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Author Topic: Please help me understand/apply "stereophonic zoom"  (Read 9838 times)

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Offline attheshow

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Please help me understand/apply "stereophonic zoom"
« on: October 12, 2009, 09:02:55 PM »
I know that stereophonic zoom is a well respected "doctrine" around here, but I can't quite wrap my head around it. If the sweet spot for audience recording of a band on stage is the point of an equilateral triangle with the other two points at each speaker/stack, then according to the SZ article the SRA would be +/- 30 degrees. Looking at Fig 3 for cards, this would have my mics very far apart and pointing much more towards the walls at the sides of the room then the stage (range of 30cm apart at 180 degrees apart to 50cm apart at 70 degrees). I would expect to get tons of reverberated sound, and wouldn't think this would sound good.

I've run ORTF close to the stage and like that sound, but going wide and somewhat spread makes more sense (logically, to me) when you're close to a wide sound source...

Am I misreading something? Does it really just work contrary to my "common sense"? Or, should I be considering the audience as part of my sound source and widening the SRA accordingly? Please help me understand.

+t in advance.
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Offline Gutbucket

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Re: Please help me understand/apply "stereophonic zoom"
« Reply #1 on: October 12, 2009, 10:02:40 PM »
The basic phenomena harnessed by the Stereophonic Zoom are a couple of things that may at first seem backwards. Both are interrelated and can be played off of each other:
 
1) The farther apart the mics are from each other, the narrower the recorded angle.  Conversely, the closer the mics are to each other, the wider the recorded angle.

2) The larger the angle between the mics, the narrower the recording angle.  Conversely, the smaller the angle between the mics, the wider the recording angle.

One way that may help to understand that relationship is to consider that the overlapping pickup zone between the two mics is what creates the Stereo Recording Angle.  When the mics are angled farther apart, or spaced farther apart, they pick up less identical information and so the Stereo Recoridng Angle is smaller.

That basic relationship is what I consider the most important lession to take from the Stereo Zoom. 


I'll post more about it after I get something to eat out of the fridge and crack a beer. But consider that AUD concert recording is a odd duck in many ways, one major one being that the mics are generally placed much farther away from the source than in most other types of recording.
« Last Edit: October 12, 2009, 10:04:25 PM by Gutbucket »
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Offline Gutbucket

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Re: Please help me understand/apply "stereophonic zoom"
« Reply #2 on: October 12, 2009, 11:11:01 PM »
OK I'm back after bailing out the swamped crisper, grrrr.

As I was saying, AUD recording of bands playing through a PA is a strange animal.  If you were recording a band plaing without any PA you would probably want to place your mics much closer to the band, even on the stage itself.  You would then find yourself in a much more typical recording situation in the eyes of the Stereo Zoom, most books on recording, and the way engineers used to record orchestras and full bands in studios and somtimes still do if they're only using a few mics verses multi-tracking.  Because the microphones would then be much closer to the band, you'd probably want a wider Stereo Recording Angle and you'd find that following the Stereo Zoom charts would give you a microphone setup that was closer to standard configurations like ORTF and its cousins.

When the same band plays though a PA system, you need to consider the PA as a contributor of sound.  Depending on the size of the venue and PA, you may have most of the sound coming off the instuments on stage and just a few things in the PA, or you may have all the audible sound coming from the PA speakers. PA speakers are usually quite directional in how they project sound out to the audience. Partly because of that diretionality, partly because there are often big subwoffers right under the stage, and partly becasue the sound guy is probably adjusting the sound from a position farther back in the room, the sound may be boomy, bass heavy and not so clear in the area just in front of the stage. Because of those things you've undoubtably noticed that the sound is often better if you think of the PA like a giant stereo and place yourself at the apex of that imaginary isocolese triangle.  You will then be much farther from the stage than you would have been if the band wasn't using a PA and the Stereo Recoridng Angle from you new vantage point would need to be much narrower, everything else being equal. And so the Stereo Zoom suggests wider spacings and wider angles between your microphones to compensate.

But is everything else really equal?  The Stereo Zoom is really about optimizing only one aspects of recording, namely getting the stereo-ness accurate.  There are other things to consider that are often more important such as the frequency balance at the mic position, clarity, the direct-to-reverberant ratio, the off-axis performance of your microphones, blathering drunks, the balcony overhang, the noisy bar on your left, the raucous mosh pit, the sqeaky chair where the guy that snores always sits, the HVAC exhaust that blows on your mics, etc. 

Retorical quetion- What does an accurate distribution of sound souces across the playback stage mean when 80% or more of the sound you're recording is from a mono PA?

The Stereo Zoom is a great tool, the trick is to understand the ideas behind it so you can decide when and how it is best applied, also realizing that there might be an better tool to do the job in some cases.

