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Author Topic: XY Greater than 90 Degrees?  (Read 6591 times)

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Offline Gutbucket

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Re: XY Greater than 90 Degrees?
« Reply #30 on: January 20, 2020, 01:53:18 PM »
With regards to adjustment of Mid/Side ratio of an X/Y recording (as well as direct recording in Mid/Side), I think it helps to think of the actual pickup pattern of the physical microphones and the setup angle between them, verses the virtual microphone patterns and angle between them which result from the Mid/Side ratio used.  If the Mid/Side ratio is altered, the virtual patterns and angle will differ from the actual microphone patterns and X/Y angle.

Using the term virtual helps keep the relationship between the physical microphone configuration and the virtual output configuration straight in my mind.

Bringing this back around to the original topic..
I found one of the most interesting things to mess around with when listening back to an ambisonic recording is changing the virtual microphone pattern/angle relationship.  With an ambisonic recording, the stereo output produced is always some virtual X/Y stereo pair. Output can be thought as being a choice between various combinations of a virtual pair of microphone patterns and inclusive angles between them.  What makes it different than Mid/Side in this respect is the inclusion of the omni microphone 3rd or 4 channel which effectively allows one to change virtual angle independently of virtual pattern, and vice-versa.  So one can hear what changing the virtual angle between microphones sounds like while the virtual pattern is held constant.

For example, one can choose a pair of virtual cardioids and play with various angles between them without altering the cardioid pattern.  Alternately one can hold the virtual angle between microphones constant while adjusting the shape of the virtual pickup patterns.

Doing this with "typical taper scenario room recordings" I found I generally ended up gravitating to virtual pickup patterns which landed somewhere between supercardioid and hypercardioid, with a typical inclusive angle of anywhere from around 110 to 120 degrees or so.  With cardioids I needed a 130-140 degree angle in an attempt to achieve a similar stereo-width and ambient-openness, yet the microphones were then typically too far off-axis from the source.   A subcardioid pattern with a 150 degrees or so angle seemed to work better than cardioids most of the time, and I'd frequently find myself going back and forth between virtual super/hypercard-like patterns with a more narrow angle between them and virtual subcardioids with a wide angle between them, homing in on what sounded best.

I ended up thinking a lot about a seemingly inherent "cardioid compromise" that defines the less attractive region between the above points on the continuum of X/Y patterns and angles. A few times I combined the two, using a wider angled pair of virtual subcardioids for low frequencies transitioning to a pair of more narrowly angled virtual hyper-cardioids at high frequencies.
« Last Edit: January 20, 2020, 02:14:22 PM by Gutbucket »
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Offline Gutbucket

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Re: XY Greater than 90 Degrees?
« Reply #31 on: January 20, 2020, 02:13:19 PM »
^My intention in posting this is not so much as an endorsement of ambisonics, but rather as a way to share what was for me a rather enlightening exploration of the general relationship between X/Y pickup-pattern and microphone angle.  Pretty much the only time I ended up liking a 90 degree virtual angle was with very hypercardioid-ish pattern that approached figure-8. 

Interestingly, I generally liked a somewhat smaller angle than the standard 90-degree specified as Blumlien for crossed fig-8s, sometimes as narrow as 70 degrees.  That helped better define a solid center, yet most of the time overall pickup of crossed-8s didn't provide enough forward sensitivity bias in typical taper scenarios, and something between supercardioid and hypercardioid tended to work better.
« Last Edit: January 20, 2020, 02:27:00 PM by Gutbucket »
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Offline EmRR

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Re: XY Greater than 90 Degrees?
« Reply #32 on: January 20, 2020, 03:17:25 PM »
^ yup to that.
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Re: XY Greater than 90 Degrees?
« Reply #33 on: January 20, 2020, 07:19:49 PM »
^ yup to that as well.

Despite certain ideal properties, in my work it has been rare that Blumlein was the most fitting solution. It has a relatively narrow stereo pickup angle, yet requires closer placement to the sound sources than most other types of setup because of the large amount of room sound that it naturally picks up--the rear lobe of a figure-8 being equally as sensitive as the front lobe, of course. And of course the closer you get to your sound sources, the wider the angle they represent from the microphones' point of view.

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Offline vantheman

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Re: XY Greater than 90 Degrees?
« Reply #34 on: February 07, 2020, 05:24:29 PM »
I took the XY 110 rig for a spin last weekend in Las Vegas for Van Morrison. All in all I’m pretty happy with the results, you can check it out here: https://drive.google.com/drive/folders/12u6Kxxg9-95TfLRjAPyYh5BwKnjxkzLg

I was front row balcony dead center in line with the center stack, and you’ll see that the music is pretty center heavy. It’s almost like a M/S recording, so mastering it was pretty straightforward once I realized this, as it harkens back to my MV88 days.

