Topic: Oddball microphone techniques - part 1

[heathen]

Perusing an old thread, I came across a comment that seems appropriate for this thread:

The not at all simple alternate answer which potentially could help adjust for being off-center has to do with adjusting the microphone configuration in combination with rotating the stand.  It's probably far more trouble than it's worth, would be hard to do precisely in the field, and is not something most tapers would want to try, but is interesting to me technically.  I won't go into it here in too much depth, but it has to do with adjusting the angle of each microphone so they are no longer in a symmetrical arrangement with the center axis of the microphone array.  Essentially, one microphone is moved forwardof the other, and that accomplishes something similar to the delay thing I described previously, "at the microphone array" itself.

That's based on the work of Michael Williams which explores the inter-relationship between pickup pattern, angle, spacing and position of a pair of microphones.  It's how he goes about "linking" multiple microphone pairs together to form multichannel surround recording arrays which are capable of seamless playback imaging between across each microphone/speaker pair sector, without gaps or overlaps.  His papers on Multi-Microphone Array Design (MMAD) explain this in depth, but are more technical than most tapers here will care to get into.

This was in the context of a question about what to do when setting up off-center from the sound source.  The bolded part has me most intrigued.  I find it really distracting when I'm listening and the whole sound is noticeably off-center.  Obviously sometimes we can't set up at the center line for whatever reason.  I've found that just raising the level of the more "distant" channel is often unsatisfactory...maybe because the volume level doesn't make up for time of arrival differences in the channels?  So, I'm curious about the practical application of the idea hinted at in the quote above.  If we're talking about a near-coincident stereo pair, would putting one of the mics closer to the sound source help mitigate being off-center more than simply raising the gain of one channel?  I'm guessing that the mic on the more distant side would need to be moved closer.  How much of a change in location are we talking about, though?  If we think of it in terms of just a simple PAS config, would each of the mics be pointed at the stacks after adjusting the forward location of the one mic?  Or before making that adjustment?

[Gutbucket]

Yes, level balancing afterwards is always helpful but can only do so much.  The practical solution at the show is to close your eyes and rotate your head to find the most balanced auditory center direction by ear, then rotate the entire mic-stand to match.   Don't trust what your eyes tell you, as often the acoustic center will not match the visual center.  Perhaps counter-intuitively, this technique will tend to point the microphones toward the louder PA, or more toward the near-side PA stack when located off-center - that is to say it will point the mics further away from the visual center and not towards it.  The result however will be a much more acoustically balanced recording.

I think this is the William's paper in which he discusses the microphone offset technique- http://www.mmad.info/Collected%20Papers/Multichannel/4997%20New%20York%201999%20(31%20pages).pdf, as one technique of several used to achieve "critical linking" between the individual recording angle segments in multiple microphone arrays - the idea being to have each segment align with the next along it's shared Stereo Recording Angle edge without excessive over-lap or a gap between them.  Obviously that's a rather involved process to figure out, but sheds light on what's going on acoustically.  You can see how simply rotating the entire microphone array to point at the apparent auditory center is sort of doing something similar, if less extreme.

^ That paper and many of his others is made generously available for non AES members via Michael William's website- http://www.mmad.info/

[Gutbucket]

Yesterday I made the comment quoted below and the one which follows in Rocksuitcase's thread Greyfox 2016 mic rig snaps where he'd posted photos of his and kindms's OMT setup. Bean had asked a good question there about phase interaction when mixing multiple mic channels which I'd overlooked until yesterday. I'm moving my response here because it addresses important aspects with regards to mixing these multichannel OMT setups, and why doing so tends to be less problematic than the more common taper technique of mixing two similar near-spaced microphone configurations that were setup above/below each other on the same stand.  This discussion is likely to close out this thread which is now nearing the 25 page limit.  If so, it makes for an appropriate book-end to the thread.

Came across this cruising old posts and living in the past!

These are good relevant questions above deserving a more in-depth discussion.

[Bean, I've snipped and edited your question quoted here..]