Hope that helps.
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Offline attheshow

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Re: Please help me understand/apply "stereophonic zoom"
« Reply #3 on: October 13, 2009, 03:49:53 AM »
Thanks! That makes sense. I wish there was more written about audio recording from our perspective. You're right, the stereo soundstage isn't all that important at a concert, in that the sound coming from the PA may very well be mono, however recording in stereo seems to add depth and sound more natural. Sounds like I just need to find the best compromise for my location between SRA, reverberation and limiting undesired noises. I think I sort of "knew" this, but was trying to hard to conform to the "expert opinion" I was reading.
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Offline Gutbucket

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Re: Please help me understand/apply "stereophonic zoom"
« Reply #4 on: October 13, 2009, 10:13:11 AM »
I do think a good sound stage is important, just not the most important thing many times.  Even if there was only one mono source such as a single un-amplified soloist on stage, there would still be plenty of stereo information from the room itself and everything in it.  'Stack tapes' made from just in front of one of the PA speakers sound more spacious, deeper, and more 'you are there' when they are recorded in stereo, even though there is only one PA speaker in front of the taper.  The decision to 'stack tape' is one that values clarity, a higher ratio of direct sound to room sound, and a higher ratio of PA sound to crowd noise over sound stage accuracy in the traditional sense. Depending on the situation, choosing to record that way might produce the best sounding recording. The more I develop an understanding of the technical knowledge behind different ways to go about recording and how those things translate to playback, the more I realize that deciding when a particular technique will be the best trade off is the true art in recording. 

I don't mean to undervalue the Stereo Zoom, it's not only a fantastic conceptual  'pointer' to understanding some important basic relationships. It's also a great practical tool in providing a range of configuration possibilities that may be narrowed down by juggling priorities and applying all that other knowledge in optimizing a recording.
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Offline Brian Skalinder

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Re: Please help me understand/apply "stereophonic zoom"
« Reply #5 on: October 13, 2009, 06:35:07 PM »
If the sweet spot for audience recording of a band on stage is the point of an equilateral triangle with the other two points at each speaker/stack

I think Gutbucket has responded quite thoroughly to the main question at hand.  SZ helped me understand why changes in mic configuration yield different results.  With that knowledge, I may then adjust my configuration with a goo idea of how the change will impact results.

I would also add that the sweet spot isn't necessarily the location that forms an equilateral triangle with the two speakers/stacks.  In fact, it's often not true.  The sweet spot may vary from venue to venue, night to night, and even band to band within the same night.
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Offline Javier Cinakowski

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Re: Please help me understand/apply "stereophonic zoom"
« Reply #6 on: October 13, 2009, 07:03:32 PM »
To the original post:   In a narrow room here in Pittsburgh I often move my mics further apart and bring the angle in significantly.  I run like 35-40cm and only 45-60 degrees apart.  Sometimes the chart just isn't realistic for the room and conditions.  But the chart is a great starting point!


This thread should be stickied, as it might best explain the zoom technique here on taperssection.  good job guys!

Quote
gutbucket: There are other things to consider that are often more important such as the frequency balance at the mic position, clarity, the direct-to-reverberant ratio, the off-axis performance of your microphones, blathering drunks, the balcony overhang, the noisy bar on your left, the raucous mosh pit, the sqeaky chair where the guy that snores always sits, the HVAC exhaust that blows on your mics, etc. 

Yeah, those factors are very important.  Though the reverberation limits are calculated on the stereo zoom chart.

I pick my microphone pattern, be it hyper card or omni on many of the variables that you list.  If there is heavy wind or hvac I might choose a lesser directional mic.  If there are drunken retards everywhere and the room is boomy, I might pick hypers.  Sometimes you need to compromise.  Once I get my microphone polar pattern and microphone stand height determined, then I choose my proper sterophonic zoom technique.  I usually measure to the outside of the stack and read the chart from there.  I improvise and try new things often, even though I really find the sterophonic zoom to be the bible of our hobby.  If you don't push things to the extreme and experiment, you wont see as many obvious changes in you work, making future decision more difficult.  my $0.02
« Last Edit: October 13, 2009, 07:07:09 PM by Javier Cinakowski »
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Offline Brian Skalinder

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Re: Please help me understand/apply "stereophonic zoom"
« Reply #7 on: October 13, 2009, 07:09:11 PM »
This thread should be stickied

Agreed.  Added to the README1st stickied post.
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Offline newplanet7

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Re: Please help me understand/apply "stereophonic zoom"
« Reply #8 on: October 13, 2009, 08:59:58 PM »
I often move my mics further apart and bring the angle in significantly.  I run like 35-40cm and only 45-60 degrees apart. 
This is what I've been doing lately as a base config.
I run 14in @ 55* to start and have been trying
other variations from there.
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Offline Gutbucket

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Re: Please help me understand/apply "stereophonic zoom"
« Reply #9 on: October 13, 2009, 09:42:00 PM »
Quote
gutbucket: There are other things to consider that are often more important such as the frequency balance at the mic position, clarity, the direct-to-reverberant ratio, the off-axis performance of your microphones, blathering drunks, the balcony overhang, the noisy bar on your left, the raucous mosh pit, the sqeaky chair where the guy that snores always sits, the HVAC exhaust that blows on your mics, etc. 