In this case I dropped the low mid range frequencies of the sides by about 6db, in an attempt to reduce the chatter, which isn’t all that bad to begin with. I lost hardly any musical information by doing this, maybe just a tad bit of “air”.

What I’m wondering is, what if anything did I really gain by going with 110 instead of 90? Yes, there is a nice stereo effect with the audience off to the sides, but I found myself wanting more center. Do you find that too? Due to where I was positioned, I only stood to capture one stack anyway.

These are things I’m considering before he comes up to Oakland next week. My approach for those shows will be pretty similar, just thinking about mic angle.
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Offline Gutbucket

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Re: XY Greater than 90 Degrees?
« Reply #35 on: February 10, 2020, 01:13:03 PM »
Sounds good here, listening on cheap Samsung phone headphones straight out of the computer.

What I’m wondering is, what if anything did I really gain by going with 110 instead of 90? Yes, there is a nice stereo effect with the audience off to the sides, but I found myself wanting more center. Do you find that too? Due to where I was positioned, I only stood to capture one stack anyway.

I suspect the 110 angle helped achieve the "nice stereo effect with the audience off to the sides" aspect you describe. I don't detect any lack of center, but I'm not listening over speakers where I'd prefer to make that kind of critical judgement.

If you plan to re-adjust stereo width in post with a M/S ratio adjustment anyway, you can let the geometry of the recording location relationship to the PA be the biggest influence in determining the X/Y microphone angle. Capture maximum clarity by keeping the microphones angled so that they are relatively close to on-axis from the PA source(s), then adjust virtual angle afterward to achieve the wider stereo effect with the audience off to the sides, but limit the ratio readjustment to the extent that sufficient center clarity and focus is retained.

If you are in a recording position where most of the energy is coming from a single PA stack position in front of you, a narrower (say 90 degree) X/Y angle combined with increasing Side ratio afterwards to achieve a similar perception of width as recording with a 110 degree X/Y angle and no ratio readjustment might be a good bet. 

Just remember to keep both the actual X/Y microphone angles and the ratio readjustments made afterwards within a reasonable range.  This method of optimizing for one aspect when recording, then optimizing for another when mixing afterwards can work quite well until it is pushed overly far.  In order for you to be able to increase either Side or Center contribution later, there has to be a sufficient amount of it in the raw recording ito start with.  If you were to start with a zero-degree X/Y angle because that would place the microphones on-axis with the only significant PA stack located directly in front of you, the recording will have no significant Side information at all, and there will be no Side information captured that you can emphasize later.
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Offline kevinsinnott

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Re: XY Greater than 90 Degrees?
« Reply #36 on: April 06, 2020, 12:19:06 PM »
My short answer is experiment and don't be too concerned about 90 versus 120 XY stereo. These are approximations I'm convinced were chosen mostly to generalize about image based upon some suppositions about how a recording might be made, which would differ completely in most of your applications.

I'm more accustomed to field audio for documentary audio, rather than recording live music, but I'm interested in this topic and otherwise enjoy this site. Historically, there have been lots of theories on stereo mic placement. A look at classical music recording philosophies, including French Radio's ORTF, Holland's NOS, so-called X-Y and Blumlein have gotten attention among sound enthusiasts, but whenever I read interviews with old sound engineers from stereo's early recording days, it's clear those are just a few. Heck, we've never really settled on how far apart speakers should be, or rather some stereo listening guides may have, but given the amount of listening done in automobiles and, particularly headphones, there's obviously no true one-size-fits-all recording methodology. I'm still astounded that after all these years the vast majority of public radio documentaries are done in plain vanilla monaural! BBC seems to have discovered stereo, but recording notes are almost nonexistent. I know a retired BBC engineer, who claims he did a lot of Mid-Side recording, but the shows I hear are all over the place.

I collect stereo microphones. One company who departs from XY slightly is Audio Technica. I own a couple of their mics that feature 120 degrees. The hole in the middle isn't a problem. Some image wavering is and can sound weird if you're, say, recording an interview and the subject wavers back and forth. Otherwise, no problem, and the backdrop is wider, which can be appealing. Sony has issued a number of Mid-Side mics, which allow you to electronically dial-in a soundstage. I find something between 90 and 120 is ideal for most of my purposes. At 120 still no problem. There is some mono incompatibility above 120. My favorite monitor phones are AKG DJ phone with a stereo-mono switch so I can judge summing compatibility in the field. 