Do you notice any phasing/cancellations when you matrix that together? Or are all of your mics/caps pretty aligned vertically to the sound source, and no cancellations like I'm hearing in my most recent recordings? Or do you normally EQ after you matrix all of your sources together? Because on their own, my [2] sources sound great separately and didn't require any EQ'ing, but as soon as I mix them 50/50, I hear some weirdness that's phasing/cancelling some frequencies out completely! To my knowledge, the 70D was NOT on phase reverse, and the 2 sources are perfectly aligned.

I just got a new SRS DINa/DIN bar, and the caps are perfectly aligned vertically, so I'm hoping that will cure what I'm hearing when I try to matrix my [2] DAUD sources with one rig about 4" farther back/higher than the other rig [mk4 rig lower than mk41 rig on my Vark Bar]. When I listen VERY closely to them once they're mixed 50/50, I can hear a lot of lowend, and a saturated/cancellation of most of the highend! I hope all of my stuff isn't like that from the last 1.5 years Ive been doing 4 channel with my 70D's?!?!

Hey Bean,  This kind of phase cancellation can happen when mixing multiple microphone channels down to fewer channels.  It arises when the sources are very similar, yet not quite perfectly in-phase with each other.  There are two fundamental ways of preventing it: 1) Arranging things so that there is no longer a significant phase difference between the sources to be mixed together  -or- 2) Arranging things so that the sources are significantly different enough that no significant phase cancellation occurs.


Approach #1 is all about positioning the mics intended to be mixed together so as to be as coincident with each other as possible.  If the mics are truly in the same point in space there will be no phase difference between them at any frequency and no destructive interference (cancellation).  In reality two or more mic elements can never occupy exactly the same point in space, but the closer they are to being truly coincident, the higher the frequency at which phase cancellations begin to occur.  The problem is that as frequency rises the wavelength becomes shorter - eventually shorter than the dimensions of the microphone elements themselves - and at that point it doesn't take much distance at all between capsules to get combing cancellation above a frequency directly related to the spacing between microphone elements.  Realistically we can get them close enough together to push those cancellations up to the top of the audible range where they are no longer problematic.  This is a fundamental problem in the design of multi-element microphones which have more than one capsule mounted in a single housing. Consider ambisonic microphones with 4 capsules which rely on mixing the output of all four capsules in various ways to produce the desired output. Ideally all four mic elements would each occupy the same position in space.  In reality the manufacturer places them as close together as practical and the resulting polar patterns and frequency responses break down above a certain frequency which is determined by the spacing between the elements.

In your case when mixing your two sources together, you describe hearing the low frequencies reinforcing each other nicely.  Those are frequencies where the wavelength is long compared to the distance between the two Left microphones being mixed together and likewise between the two Right microphones being mixed together.  You also describe hearing cancellation in the high frequencies.  That's where the wavelength becomes short in comparison to the distance between mics on each side and the resulting comb-filtering begins to be audible.  If you were to move one pair of mics slightly with respect to the other pair while mixing them together and listening at the same time, you'd hear that cancellation shift around in frequency as you varied the distance between the mics, in a way similar to a chorus pedal effect.

To take advantage of this approach, you want to get the two left microphones closer together and the two right microphones closer together.  How close is close enough?  The practical answer is you'll have to mix them and listen to the result to determine if its good enough to satisfy your ear, but let's do some simple math on a very simplified model to illustrate- You mention that without the new mic bar, one set of mics was about 4" above and behind the other pair.  Let's ignore the vertical dimension and figure one pair as being 4" directly in front of the other, and let's only consider sound arriving from directly in front.  Sound arrives at the forward-most microphone first, then at the one located 4" immediately behind the first.  4" is one wavelength of a 3,350Hz audio frequency or right about.  So 3,350Hz sound encounters the first microphone and then the second exactly one full wavelength afterwards.  The sound is in-phase at both microphones at that frequency.  When the two signals are mixed together later in equal proportions that 3,350Hz frequency interferes constructively (adds together).  At twice that frequency the two signals arrive only one-half of a wavelength apart (180 degrees out of phase) and destructively interfere (they cancel out creating a notch).  At higher multiples of that frequency the same things happen and the resulting response is a comb-filter, alternating between the frequencies where the two signals constructively and destructively interfere with each other.

