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Author Topic: Oddball microphone technique (OMT) - part 2  (Read 15391 times)

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Offline kindms

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Re: Oddball microphone technique (OMT) - part 2
« Reply #165 on: October 08, 2018, 06:58:00 PM »
Kindms, thanks to you and Rocksuit for your recording. I am listening it via speakers now. It has something that it makes me feel like I am right there. It sounded very pleasant because this.

I also tried to mix near spaced omnis (because it was stealth) with a center microphone. I was afraid of the comb filter. In the end, I mixed very little of center mic. I'm not sensitive for comb filtering, so I was rather careful. Still, the small addition of the center has improved the recording.

I was playing with my old oddball recording. Here is my little observation. Gutbucket advised us to try more methods to mix rear channel. One was just to mix it in. Another was to stereoise the rear channel by M/S with forward mic. Or we can stereoise the rear channel by some other artificial effect (eg.reverberation). I was trying M/S and it was not working well in my recording. It sounded somehow sharp, unpleasant. I remembered that Gutbucket also mentioned that we can try to make artificial stereo of rear channel by plugin for phase shift. I found this plugin. This plugin shifts phase of all frequencies by constant angle. The rear channel is mixed in M/S by phase shift of +-180° . But we can set lower shift by this plugin.
https://www.kvraudio.com/product/phasebug_by_betabugsaudio

I think I used +-55°. It sounded better than M/S. It was not so aggressive. I mixed the rear mic into the right channel with positive phase shift and into the left channel with the opposite phase shift. There are two variables for setting - level of rear channel and phase shift. I set up the level as in the previous methods. Phase shift, I set first +-180 ° as in M/S, and then I was lowering it than it sounded pleasantly.

Interesting. And Im glad you dig the recording. I think the proximity of the omnis makes the recording sort of "shine" in headphone playback. We kind of envisioned the human head when setting up the array if you can call it that.  So Omnis as the ears c426 as the eyes, the ck61s as the 3rd eye

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Offline ycoop

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Re: Oddball microphone technique (OMT) - part 2
« Reply #166 on: October 08, 2018, 08:06:10 PM »
^ Talk about oddball  ;)
Mics: Avantone CK-1s, CA-14 omnis
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Recorders: DR-60d mkII, DR-22wl, DR-07

Offline kuba e

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Re: Oddball microphone technique (OMT) - part 2
« Reply #167 on: October 09, 2018, 06:49:20 AM »
^ Talk about oddball  ;)

Great, this picture should be the title page of Gutbucket's oddball pdf.

I think the proximity of the omnis makes the recording sort of "shine" in headphone playback. We kind of envisioned the human head when setting up the array if you can call it that.  So Omnis as the ears c426 as the eyes, the ck61s as the 3rd eye

Yes, it sounds great in headphones.

I downloaded for comparison second recording of the same show made by Michael - pair of Schoeps, supercardiod . It is excellent recording. It has very nice stereo separation and everything is sounding clear and upfront. But for some unknown reason the omt recording sounds little more pleasant to me. Maybe it is because openness or just different sound color (eq).
http://bt.etree.org/details.php?torrentId=601963

Offline Gutbucket

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Re: Oddball microphone technique (OMT) - part 2
« Reply #168 on: October 09, 2018, 10:54:16 AM »
We kind of envisioned the human head when setting up the array if you can call it that.  So Omnis as the ears c426 as the eyes, the ck61s as the 3rd eye

Love it!

I was playing with my old oddball recording. Here is my little observation. Gutbucket advised us to try more methods to mix rear channel. One was just to mix it in. Another was to stereoise the rear channel by M/S with forward mic. Or we can stereoise the rear channel by some other artificial effect (eg.reverberation). I was trying M/S and it was not working well in my recording. It sounded somehow sharp, unpleasant. I remembered that Gutbucket also mentioned that we can try to make artificial stereo of rear channel by plugin for phase shift. I found this plugin. This plugin shifts phase of all frequencies by constant angle. The rear channel is mixed in M/S by phase shift of +-180° . But we can set lower shift by this plugin.
https://www.kvraudio.com/product/phasebug_by_betabugsaudio