Offline datbrad

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Re: XY Greater than 90 Degrees?
« Reply #37 on: April 06, 2020, 06:50:32 PM »
I agree, don't get stuck on angles, unless SRA is a big concern to you. The key trait of XY stereo  is the pure coincidence of the capsules (intensity). When capsules are positioned on the same vertical plane, usually one over the other, there is no comb filtering or phase issues when summed and played back in mono. Since spaced and near coincidence patterns are known to produce audible comb filtering, for decades in traditional broadcast radio, XY was usually favored because it made tracks playable on single speaker radios without loss of fidelity.
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Offline Gutbucket

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Re: XY Greater than 90 Degrees?
« Reply #38 on: April 06, 2020, 08:18:34 PM »
With wide X/Y angles there isn't really any mono incompatibility or hole in the middle issue in the traditional sense of those terms.  However, because the microphones are pointed further off-axis with respect to centrally located sound sources there is somewhat less sensitivity and possibly less linear response across the center, in combination with increased pickup from the sides and an increase in Side in comparison to Mid information in the resulting recording.

Pushed too far, the center may become somewhat less distinct while the sound arriving directly from the sides and around back becomes louder and more clear.
« Last Edit: February 23, 2022, 01:08:08 PM by Gutbucket »
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Offline Gutbucket

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Re: XY Greater than 90 Degrees?
« Reply #39 on: February 23, 2022, 12:58:37 PM »
Revisiting this thread a few years later, I would like to add..

In the same way that we can EQ the Side channel of a Mid/Side pair, or an X/Y pair converted to Mid/Side (typically boosting low-frequencies to improve spaciousness), it can be useful to EQ the Mid channel differently from the Side channel.  This will modify the sound of the content dominating the central portion of the stereo image, which can be useful for things like clarifying vocals.

Here's something to be aware of when doing this- Similar to the way that making a Mid/Side ratio adjustment modifies both the virtual pickup pattern of the microphones and the inclusive angle of the pair simultaneously, EQ'ing the Mid and Side channels separately will simultaneously effect both width and level in the frequency range being EQ'd.  If you boost the lows in the Side channel to increase spaciousness you will also be increasing the overall low frequency content of the recording.  Likewise, if you make a boost in the Mid channel to increase vocal clarity, you will simultaneously be narrowing the stereo width in that frequency region.

However, we can use a second EQ instance in L/R mode to re-adjust the overall frequency balance to whatever we want. The combination of using two EQ instances in this way is powerful. For instance, we might make a low frequency boost in the Side channel to increase spaciousness, only to find that the spaciousness is now much better but there is too much low frequency content overall.  We can fix that by making a low frequency cut in the L/R EQ to corrects the overall frequency balance without changing stereo width.  Or, if we have a problem with vocal clarity in the center verses the clarity of audience chatter to the sides, we might boost the Mid channel for improved vocal clarity in the center a bit more than we really want, then correct the overall timbrel change by making a corresponding cut in the L/R EQ.  The L/R EQ brings the timbre of the center part of the image back to what we want, and effectively reduces the same frequency range in the wide portions of the image.  You can work this balance between the two EQ's to your advantage.

Extra credit-
Instead of following the Mid/Side EQ with a L/R EQ to "undo the timbrel change" the same can be achieved in a single Mid/Side EQ instance by applying an inverse curve to the other channel.  If the exact inverse curve is applied to the other channel, only stereo-width/pickup-pattern is altered while overall timbre remains about the same.  That's great if using an EQ which can be set to automatically apply the inverse curve to the other channel, in which case the Mid/Side EQ is pretty much only effecting width, and the difference in timbre of the center verses the sides, while not strongly effecting the overall frequency response of the recording.  Follow that by L/R EQ to fine tune the overall frequency balance as desired.  Cool stuff, however that kind of automatic application of inverse curve in a Mid/Side EQ just a nicety.  It works just as well to first make a regular, non-inverse curve Mid/Side EQ adjustment followed by a L/R EQ adjustment for correcting overall frequency response and adjust the two in an iterative way to achieve the desired result. 
« Last Edit: February 23, 2022, 01:08:56 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)

Offline kuba e

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Re: XY Greater than 90 Degrees?
« Reply #40 on: March 29, 2022, 04:51:16 PM »
In the same way that we can EQ the Side channel of a Mid/Side pair, or an X/Y pair converted to Mid/Side (typically boosting low-frequencies to improve spaciousness), it can be useful to EQ the Mid channel differently from the Side channel.  This will modify the sound of the content dominating the central portion of the stereo image, which can be useful for things like clarifying vocals.

If you boost the lows in the Side channel to increase spaciousness you will also be increasing the overall low frequency content of the recording.  Likewise, if you make a boost in the Mid channel to increase vocal clarity, you will simultaneously be narrowing the stereo width in that frequency region.

Gutbucket, thank you very much for continuing this thread. I started thinking about it more. I'm just not sure about the equalization. I don't know what is overall frequency balance. Is it sum of left and right channel? Then that would be the mid channel. When we do the eq of side, we should just emphasize the difference in the right and left channel but the overall frequency balance (mid channel) should be maintained.

I was thinking, when we equalize the mid (or side) and add the second eq in the stereo left/right, it's the same result as equalizing only the side (or mid).