Yeah, those factors are very important.  Though the reverberation limits are calculated on the stereo zoom.

Just to clarify, the reverberation limits in the Stereo Zoom aren't refering to the direct-to-reverberant ratio of sound at the microphone position or the total amount ot reverberant sound in the resulting recording, but instead refer to where the reverberant room sound appears in the resulting payback image. It's more of a warning that with some configurations the center might sound more reverberant than clear and direct, or that most of the reveberant sound of the room will seem to be coming mostly from the speaker positins and not spead evenly acrsoss the stage between the speakers.

That distribution of the reverberant room sound is one aspect of 'stereo-ness' that the Stereo Zoom aims to optimize, the other two are the Stereo Recording Angle itself which is the angle as seen from the microphone position that will fill the space between the speakers on playback (that's the big one and the main point of all this) and the other is how evenly sound sources located within that Stereo Recording Angle are distributed between the speakers on playback- are they mostly bunched up in the center, smooshed out to the speaker positions with a 'hole in the middle' or evenly spaced between the speakers with the same relative positions that they they had when the recording was made.

Great point about trying a variety of things to hear what's going on with all this.
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Offline DATBRAD

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Re: Please help me understand/apply "stereophonic zoom"
« Reply #10 on: October 14, 2009, 10:52:03 AM »
If the sweet spot for audience recording of a band on stage is the point of an equilateral triangle with the other two points at each speaker/stack

I think Gutbucket has responded quite thoroughly to the main question at hand.  SZ helped me understand why changes in mic configuration yield different results.  With that knowledge, I may then adjust my configuration with a goo idea of how the change will impact results.

I would also add that the sweet spot isn't necessarily the location that forms an equilateral triangle with the two speakers/stacks.  In fact, it's often not true.  The sweet spot may vary from venue to venue, night to night, and even band to band within the same night.

Adding to what Brian said, the "sweet spot" also may not necessarily be centered between the PA stacks. At some shows, the FOH postion is off center, and therefore the mix may be also.

I have taped at shows where I walked around listening from different spots during an openers set, or even just to the CD being played on the PA before the show, and found the audible sweet spot to be in a place I did not expect it to be just by looking at the venue and stage/PA layout.
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Re: Please help me understand/apply "stereophonic zoom"
« Reply #11 on: October 14, 2009, 01:32:34 PM »
Even if there was only one mono source such as a single un-amplified soloist on stage, there would still be plenty of stereo information from the room itself and everything in it.  'Stack tapes' made from just in front of one of the PA speakers sound more spacious, deeper, and more 'you are there' when they are recorded in stereo, even though there is only one PA speaker in front of the taper.

Just to add to the complexity of this subject, I think that using the two mic method on a stack helps to approximate how/what humans hear, with our two ears that are spread approximately six inches from each other.  So, perhaps it's semantics, but I think it's less about 'plenty of stereo information in the room itself', and more about how time delay (between our two ears and thus two mics) affects how our brains process the information that's laid down on each channel of a two channel recording.  So, what is captured on tape is pleasing to our senses because it provides a better simulation (than a single mic) of how/what each of our ears hears when we're standing in front of the stack during the show.

This is a similar, but different, phenomenon than is described in the SZ article where mic separation and incident angles are optimized for obtaining stereo effect through the time delay.

« Last Edit: October 14, 2009, 02:00:52 PM by tonedeaf »

Offline Gutbucket

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Re: Please help me understand/apply "stereophonic zoom"
« Reply #12 on: October 14, 2009, 04:23:02 PM »
Confirmation on the last two posts-
I was listening lastnight to a recording I made a week ago in a large venue which has rather muddy acoustics.  I did the pre-show walk around listening to the opening DJ, decided that a central position slightly closer than I expected sounded best and staked out a position to recoded from there.  When the main act began the sound went from decent to horrible- subs distorted, instruments muddy, vocals muffled- the typical too close to the stage without enough fill from the stage amps or side fills sound problem.  It was crowded and I stuck it out for a good while, hoping the sound would get dialed in, but finally bailed for a position off to the right that was close to and in-line with the right PA hanging line array.  Basically a stack tape position.  The sound there was much cleaner and better balanced than anywhere else in the hall according to friends who walked around during the main act.  That was confirmation of DATBRAD's point to an extreme- there was a recoding sweet spot, though no traditional stereo location.