I think I used +-55°. It sounded better than M/S. It was not so aggressive. I mixed the rear mic into the right channel with positive phase shift and into the left channel with the opposite phase shift. There are two variables for setting - level of rear channel and phase shift. I set up the level as in the previous methods. Phase shift, I set first +-180 ° as in M/S, and then I was lowering it than it sounded pleasantly.
^
Great that you've tried this!  Thanks for your report.  This is one of the things I've wanted to explore for years- constant-angle opposite phase-shifts routed Left and Right (guessing a shift of somewhere between 40° and 90° will be most appropriate).  I'll have to check out that kvraudio plugin.  Most of the others I came across did other stuff than simply shifting phase by a constant amount across all frequencies- and those intended as "pseudo-strereoizing" mono-to-stereo plugins are mostly crap.  The one I do know can do the constant phase shifting without other unwanted tricks is Voxengo PHA-979.

A few reasons this approach is conceptually attractive for me-
It doesn't change the timbre of the rear-facing channel content.  The solo'd rear facing channel(s) sound the same before and after this phase-shifting treatment.  It only changes the how the separate channels interact with each other. 

When summed to mono, all opposite polarity content cancels out.  So with a standard Mid/Side 180° phase shift of the Side channel (in this case Rear channel), the Side/Rear content goes away entirely and only the Mid + omni sum content remains.  But with less than a 180° shift, only some of that content is cancelled, so by gaining control over the degree of phase shift, we gain control over how much rear-ambience remains in the mono sum.. which is also how much rear-ambience is present across the center portion of the 2-channel stereo image.  In other words, we gain control over rear ambient stereo width without effectively reducing the center channel level at the same time.

This is the primary principle upon which old-school matrix surround is based.  Content with a total phase shift of 180 degrees between Left and Right channels is routed to the back center.  Content with no phase-shift is routed to the front center.  Content with a phase shift somewhere between those two extremes is steered somewhere between front and rear with the degree of phase shift determining how far front/rear and the polarity of the shift determining if that is sent towards the Left or Right sides.  So in addition to making better sounding 2-channel stereo output, this is also effectively modifying the defacto matrix surround encode created by your mix.  Anyone playing your 2-channel stereo file back through a surround matrix decoder will hear the rear ambience spread around the back between the Left/Right and surround channels.

A +/-90° Left/Right phase shift = 180° total (rear ambience routed to the back in surround, to the far sides in stereo, and cancels out entirely in mono).
A +/-45° Left/Right phase shift = 90° total (rear ambience routed to the sides in surround, halfway between the far sides in stereo (less out-of phase stereoness), and is reduced by 6dB in the mono sum rather than canceling out entirely).
No Left/Right phase shift = 0° total (rear ambience routed to center in surround, to the middle of the image in stereo (Mid), and is not reduced in level at all in the mono sum.

Your +/-55° Left/Right phase shift = 110° total, which likely represents the most appropriate mid-way point between +/-90° and +/-45° for that particular recording.
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Offline Gutbucket

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Re: Oddball microphone technique (OMT) - part 2
« Reply #169 on: October 09, 2018, 11:21:20 AM »
I'm also interested in other ways of decorrelating the Left/Right routing of the single rear-facing ambience channel which may be more effective than phase-rotation.  In my discussions with wforwumbo (who just completed a doctorate in acoustics), IIRC he suggests trying a plate-style reverb with zero (would that work?, maybe an especially short) reverb time.  Not sure of the specific settings, but the intent is not adding 'verb which extends the time envelope of the sound, even if very short, but rather sufficiently "different-izes" the Left/Right feeds from the single channel without adding any specific sound character other than increased spatialization.  Perhaps he'll chime in here.
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Offline kuba e

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Re: Oddball microphone technique (OMT) - part 2
« Reply #170 on: October 10, 2018, 06:30:25 PM »
When summed to mono, all opposite polarity content cancels out.  So with a standard Mid/Side 180° phase shift of the Side channel (in this case Rear channel), the Side/Rear content goes away entirely and only the Mid + omni sum content remains.  But with less than a 180° shift, only some of that content is cancelled, so by gaining control over the degree of phase shift, we gain control over how much rear-ambience remains in the mono sum.. which is also how much rear-ambience is present across the center portion of the 2-channel stereo image.  In other words, we gain control over rear ambient stereo width without effectively reducing the center channel level at the same time.

