I should simply high pass at the preamp stage for the band-facing mic array to eliminate low frequency standing waves before the bits are even saved. I’ll have the bass close miked so I can easily center that later. If there’s any logic to this, then what would matter most is the placement of the guitar and drums. Having sat down in front of my stereo the past couple days to listen to some of my favorite trio albums, drums tend to lean a little left or right, and guitar or other soloist a little more firmly left or right.
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Re: Relationships between spaced omnis and center mic
« Reply #66 on: January 19, 2023, 09:51:41 AM »
^ All viable options.  I like to retain something close to what the placement on-stage actually was in the recording, but work the balance however is needed.  Partly because retaining the same general positioning arrangement keeps the reflections and ambient sound positioned correctly, making it easier to preserve a good sense of space and depth.  I've plenty of great recordings where the drums or bass or are over to one side yet the overall soundstage remains balanced, which can be refreshingly interesting because its not the same image geometry every time.  I try to stay open to whatever works in a gestalt sense, and nudging things around a bit without gross repositioning seems to be the most productive way for me to get there without losing depth and naturalness.  In that tiny-stage trio recording I mentioned above, the bass is left, guitar right, and drums centered (as the band was on stage) and it works really well.  I did intentionally setup slightly to the snare side of the kit, primarily to assure an unobstructed direct line from the snare to my center mic pair like I always want when close enough, yet angled the array slightly such that in the recording the drums are fully centered, with snare clear and nicely represented in the center.

I try to get a relatively equal amount of low-frequency energy in both channels of the recording, but perceptually the bass and kick drum might not actually be heard as centered.  What I usually want perceptually centered most of all are main vocals.  When making classical orchestral recordings that feature a singer I've gone both ways with regard to the singer being either perfectly centered, which is not accurate but more expected in a recording, verses slightly to the left of center, reflecting the actual position in performance standing just stage-right of the conductor.

A bit like your idea of high-passing the band-facing array at the preamp, in my array the directional mics are supercards which have a sensitivity that begins to drop off below about 200Hz, yet are still sensitive to content all the way down.  This effectively creates a gentle cross-over hand-off to the omnis supplying the meat of the bass, and that seems about right to me.  I don't like a sharp high-pass cutoff as much, but will switch one in if there are low frequency windnoise rumble problems that the windscreens alone are not taking care of, or pickup of vibration through the stand or something, ideally setting the corner frequency of the high-pass only as high as is necessary to take care of that problem.  The omnis make doing that work fine, but I prefer the "right amount" of bass in the directional channels where possible because in my experience the potential for a really great recording is just greater that way.  Sort of a Goldilock middle-way thing.
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Re: Relationships between spaced omnis and center mic
« Reply #67 on: January 19, 2023, 10:51:48 AM »
I disagree with this statement 100%.

snip <NOS's 12" spacing is optimized for use as a single near-spaced pair without the inclusion of a center mic or coincident pair, but isn't spaced enough to work especially well in combination with a center mic position >snip

NOS with directional mics can very much benefit from a center forward facing single directional mic. I've made a bunch of recordings using this arrangement and it's been great more often than not.

How about 50%?  ;)

Ok bad choice of words there on my part.  I'm not saying that can't work and produce great recordings, and since many tapers feel NOS on its own is a bit over-wide, a center mic would help correct that.   I really didn't intend to pick on NOS there, just using it as an example of a typical near-spaced configuration after vantheman mentioned using it. 

Let me restate it a better way- Named two channel near-spaced configurations were standardized by the organizations that originated them and went on to became popularly used and preferred because they represent good balances of angle and spacing for a stereo pair of two microphones in common recording situations.  That balance is going to change when introducing a third center microphone.  If that change is what you want and improves things in your situation, great.  That's the right choice, moving things in the right direction.  However, if your intent were to retain the same recording angle that you previously had with two microphones, you'd want to increase the spacing and angle of the original pair to do that.  Even if your intent is to change the recording angle (along with the other changes introduced by the 3rd microphone position, which may well be more important to you) there may be an even more optimal way to do it.

If open to trying it when including the center mic, I think you might like what you hear with somewhat more spacing than NOS between the L/R pair, retaining the same angle.  It's just makes for an even more optimized arrangement of 3-microphones, while most likely retaining what you like from the inclusion of the center mic to begin with.  If you try it let me know your thoughts.  If your intent is to retain standard NOS as a safe backup that won't necessarily require the inclusion of the center microphone, I get that too.  Totally legit choice.

Of course some of this comes down to preference, but some of it is arranging things to optimally pursue that preference.  Taper recording is an odd thing.  There is not really any one right  microphone configuration arrangement to rule them all, but a lot of latitude in what can make a good recording.. and potential for further optimization of what makes a recording a bit better or not.  I'm generally looking for and suggesting trends toward further optimization of what may already be working well that may be usefully applied.
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Re: Relationships between spaced omnis and center mic
« Reply #68 on: January 19, 2023, 11:06:15 AM »
Goodcooker, thanks for calling that out.  I went back and edited my previous post in light of your comment.