Bean- Did your new mic-bar arrangement which places the mics in closer vertical alignment take care of the phase-interaction problems sufficiently for you?  For sound arriving from in front the more
 closely vertically aligned should shift the comb filtering I'm describing upwards in frequency as the mics are positioned closer to each other.  Keep in mind there still may be other phase-interaction stuff going on with the ambient, reflected, and audience reaction sound arriving from all other directions.


Approach #2 is to make sure the microphone signals are different enough so they won't destructively interfere significantly when combined.  That's basically what OMT is doing for the most part.  Unlike approach #1 there are numerous ways to achieve this- the mics can be placed far enough apart, or be directional enough and pointed far enough away from each other, or have significantly different responses in the high frequency range in question, or one of them could be delayed slightly compared to the other, or other types of manipulation can be done to make the phase relationship between signals different enough that they won't destructively interfere in a negative way when mixed together.

In practice, I find it more difficult to assure that multiple microphone sources intended to be mixed together are coincident enough to eliminate the problem, than it is to for me to make sure they are different enough from each other to avoid the problem.  Also, for numerous other reasons I find it more useful to combine microphone sources which are each providing something really significantly different, rather than combining microphones which are each providing very similar if slightly different content.  To really make the effort fruitful and worthwhile,  IMHO the sum should be significantly greater than the individual parts.

With all that rambling I hope I didn't loose you.

Here's something interesting to consider- all non-coincident microphone setups defy both of the fundamental approaches I describe above as ways to avoid phase problems!  But that's okay since the Left and Right channels aren't intended to be mixed together.  This is the the mono-compatibility thing.  Non-coincident stereo recording is phasy by nature and intentionally so. The left and right channels need to be kept separate from each other to completely avoid phase cancellation problems.   When you mix your two separate near-spaced stereo recordings together, you are essentially experiencing two mono-compatibility problems, one on each side.  The two Left channel microphones mixed together produce phase cancellations in the Left output channel, and the same goes for the to right channels. 

Here's another thing to consider- stereo playback via speakers also defies these fundamental aspects as well and is fundamentally phasy.  This is a deeper problem as it relates to the basis upon which stereo playback over speakers works.  It's basically the same problem as described above, but in reverse: two speakers > two ears, via four paths between those points, with the cross-talk signals interacting at each ear causing constructive and destructive interference.  The only way around that is binaural reproduction (one speaker > one ear) either via headphones or via cross-talk cancellation techniques which attempt to cancel the interfering signal from the opposite side at each ear.

[Gutbucket]

I've assembled a little PDF booklet intended to be an visual introduction to OMT which graphically illustrates suggested OMT setups for microphone arrays of 3 to 8 microphones.

Find it attached below and let me know what you think.  I'm hoping the graphic illustrations will make the OMT microphone setups easier to understand for some of the more visually oriented folks interested in it.  We've thrown plenty of words around in this thread, time for some graphics.  All of the microphone setup illustrations are shown in "bird's eye view" perspective as if looking down on them from above (as standard stereo microphone setups are typically drawn), with the stage located at the top of the page and the audience located around the sides and bottom of the page.

It was fun to put this together.  Info-graphics are cool.


[edit]- (ironically adding yet more words here, in support of the booklet of illustrations)

What is OMT?  I started this thread almost exactly 10 years ago, with the original intent of discussing any unusual (oddball) microphone technique folks here at TS had found useful.  However it quickly became focused primarily on my own exploration of open recording techniques, combined in such a way as to achieve the goals I found most important in making the most believable and enjoyable sonic recreation of the event I could manage, within practical constraints. 