Thanks for this explanation. It's good to look at this issue from perspective of mono and stereo. I like the idea of mixing Mid/Rear. It is bringing something special to the recording. When I mix the rear channel just as mono, the effect is less significant. I recorded only couple of recordings by omt. Maybe it's just a coincidence and the next recordings will sound better with mono rear channel. I am curious how would compare the Mid/Rear and a pair of rear mics in XY. Also, I am curious about wforwumbo reverb effect too. Thanks for the basic surround matrix theory too.

I update the link for phase plugin. I don't know if the plugin is high quality. But I tested it on the sum of mono. It is working very well. When decreasing the phase, the mono signal increases. And all is canceled for 180°. I do not have a good listening skills. Maybe it is creating some unwanted artifact. But it should be negligible for our rear mic.
http://www.pcjv.de/vst-plugins/betabugs-plugins/
« Last Edit: October 10, 2018, 06:36:47 PM by kuba e »

Offline Gutbucket

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Re: Oddball microphone technique (OMT) - part 2
« Reply #171 on: October 11, 2018, 12:44:26 PM »
Yeah I really dig the rear-facing channel.  I've said it here many times, but almost every time I'm tempted to leave it out, I find that things are just not quite as good without it.  I keep meaning to experiment more with an additional rear-facing channel, but I value what the addition of other microphone channels do more, so tricks such as we are discussing which "de-monoize" a single rear-facing channel will remain attractive unless I move to recording more than 6 channels. 

"De-monoizing" the rear ambient material, either by manipulating the single recorded channel or by recording more than one rear channel seems to help with a couple things- It keeps that material from imaging strongly in the middle and thus directly competing with the forward-facing channel intended to monopolize the monophonic center-portion of the image; and it conveys that material in a more more diffuse, wide and enveloping way, which is appropriate as it ideally consists of all reverb, reflections, and audience reaction, with as little direct sound leakage from the front in it as possible..

I am curious how would compare the Mid/Rear and a pair of rear mics in XY.

Stick with me here and I'll get to that..
TLDR- Other than one test mentioned below, I've yet to move beyond a single rear-facing channel mainly because any additional recording channels have proven more valuable to me when pointed elsewhere, at least when recording up to 6 channels total. 

IME, the primary requirement of the rear-facing channel is minimizing pickup of direct sound arriving from the front.  Next is appropriate time-alignment, such that the direct sound from the front which "leaks in" to that channel will not be smear the imaging provided by the the primary microphones when everything is mixed together.  That happens automatically if all the microphones are on the same stand and the rear-facing microphones aren't distant from the others.  It doesn't have to be perfectly aligned though, as it does for a typical matrix of multiple forward-facing mics- and I suspect a few milliseconds (corresponding to a foot or two between the front/rear pair) is helpful for differentiating the front from back, "keeping the front sound in front" via that slight time of arrival difference.

^That's most definitely the case for discrete surround playback where the rear channel is routed to speakers behind the listening position.  In that case the front-bleed into the rear channel, combined with insufficient front/back spacing perceptually pulls too much of the front direct sound into the rear surround speakers.   A little spacing provides time-of-arrival cues which helps keep the direct sound from the front sounding like its coming from the front, such that more rear facing channel ambience can be used before the front imaging shifts rearwards.  And for quieter sections where the front sound isn't considerably louder than the sound from the rear, it also helps keep the "rear sound in back", and better differentiated from the front. 

However, I'm not as certain about the front/back microphone spacing when mixing to 2-channel L/R stereo.  I know that the spacing I am using works nicely in my 2-channel mix, so I stick with it because it works well for both surround and stereo, but I'm not certain if less spacing might be better or not for 2-channel stereo alone.  It could be that having only level differences between the front/rear channels and no time of arrival differences will make for cleaner, less smeared front imaging.  It's probably a trade off depending on how much front/back level isolation can actually be achieved, because in a practical sense, the greater the front/back level difference there is, the less time-of-arrival difference should be required.  Varying the front/rear microphone spacing is something I encourage those of you trying these OMT setups to experiment with, including trying a coincident front/rear arrangement.  If you do so, please let me know.  From a practical point of view, reduced front/rear spacing will make for a more compact and potentially less visually intrusive setup.