I'm confused. Maybe I'm looking at it wrong. I'll think about it more.
« Last Edit: March 29, 2022, 05:10:40 PM by kuba e »

Offline vantheman

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Re: XY Greater than 90 Degrees?
« Reply #41 on: March 29, 2022, 05:31:29 PM »
Yeah, I'm the thread starter and just saw this recent reply myself. I really want to experiment with that mid/side EQ technique. It hadn't occurred to me but it makes sense. I have a show coming up next week so I'll plan on doing 2 versions so I can A/B them (also using it as practice because I generally suck at EQ).
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Offline Gutbucket

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Re: XY Greater than 90 Degrees?
« Reply #42 on: March 29, 2022, 06:33:47 PM »
Hi guys, yeah this Mid/Side EQ stuff can be a challenge to wrap one's head around.. let me try to explain it better.

Kuba e-
By overall frequency balance I mean the general frequency balance or tonality of the entire recording.  That would be both Left and Right channels, or both Mid and Side channels in the overall collective tonal sense.

We essentially have 3 variables in play here: Stereo width, the tonality of the center verses the sides, and the overall collective tonality of the recording as a whole.

If we EQ Side only, we affect both width and the tonality of the sides of the playback image differently than the center of the playback image.  If the Mid remains the same as it was before when we do that, we also affect the overall tonality of the recording (say as heard from an adjacent room through an open door, devoid of all stereo imaging).  One way to avoid that overall tonality change is to make a complementary inverse EQ adjustment to Mid which offsets that overall tonal change.  If we do that, the overall tonality of the recording (as heard from the adjacent room) will remain the same, and only the stereo image width will be changed - actually by twice as much as it was when only EQing Side only (in this case of boosting one channel and cutting the other by the same amount, both work together in a complementary way in altering stereo width, while working against and offsetting each other with regards to the overall frequency balance as a whole). 

But unless the Mid/Side EQ is set to do so automatically, instead of carefully EQing the Mid with an exact inverse of the curve applied to the Side, another way of achieving the same thing is to use a Left/Right EQ with the same EQ setting applied to each channel, adjusted so as to "undo" the overall tonality change.. or push it toward whatever overall tonality you want.  This is simpler in practice.  Adjust stereo width (along with center/side tonal balance) with a Mid/Side EQ, and the overall collective tonality of the recording with a separate Left/Right EQ.

Here's the thing- If we EQ either Mid or Side differently from the other, we change stereo width in the affected range- Boosting frequencies in the Mid channel not only emphasizes them across the center, it also reduces their stereo width, while cutting frequencies in the Mid channel not only reduces them in across the center, it simultaneously increases their width. Likewise, boosting frequencies in the Side channel emphasizes them to either side while also increasing stereo width, while cutting them reduces them to either side and reduces stereo width.

It's all about the ratio between the Mid and Side channels in the frequency range of interest, regardless of exactly how that ratio is changed.

Not sure if that helps, but I hope it does.
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Re: XY Greater than 90 Degrees?
« Reply #43 on: March 29, 2022, 07:38:35 PM »
 :coolguy:

I have a stereo mic or two (card patterns @ 100'), and have observed a image improvement since running them that way.  It's like what you can accomplish with a smidgen of Ozone Imager width in post.
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Offline kuba e

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Re: XY Greater than 90 Degrees?
« Reply #44 on: March 30, 2022, 04:38:01 AM »
Thank you Gutbucket! All is starting to be clear to me now. I misunderstood tonality. I mistakenly thought that tonality was the sum of the left and right channels (the mid channel). But now, I realize that tonality should correspond to the sum of the absolute values ​​of the right and left channels. And that's something different. For example, when I sum two identical signals of opposite phase, I get zero. It is mid component. But when I sum the absolute values ​​of these signals, I get a double signal. This could correspond to the tonality.

I am sorry, I look at it more from the view of the numbers. I hope my idea is correct. When I eq for example +2db bass in stereo left/right, the same result is as when I add +2db bass in the side and +2db in the mid. But when I eq +2db bass only in the side component, it will expand the stereo width for bass (it increase the difference between the left and right channels) and also eq stereo image sides by some amount (if I understand right, stereo image sides contain the side and also the mid component, the mid component is zero only when left and right channels are in oposite phase). The tonality influence of the side eq is reduced towards the center of the stereo image because the influence of the side component decrease there. The very center of the stereo image will not be affected, because it is created only by the mid component.

I never realized the tonal influence in M/S editing. But now it reminded me that sometimes when I expanded the stereo image for bass, the center of the recording was cleaned up from bass hum and everything there was more clear. I thought it was because the change in the stereo image, but there's also effect of eq. I will pay more attention to the change in tonality on the sides and center.
« Last Edit: March 30, 2022, 05:12:20 AM by kuba e »

 

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