OMT is designed specifically for location live music recording from an audience or stage-lip location, intended to convey a natural sense of being there surrounded by that space and audience, without sounding overly distant or reverberant but with good up-front presence and apparent proximity to the source.  It is not a microphone configuration intended for studio recording in smaller spaces close to the source, or even in controlled live music hall conditions where one has more control and can listen then carefully readjust things before recording, although I believe it would work well there.  It's designed with the intent of quickly setting it up and letting it roll at the event without having to strategize too much on the spot, collecting the appropriate signals which allow one sufficient freedom to mix and manipulate those signals afterwards, arranging and fine-tuning things after the recording was made.  In that way the efforts go into the initial design behind these microphone arrays and what one does with the resulting recordings afterwards.  Making the recording at the event has enough distractions and pleasures without overly complicating that part of the whole undertaking with second guessing which microphone arrangement is appropriate.  Besides describing OMT setups visually, what the booklet does is allow you to pick from just a few variants based primarily on how many channels you are willing to record and how many of the appropriate microphone types you have on hand.

I now define OMT setups as live music recording microphone arrays which have these common traits-
> A multi-microphone array technique using more than two microphones
> Generally incorporates and builds atop a foundation of spaced omnis
> Is capable of being supported by a single taper-style microphone stand (typically a repurposed light-stand)
> Has a primary aim of producing a believable sensation of being in the space in which the recording was made
> Aims to emphasize the good qualities and minimize the bad qualities of the live sound, such that the recordings are capable of sounding better than it did live
> Does so by analyzing and determining what is important about live and reproduced sound, and designing the microphone array to specifically address those phenomena
> Splits the workload between sub-units optimized for each role, rather than searching for the best compromise intended to serve all roles - the sum is greater than the individual parts
> Provides for greatly increased flexibility and choices during post-processing and mixing
> Provides some welcome redundancy (should one microphone channel go bad) without detrimental overlaps between microphone channels
> Excels at all three primary forms of playback via different post-processing technique decisions, from the same OMT source recording- Binaural (headphones), 2-channel stereo, and multichannel surround reproduction

[Edit 2]- The illustrated OMT booklet is available in sections linked below, because when combined they exceed the site's 750MB upload limit.  This is currently a work in progress and I'm updating these sections as I revise things.  If you have already downloaded this, please check the edit date on this post.  If I've edited the post since you last downloaded, please re-download the newer corrected, revised version.  Once complete, I'll reassemble the entire document and ask the admins to host it.

[Gutbucket]

I found and corrected a few mistakes on page 4 (6 channel OMT variants), then re-uploaded the PDF above a few moments ago.  14 people have downloaded this since my original posting last night, so if you are one of them you may want to download the new corrected version.

[dactylus]

^
Thanks Lee!

[Moke]

+ to you.

[rocksuitcase]

Awesome! I am glad we spoke about the visual aspect of this and that you were able to put so much time and effort into creating this pdf. This will be very useful for discussing with other recordists or sound/PA operators.
I still need to add an appendix of all the variants kindms and I have used along with some mixing tips/ideas
I offer a spelling correction from your descriptive post above- in this line replace the strike-thru's with the bold:
Splits the workload between sub-units optimized for each roll role, rather than searching for the best compromise intended to serve all rolls roles - the sum is greater than the individual parts

Thanks for creating this. I have it in my music documents folder now.

[thatjackelliott]

This is very cool. I don't know whether this is something that should be visible by glancing at the diagrams (and should be obvious to all) but is there a way to determine how mono-compatible the varied and wondrous arrays are?

[voltronic]

Thanks!

I'm considering working up something similar covering working with the resulting channels afterwards- the mixing post-production stuff.

In thinking about that this morning I realized I forgot a few important variants which I should include in the booklet, primarily those using a Dual-Mid/Side or Ambisonic microphone in the center.  Expect an addendum page on that soon.  I'll probably post that separately before rolling that and the mixing section into the booklet at some point.

This is really great!  Thanks so much for your hard work on this.