After minimizing pickup of direct sound arriving from the front and achieving appropriate time-alignment, the third most important thing is the "de-mono-ization" of the rear-ambient pickup we've been talking about.  I'd probably not choose a rear-facing X/Y pair for "de-mono-izing" the rear ambience because X/Y with cardioids is center mono-heavy unless you use a very wide inclusive angle, and then you are going to get lots of front leakage into the sides of the cardioids.  X/Y supercards would be better, angled such that their shared null-axis points directly forward, but if using a pair of cardioids or supercards, I'd rather just space them out on the same bar which is supporting the omnis and point them directly rearward or maybe with a slight outward angle, keeping their minimum sensitivity axis facing towards the front.

I make a practical exception for a Mid/Side rear-facing pair (by the addition of a coincident Side fig-8 to the rear-facing cardioid/supercard), partly because it means the simple addition of the figure-8 to the already existing rear-facing microphone without otherwise changing it.  If the rear-microphone is a cardioid, the pattern nulls of both the cardioid and the 8 face directly forward and regardless of the Mid/Side output ratio all virtual output pair combinations will likewise feature a forward facing null, helping to limit pickup of direct sound from the front.  This is essentially the same as rear-facing X/Y supercards/hypercards angled such that their shared null angle faces forward.

^ I made some test recordings at Wanee year before last to experiment with front and rear facing Mid/Side pairs, but have yet to do anything with the rear facing pair.  In that case I placed Naiant X-8S fig-8's coincident with my standard front-facing supercardioid and rear-facing supercardioid, in order to turn them into front and rear facing Mid/Side pairs.  I recorded the rear-facing pair to the stereo channel of the DR-680 through a V3 feeding the digital input of the DR-680 so as to be able to record all 8 channels time-aligned on the same machine, and only messed around with comparison between the split omnis + a coincident Mid/Side stereo center, versus split omnis + a near-spaced 3-channel L/C/R center setup, as well as various combinations of the two.  I did so via a 2-channel mix directly out of the recorder using its Mid/Side playback function option.  The conclusion was that either center setup works well in 2-channel mix, they sound different from each other, both represent a significant improvement over a single center channel, and both together in a 2-channel mix is not necessary.  Also that the 3-channel near-spaced L/C/R center setup is superior for playback over 3 front speakers, which is not surprising.

In doing that comparison, I was struck by the absence of the rear-facing ambience channel, since in the direct-off-the-DR-680 monitor mix the stereo channel cannot be played back simultaneously along with the other 6 channels. I missed it!  And I've yet to get around to playing around with all 8 channels in the DAW including the rear-facing Mid/Side pair.

Quote
I update the link for phase plugin. I don't know if the plugin is high quality. But I tested it on the sum of mono. It is working very well. When decreasing the phase, the mono signal increases. And all is canceled for 180°.

Good test, which seems to confirm it is working correctly.

wfowumbo reminds me that this kind of equal phase rotation by frequency manipulation is called a Hilbert filter.
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Offline Moke

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Re: Oddball microphone technique (OMT) - part 2
« Reply #172 on: October 13, 2018, 04:21:36 PM »
OK,... Impr0v Oddball,...
Acoustic music, string quartet, w/ amplified vocal.  6' out from stage.
Binaural + Line Audio CM3 as 0º forward and 180º rearward.  The 0º forward mic is synched with the binaural. The ambient 180º mic is slightly rearward of the main three.
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Offline kuba e

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Re: Oddball microphone technique (OMT) - part 2
« Reply #173 on: October 14, 2018, 06:11:13 AM »
Mike, you have started to use rear mics too! I am looking forward to hear from you how the rear channel complements Gude's head. Is it an improvement or is the rear channel negligible? I am curious too what you will say about using the rear mic for acoustic music. I don't tape a lot now. But if I have chance I always use OMT with rear channel for audience recording. Big thanks to Gutbucket.

Mike's setup brought me a question. Have somebody tried rear channel for onstage or stage lip? It can revive the recording, bring more audiences. But does it have any plus for music?

Gutbucket, thanks for your great explanation. I'm already infected like Moke, Kindms, Rocksuit, Heathen ... I'm thinking that I will try 6 channels next occasion.

