Become a Site Supporter and Never see Ads again!

Author Topic: Relationships between spaced omnis and center mic  (Read 8588 times)

0 Members and 1 Guest are viewing this topic.

Offline Gutbucket

  • record > listen > revise technique
  • (15)
  • Needs to get out more...
  • *****
  • Posts: 14383
  • Gender: Male
  • "and the rowers keep on rowing!"
Re: Relationships between spaced omnis and center mic
« Reply #45 on: November 02, 2021, 10:42:34 AM »
DSatz raises a good point about potential phase interaction complexities as multiple microphone channels are mixed together.  I've found using more than two microphones in an array to be beneficial for many taper situations and provides a lot of flexibility, but it does require thought and attention regarding how those microphones are arranged and used in order to minimize potential problems and maximize the potential benefits. In light of that, limiting a multichannel microphone arrangement to three or four channels is likely to represent the sweet spot for most tapers who want to play around with what a multiple microphone array can do, as it strikes something of a reasonable balance between simplicity and complexity in providing additional degrees of freedom without the complexity getting out of hand.

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

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

Keep your ear open for phasiness with any spaced configuration.  Especially those using more than two physically spaced apart microphone positions that will be summed together.  But don't let the potential for that keep you from trying multi-microphone spaced array techniques if they are interesting to you.
musical volition > vibrations > voltages > numeric values > voltages > vibrations> virtual teleportation time-machine experience
Better recording made easy - >>Improved PAS table<< | Made excellent- >>click here to download the Oddball Microphone Technique illustrated PDF booklet<< (note: This is a 1st draft, now several years old and in need of revision!  Stay tuned)

Offline dyneq

  • (12)
  • Taperssection Member
  • ***
  • Posts: 690
Re: Relationships between spaced omnis and center mic
« Reply #46 on: November 02, 2021, 11:13:06 AM »
I stumbled upon a recent example: http://phish.in/2021-10-23
Quote
Source: DPA 4006a w/30mm APE spheres(split 7.5'@9')->Portico 5012 + DPA 4011c(xy - centered between 4006s/-9.5db)->SD788t(24bit/96kHz)->MBit(16bit/44.1kHz)->FLAC

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

Offline Gutbucket

  • record > listen > revise technique
  • (15)
  • Needs to get out more...
  • *****
  • Posts: 14383
  • Gender: Male
  • "and the rowers keep on rowing!"
Re: Relationships between spaced omnis and center mic
« Reply #47 on: November 02, 2021, 11:52:54 AM »
Some more in-depth dirt on phase interactions between channels-

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

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

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

When using a coincident pair in combination with a wide-spaced pair we can set things up so as to have each pair better optimized for what it can best contribute.  A coincident pair produces no phase difference between channels and does a great job in translating clear and distinct directional imaging of direct arrival sound. A wide omni pair randomizes phase across a significant portion of the frequency range, which does a great job of translating a big lush open reverberant sense of space, and at lower frequencies can provide useful non-random phase differences that makes for more enveloping, non-monophonic bass.  There is still a compromise to be made however, although now it is a different one - managing the phase interaction complexity between both pairs.  Using a coincident pair in the middle helps with that, as does spacing the omnis far enough apart that they won't produce objectionable phase interactions with the coincident center pair (yet not too far), and a somewhat wider spacing that can be used when a center microphone or pair is included also helps with portrayal of the reververant aspects of the recording.
« Last Edit: November 02, 2021, 03:56:54 PM by Gutbucket »
musical volition > vibrations > voltages > numeric values > voltages > vibrations> virtual teleportation time-machine experience
Better recording made easy - >>Improved PAS table<< | Made excellent- >>click here to download the Oddball Microphone Technique illustrated PDF booklet<< (note: This is a 1st draft, now several years old and in need of revision!  Stay tuned)

Offline Gutbucket

  • record > listen > revise technique
  • (15)
  • Needs to get out more...
  • *****
  • Posts: 14383
  • Gender: Male
  • "and the rowers keep on rowing!"
Re: Relationships between spaced omnis and center mic
« Reply #48 on: November 02, 2021, 01:05:11 PM »
As mentioned above, although a wavefront arriving from somewhat off axis will produce phase differences across two spaced microphone positions, particularly so at higher frequencies, a wavefront arriving absolutely directly on-axis, perpendicular to the spacing axis will not produce a significant phase difference between channels.  This is an interesting phenomena.

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

One interesting implication of this may be it's potential application to the positioning of the center microphone or pair. We've talked here or in other threads about either placing the three microphone positions directly inline with each other, or shifting the center position forward somewhat, and if so by how much?   It might be useful to shift the center position forward just enough so that the path from the left PA stack arrives perpendicular to the axis between the left omni and center pair, and vice versa with the PA stack and omni/center-pair axis on the other side.  That seeks to minimize phase difference between the wide omni and center position for pickup of the PA on each side.  The farther back the recording position, and the narrower the distance between PA stacks, the less far forward the center microphone position would be placed in front of the omnis.
« Last Edit: November 02, 2021, 03:59:16 PM by Gutbucket »
musical volition > vibrations > voltages > numeric values > voltages > vibrations> virtual teleportation time-machine experience
Better recording made easy - >>Improved PAS table<< | Made excellent- >>click here to download the Oddball Microphone Technique illustrated PDF booklet<< (note: This is a 1st draft, now several years old and in need of revision!  Stay tuned)