One suggestion: I think it would be helpful to include mic spacing measurements, or at least acceptable ranges thereof.  All of the standard arrays on the first page other than AB omnis have specific dimensions, and while most of us that would be attempting your "oddball" setups will know that any such setups will need to be experimented with and adjusted by ear, it would be helpful to have what has worked for you as a starting point at the least.

[Gutbucket]

Thanks for the thanks and suggestions, I'll work them in as I revise this over time.

As for mono-compatibility, in some ways I don't really care but in others I really do!  A few words of explanation on that- A primary goal in developing these arrays is that they mix-down nicely avoiding phase interaction problems, which is an important aspect that differentiates OMT from the more typical multi-channel AUD taper technique of mixing multiple closely-positioned near-spaced stereo arrays.  Mixing more than 2 microphone channels down to 2-channel stereo can be thought of as two separate mono-compatibility problems- one in the left channel and one in the right channel.  I care very much about that.  I don't really care much about mixing down to a single monophonic channel so I don't worry too much about that part.  We can explore how mono-compatible our resulting stereo (and surround) mixes from these arrays are, but its not something super high on my list or important to me personally.  That is to say I don't care to make compromises in how I make a stereo or surround mix in order to achieve good mono-compatibility, if were to come at the expense of how good those mixes sound in stereo or surround.

Also, in some ways mono-compatibility is an end-user format problem we can avoid entirely.  As mixers, we can work around the problem in numerous ways by making dedicated mono-compatible mixes from the available recorded OMT channels when necessary.  In it's simplest form consider how a simple pair of spaced omnis is generally considered to be non-mono-compatible.  Yet as mixers, we can simply use only one of the two omnis channels and route that to both Left and Right channels for a fully mono-compatible output from that spaced omni pair. A single non-directional omni is as mono-compatible as it gets. 

With OMT we have many more mixing options available which are likely to be more attractive.  The single center forward-facing microphone or center coincident pair makes the OMT center components extremely mono-compatible.  One can always discard the omni channels and use the center mic or pair alone if the omnis aren't folding down to mono in an appropriately mono-compatible way.


As for setup dimensions, my original intent was to keep the illustrations simple and as visually uncluttered as possible.  I do agree that that information is important and I'll probably try to work it in.  One thing I wish to avoid for folks following all this only by rote rather than really getting a handle on the underlying relationships, is not placing a false emphasis on exact spacings where it doesn't matter so much as well as avoiding loose spacings where it does matter.  That's really down to my own communication of those points in a clear and concise way.

I'm considering putting together something similar covering working with the resulting channels afterwards- the mixing post-production stuff.  In thinking about that this morning I realized I forgot a few important variants which I should include in the OMT booklet, primarily those using a Dual-Mid/Side or Ambisonic microphone in the center, as well as some setups which I don't particularly recommended.  Expect addendums and revisions..

[Gutbucket]

The two-page addendum I posted yesterday (showing setups using dual Mid/Side or ambisonic microphones in the center, and/or Polar-Flex-like "Strauss-Packet" arrangements for the omis, plus a couple non-recommended setups with some explanation as to why the aren't) has now been reworked into the document.  I've also made other corrections, changes, and layout improvements.  As mentioned, this is a work in process.  Thanks for your patience, comments, and for downloading the most current version.

See the note in bold at the bottom of the post with links to the booklet, divided into in sections as a work around the file upload size limit.  If the edit date on that post is later than the version you've already downloaded, please download the newest revised version.  Link to that post- https://taperssection.com/index.php?topic=96009.msg2250377#msg2250377

Thanks all and Happy New Year!

[Gutbucket]

Alright, I just uploaded the last revisions that will be made until after the new year.  Apologies to the 4 or 5 of you who already jumped on the previous updates over the past few hrs.  No plans to make further updates until I start getting into the section on mixing.

[down2earthlandscaper]

Alright, I just uploaded the last revisions that will be made until after the new year.  Apologies to the 4 or 5 of you who already jumped on the previous updates over the past few hrs.  No plans to make further updates until I start getting into the section on mixing.

Thanks so much for sharing this.

[dactylus]

^^
Thanks GB!