Varying the front/rear microphone spacing is something I encourage those of you trying these OMT setups to experiment with, including trying a coincident front/rear arrangement.  If you do so, please let me know.  From a practical point of view, reduced front/rear spacing will make for a more compact and potentially less visually intrusive setup.[/i]

How we can setup coincidence arrangement? It is easy with vertical capsules. But the mic's bodies should influence sound for horizontal capsules.

wfowumbo reminds me that this kind of equal phase rotation by frequency manipulation is called a Hilbert filter.

I google it. Some very complicated math behind this (probably behind most of plugins we are using). Kudos to programmers.
« Last Edit: October 14, 2018, 06:17:13 AM by kuba e »

Offline Moke

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Re: Oddball microphone technique (OMT) - part 2
« Reply #174 on: October 14, 2018, 10:20:01 AM »
In a quick throwdown mix, I like it.
The 0º CM3 brought the vocal amplifier from its natural 1 o'clock position in the sound field, and, dragged it to the center at 12.  That was actually kind of a nice bend.  The binaural stereo aspect remained strong in headphone listening; rightfully. I've only listened to a few songs, and via headphones. But I have had a chance to compare binaural and bino-mix that way.  The binaural stereo is a bit softer presentation; the Mix is more dynamic, especially in vocal presentation.
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Offline Gutbucket

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Re: Oddball microphone technique (OMT) - part 2
« Reply #175 on: October 15, 2018, 09:20:11 AM »
Cool setup Mike.  Thanks for sharing it here.  Let me know what you think about the rear facing channel in that setup.  Its usefulness may depend on how ambient the binaural recording is to begin with.  Try it simply panned center, and you might alternatively try it as Side channel matrixed with the Front as Mid.
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Offline Gutbucket

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Re: Oddball microphone technique (OMT) - part 2
« Reply #176 on: October 15, 2018, 09:42:07 AM »
I am curious too what you will say about using the rear mic for acoustic music.

Have somebody tried rear channel for onstage or stage lip?

How we can setup coincidence arrangement? It is easy with vertical capsules. But the mic's bodies should influence sound for horizontal capsules.

Works great for acoustic music.  I pretty much always use the rear facing channel(s) when recording both onstage and out in the audience.  At the lip I take advantage of the stage edge itself as an acoustic barrier, mounting either a directional pair below the lip facing out into the room, or a wide spaced pair of omnis boundary-mounted onto the front edge of the stage facing out into the room.  That way the stage edge helps limit how much direct sound radiating from the stage sources reaches those microphones, yet the direct sound which does refract around the edge and leak in remains closely enough time-aligned that smear or echo isn't a problem.

To make the front/rear facing mics coincident just mount them side by side or one over the other, such that the capsules are aligned with each other in the fore/aft plane.  I posted photos of the side by side arrangement in the previous OMT thread.  In that case the microphones were not coincident, but tight enough to be able to fit under a single umbrella.  I'd shift them further forward/backward in the mounts to increase the spacing as long as the weather was good.

Here are a couple of those photos again-



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Offline Moke

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Re: Oddball microphone technique (OMT) - part 2
« Reply #177 on: October 15, 2018, 09:47:30 AM »
there are two Axis-Reference stereo bars on this page.
First time ever for two in the same thread/page?
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Offline Gutbucket

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Re: Oddball microphone technique (OMT) - part 2
« Reply #178 on: October 15, 2018, 10:00:10 AM »
Axis-reference, nice. Is that what you call your DIY mic-bars with the angle markings?

For those following, Moke made the bar supporting the microphones in the photos I just posted above, which features easy to read 90-degree inclusive markings at each mount point.  I've gotten a lot of good use out of that bar.  Thanks Mike!
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Re: Oddball microphone technique (OMT) - part 2
« Reply #179 on: October 15, 2018, 10:27:04 AM »
I've built three or four of them.  SCB has one I made for DPA4028 pair.  Yours. and the one I'm presently using. 
I threw the one I'm using together on a rush, and then never followed up with the white markings. Its all scribed in, just not painted lines.
makes note to self,....
If Scott posts his in this thread, world balance might just change.

regarding the rear mic, and the binaural rig.
Yeah, probably not a necessary addition.  It did add something to the sound, didn't seem to detract, so I left it. 
The library did all of these upgrades to the room over the summer, since the last time we were in there. but, they still have an HVAC whoosh going on.
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