Offline rocksuitcase

  • (3)
  • Needs to get out more...
  • *****
  • Posts: 7176
  • Gender: Male
    • RockSuitcase: stage photography
Re: Relationships between spaced omnis and center mic
« Reply #49 on: November 02, 2021, 01:19:50 PM »

One interesting implication of this may be it's potential application to the positioning of the center microphone or pair. We've talked here or in other threads about either placing the three microphone positions directly inline with each other, or shifting the center position forward somewhat, and if so by how much?   It might be useful to shift the center position forward just enough so that the path from the left PA stack arrives perpendicular to the axis between the left omni and center pair, and vice versa with the PA stack and omni/center-pair axis on the other side.  That seeks to minimize phase difference between the wide omni and center position for pickup of the PA on each side.  The farther back the recording position, and the narrower the distance between PA stacks, the less far forward the center microphone position would be placed in front of the omnis.
I've tried this once without a scientific measurement between the three sides of the triangle.
This paragraph would suggest you are getting toward a Decca Tree type arrangement for the center (fwd) channel(s). Am I headed in the correct direction as to your intent?
music IS love

When you get confused, listen to the music play!

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

Offline Gutbucket

  • record > listen > revise technique
  • (15)
  • Needs to get out more...
  • *****
  • Posts: 14383
  • Gender: Male
  • "and the rowers keep on rowing!"
Re: Relationships between spaced omnis and center mic
« Reply #50 on: November 02, 2021, 02:21:43 PM »
Sort of.. in a way, yes.  It would imply a more triangular arrangement when recording from closer in (as the apparent angle to the PA stack on each side becomes wider) as the opposite extreme to a very distant position where the wavefront arrival is flat and all three microphone positions would be arranged in essentially a straight line.

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

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

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

I'll say this- In moving to a directional center mic or pair I tend to not want the center coincident pair position out too far forward because I feel like I want the impulses from a front arriving wavefront well aligned across all forward facing mic channels, and I don't want to have to deal with the hassle of delaying the center channels to achieve that.  Yet I used to frequently use four omnis in a spaced diamond configuration from an FOB position in the center of the audience relatively close to the stage, using anywhere from a meter or two of horizontal spacing and a half-meter to a meter or so of front/back spacing.. and I never noticed a problem with smeared transients doing that.  In that case the axis between the center omni placed forward of the L/R omnis and the omni over on one side was probably close to being perpendicular to on-axis to the PA on that side much of the time.  I didn't think of it this way back then, though.  I thought of it more in terms of just achieving more spacing between the center and side omnis than I could otherwise, by pushing the center position forward, and also as a modification of the traditional Decca tree arrangement with a fourth microphone spaced out to the rear forming a diamond arrangement.
« Last Edit: November 02, 2021, 04:05:23 PM by Gutbucket »
musical volition > vibrations > voltages > numeric values > voltages > vibrations> virtual teleportation time-machine experience
Better recording made easy - >>Improved PAS table<< | Made excellent- >>click here to download the Oddball Microphone Technique illustrated PDF booklet<< (note: This is a 1st draft, now several years old and in need of revision!  Stay tuned)

Offline heathen

  • (23)
  • Needs to get out more...
  • *****
  • Posts: 3306
Re: Relationships between spaced omnis and center mic
« Reply #51 on: November 24, 2021, 09:53:05 AM »
Here's an example I made:  https://archive.org/details/garajmahal2018-05-11.AT4031.CA14

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

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

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

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

Post: Mixed sources 1&2 (source 2 at -7 dB compared to source 1), amplify, fades, track splits, and conversion to flac using Audacity; File tagging using Mp3tag; ffp created using Trader's Little Helper
Mics: AT4050ST | AT4031 | AT853 (C/SC) | Line Audio CM3 | Sennheiser e614 | Sennheiser MKE2 | DPA 4061 | CA-14 omni Pres: CA9200 | DPA d:vice Decks: Zoom F8 | Roland R-05

Offline dyneq

  • (12)
  • Taperssection Member
  • ***
  • Posts: 690
Re: Relationships between spaced omnis and center mic
« Reply #52 on: November 24, 2021, 02:46:35 PM »
Here's an example I made:  https://archive.org/details/garajmahal2018-05-11.AT4031.CA14

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

Did you choose 100* in order to PAS?

Offline heathen

  • (23)
  • Needs to get out more...
  • *****
  • Posts: 3306
Re: Relationships between spaced omnis and center mic
« Reply #53 on: November 24, 2021, 08:57:55 PM »
Yeah, I was aiming for PAS and 100* was my estimate of what that was.  The driving force was the PAS part not the numerical part, if that makes sense.

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

Also note there are other sources available, like Scott's on stage DPAs.  That could make for interesting comparison, even if it's not necessarily like-for-like.
Mics: AT4050ST | AT4031 | AT853 (C/SC) | Line Audio CM3 | Sennheiser e614 | Sennheiser MKE2 | DPA 4061 | CA-14 omni Pres: CA9200 | DPA d:vice Decks: Zoom F8 | Roland R-05

 

RSS | Mobile
Page created in 0.041 seconds with 32 queries.
© 2002-2021 Taperssection.com
Powered by SMF