Quote
NOS's 12" spacing is optimized for use as a single near-spaced pair without the inclusion of a center mic or coincident pair, but isn't spaced enough to work especially well in combination with a center mic position. EDIT- (that statement is too strong) NOS's 12" spacing was originally intended and optimized for use as a single near-spaced pair without the inclusion of a center mic or coincident pair. It isn't spaced enough to work really optimally in combination with a center mic position, even though the inclusion of a center mic may very well serve to improve the recording over 2ch NOS alone.  Your call on whether 2-ch NOS or 3 or 4chs consisting of 28" wide L/C/R using a coincident center pair is the way to go. If you are able and its not a hassle to do so, record six channels including the omnis as well.
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Re: Relationships between spaced omnis and center mic
« Reply #69 on: February 09, 2023, 07:51:28 PM »
I thought I’d follow up on the side discussion we were having. The event came and went, and I’m not at liberty to share anything yet but I can share some observations, now that the event is over.

I can see myself getting a lot of mileage out of this spaced omni and XY cardioid rig. The XY pair gives you a sort of variable center in the way that mid side gives you variable side. I almost couldn’t dial up the XYs too much. It sounded good at any volume, giving me the stylistic choice of how much I wanted. In this case I went with no XY in my final mix for reasons I’ll explain but it’s going to work well in a lot of situations.

There’s always a curveball and this time it was that the drummer unexpectedly used the house drum kit, which was already set up at the venue, meaning he was center when I would have wanted him on either side. The bassist was sick and avoiding people so I didn’t mic his bass cab, and so I went with it and just had the array directly in front of them. I should really take more pics when I do this. It’s always the last thing on my mind.

I was deeply worried that I was going to have to fight the dominant pair and rebalance things to the detriment of the overall sound in order to make the mix sound more even. I had the multitracks, so I actually did go down that road to sort of discover on my own why I shouldn’t do that. But the problem remained that the mix was very left leaning - guitar left, drums center, and bass right. What I ended up doing which to my ears seems to avoid phasing or other weirdness was two small changes - I added the toms and snare ISOs a little center right, and left out the overheads ISO as the added cymbals were sounding phasey. I also added the bass ISO and panned right to reflect where he was actually standing, gave him a bump in the 1000-4000khz register to bring him out, and high passed at about 360 to let the omnis take over on the lower register.

I think the final result came out great. It completely evened things out with the lightest possible touch, and I might actually prefer it to having the drums panned to the side. I don’t think the approach would have worked as well without the multitracks, but I was delighted that it was possible to come out with a straight up great pull, no asterisks. I hope it’ll see the light of day soon one way or another.

But moreover I love this rig and this approach to stage lip taping, it allows for a lot of flexibility. Maybe one takeaway for others is to grab the bass ISO if you can and if you have limited inputs and need to make a choice.



« Last Edit: February 10, 2023, 10:36:34 AM by vantheman »
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Re: Relationships between spaced omnis and center mic
« Reply #70 on: February 10, 2023, 10:03:53 AM »
Good to hear.  Yeah, setting things up so as to gain increased flexibility is one of the best counters to the fundamental taper predicament of not really being able to adjust the setup while listening nor control the situation other than throwing mics up in a considered way and grabbing SBD/ISOs if/when available.

I can see myself getting a lot of mileage out of this spaced omni and XY cardioid rig. The XY pair gives you a sort of variable center in the way that mid side gives you variable side. I almost couldn’t dial up the XYs too much. It sounded good at any volume, giving me the stylistic choice of how much I wanted. In case I went with no XY in my final mix for reasons I’ll explain but it’s going to work well in a lot of situations.

That all rings true with my experience of the two configurations usually combining really well, complementing each other and offering a good range of adjustability.
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Re: Relationships between spaced omnis and center mic
« Reply #71 on: March 13, 2023, 12:39:43 PM »
Following up on this discussion..

I've been putting some time in modeling three-point microphone array setups in Schoeps Image Assistant, playing around with the variables and mapping out the results, in an effort to produce something similar to the 2-channel Improved PAS table except extended to 3-position arrangements, using either a single microphone in the center if recording three channels total, or better, a coincident pair in the center (Mid/Side or X/Y), recording four channels total.

In these 3-position arrays I'm modeling I'm keeping the spaced-pair PAS (pointed at the PA speakers).  The idea is that the user can either can use omnis when appropriate or switch to more directional microphones when recording from farther back in a less than excellent acoustic room.

The first take away I can share with you all is that it appears best to position the center microphone or coincident pair 17-22cm (7"-9") forward of the spaced pair. To reduce setup complexity, I'm attempting to solve for keeping that forward spacing constant at around 20cm (~8") so that the only spacing change that need be made is between the wide pair as the distance from the stage increases / PAS angle decreases.  Until I can get around to translating all this into an easy to use table, that forward spacing thing is the main thing I'd like to share here at present.

To give a hint of the a general range of spacings between the wide-pair, I'm attempting to solve for a range of PAS angles between 120 degrees down to 40 degrees, and to accommodate that, the spacings I'm coming up with cover a range of around 37cm (15") to 220cm (86").  Recording positions that are farther back will equate to a narrower PAS angle which requires greater spacing.

Stay tuned..
musical volition > vibrations > voltages > numeric values > voltages > vibrations> virtual teleportation time-machine experience
Better recording made easy - >>Improved PAS table<< | Made excellent- >>click here to download the Oddball Microphone Technique illustrated PDF booklet<< (note: This is a 1st draft, now several years old and in need of revision!  Stay tuned)

 

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