Listening back to the two separate stereo pairs last night I had a realization that confirmed something akin to what tonedeaf mentions above.  the L/R pair from the revised position sounded clear and balanced, but perhaps a bit flat and 2-dimensional.  Not as much as a SBD pull of course, but lacking space, depth and interest.  However, the Center/Back pair had a almost as much clarity and balance in the Center channel, yet the baffled Back channel added a great sense of dimension and depth- exactly what is so often 'missing' in a more direct stack tape vs a good AUD position.  Some of that was probably due to increased sense of room space and verb picked up by the back mic but I think some of it was also due to the slight delay and timbre differenc between channels that is not present in the L/R pair.  I found myself enjoying listening to that pair more than the L/R pair, even though the playback soundstage was funky with the band primarily in the left speaker or headphone and the room in the right.  If I had to choose from just one or the other of the two pairs I'd be hard pressed.  But I don't have to choose, I can mix them or play them back discretely as a surround recording.  I mention it because what tonedeaf posted rang true in this case. 

We can't actually recreate the sound field that was present at the event, but we can trick our brains and fool ourselves into the suspension of disbelief.  It's all about creating an illusion.  Sometimes weirdo oddball stereo can be a more convincing and better sounding compromise than trying to make purist stereo techniques work in a compromised recording situation.
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Offline mdogbucket

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Re: Please help me understand/apply "stereophonic zoom"
« Reply #13 on: November 17, 2009, 12:50:43 AM »
So....to help dummies like me:

If I am always recording open with a pair of cardioid condenser mics and recording things without a P.A.  (live jazz, acoustic bands etc..) and I don't want to do too much calculation but like the results I get with ORTF and similar things, can you get good even stereo imaging by just keeping mics around 6-8 inches apart and modifying ORTF spread to keep the band outer players just inside of where the mics are aimed?

And, if I understand some of the above posts, the more overlap of information between the what the two mics are picking up, the more center heavy the resulting stereo image will be.  The less overlap, the more ping pong (sound clustered around the speakers) with a hole in the middle the stereo image will be.  Is that basically it?

One last thing.  I intuitively run my mics spaced 6-8 inches and crossed (toed in) rather than ORTF when there are close sidewalls or a P.A. that will make a natural stereo image hard to achieve.  Does this make any sense?

Thanks.

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Re: Please help me understand/apply "stereophonic zoom"
« Reply #14 on: November 17, 2009, 11:56:30 AM »
So....to help dummies like me:

If I am always recording open with a pair of cardioid condenser mics and recording things without a P.A.  (live jazz, acoustic bands etc..) and I don't want to do too much calculation but like the results I get with ORTF and similar things, can you get good even stereo imaging by just keeping mics around 6-8 inches apart and modifying ORTF spread to keep the band outer players just inside of where the mics are aimed?

And, if I understand some of the above posts, the more overlap of information between the what the two mics are picking up, the more center heavy the resulting stereo image will be.  The less overlap, the more ping pong (sound clustered around the speakers) with a hole in the middle the stereo image will be.  Is that basically it?

One last thing.  I intuitively run my mics spaced 6-8 inches and crossed (toed in) rather than ORTF when there are close sidewalls or a P.A. that will make a natural stereo image hard to achieve.  Does this make any sense?

Thanks.

When you are running coincidence, or near coincidence patterns, you will not get a "hole in the middle" unless the angle is extremely wide, say 120 degrees and beyond.

Cards setup in DIN, DINa, ORTF, as well as good old XY have probably 70% to 80% the same information, with only a slight difference noticable whenever left/right intensity differences in the source are recorded.

The "hole in the middle" effect usually comes from too wide spacing, regardless of angle. Take a pair of hypers and space them more than a foot apart, and the center of the image will collapse, and the sound will seem clustered around the speakers on playback.

For the type of recording you are doing, without a PA, I would stay with 90 degree patterns, and if you are real close, go with pure XY.

90 degree XY is the most accurate in terms of replicating the sound stage. The spacial position of an instrument in the live sound stage will be most faithfully reproduced using 90 degree XY. 90 degree classic DIN, DINa, and NOS are great to use when far afield, because the slight time delay will offset the "too centered" image that XY gives you when you are somewhat distant from the source. The angular accuracy remains.

ORTF is the pattern designed to most closely approximate human hearing, which is great when the placement of the pair is ideal, but not so great when placement is not ideal. I rarely run ORTF indoors, not because of a "hole in the middle" sound, but because the direct to reflected sound ratio is too low for my tastes, but that is just me.
 
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