Gear / Technical Help > Microphones & Setup
Oddball microphone technique (OMT) - part 3
Gutbucket:
OMT Part 1 thread- http://taperssection.com/index.php?topic=96009.msg1279052#msg1279052
OMT Part 2 thread- https://taperssection.com/index.php?topic=184876.msg2251211#msg2251211
Gutbucket:
--reserved--
Updated OMT PDFs attached. Still need to roll these into a revised complete booklet along with a few additional sections I'm still working on. These currently supercede the booklet linked in my signature.
Gutbucket:
I'm shifting a discussion here that started in another - ORTF + Central Omni thread at GS - then drifted into more oddball topics.
It started getting into new OMT territory around the end of this post, when we started discussing ideas about interference tube "shotgun" mics as Mid channel center mics. The first part of that post was a rehash of conclusions I'd made concerning "ambient pair" mics over the course of the first two OMT threads.
then EmRR posted this oddball setup which sealed the deal!
Gutbucket:
--- Quote from: EmRR on September 18, 2019, 11:42:12 AM ---When looking at your OMT pdf’s, I took it as a general approach to have the center somewhat forward of the spaced omni’s. Trying to parse your thoughts above in that context. I’m guessing some offset for minimal time of arrival, but not a lot? I'm remembering 3-12" from somewhere.
--- End quote ---
That's right. In my previous post in the ORTF + Central Omni thread thread I was referring to the common practice of positioning ambient mics significantly further back in the room than the main array - several meters or more. Based on that I used to be concerned about how to achieve sufficient front/back spacing between main mics and ambient mics, but I'm no longer concerned with that and typically arrange things so the ambient/surround mics are only about 12" behind the others.
I do generally position the center microphone forward slightly. That emulates other LCR microphone setups like OCT and Michael William's extension of Stereo Zoom to 3 or more channels, where doing that in specific combinations with L/R angle, spacing and pickup pattern is intended to provide "image linking" between the Left-Center and Center-Right sectors without excess image overlap or gap. The ideal is that the "edge" of the imaging segment captured by the left/center stereo pair of channels should not extend over to the right side, but sort of seamlessly "hand-off" the image to the center/right stereo pair of channels. This kind of thing may be more important for surround playback where it can achieve continuous image linking across speaker sectors, and where directional imaging can extend seamlessly beyond the L/R speakers out into the L-Ls and R-Rs sectors when things are setup correctly for it. I can clearly hear this kind of smooth and solid image linking in surround playback when it's right. Really cool and one of the reasons I keep my mic setups compatible with good surround playback even though I don't currently have any surround playback set up.
This is visually indicated in Image Assistant when playing around with 3 mic L/C/R arrays, where it becomes evident that it takes some degree of center forward spacing to cause the imaging curves for the L/C and C/R sectors to line up correctly across the center. (Note: Image Assistant is a bit complicated and takes a bit of familiarization to be useful. I prefered the old Java version which presented all the information on a single display instead of more modern "sparsely populated tabs". The Sengpielaudio Visualiser is easier to understand making it a better resource for many, yet is limited to the visualisation of 2-channel stereo microphone setups)
But does that matter in a 2 channel stereo mix where those discrete imaging sectors are sort of being overlaid upon and woven against each other, and the center is a phantom position rather than a physical speaker corresponding to a discrete microphone channel defining the edge of each sector? It's certainly not obvious in the same way when listening back as there is only one Left-Right sector between the two speakers (not counting the non-imaging "beyond the speakers" region of diffuse sound). My feeling is it's less likely to matter, partly because of that but also because I'm typically manipulating the level of channel pairs against each other more in a 2ch mix to sort of layer them, and as channel gains are changed relative to each other, the image linking relationships shift. In surround I'm balancing pairs and the level of sectors against each other, but the separate sectors are intended to be "contiguous" rather than "overlaid".
I'll note that for practical reasons, sometimes over the past year or so I've ended up angling the telescopic arms which support the omnis and L/R supercards slightly forward, causing the center to no longer be forward of those mics. Usually that's done to squeeze the array into an odd spot, or as a way to angle the L/R facing mics more forward when necessary. Since I'm not currently set up for surround playback set up I've not listened to these particular recordings in surround mode to determine how much impact that lack of forward-spacing on the center mic has on image linking across segments. Doesn't seem to pose any problem in the 2-ch mix.
In any case, I don't think forward spacing of the center in relation to the omnis is a major factor. It's more important in keeping an appropriate geometric relationship between the center any near-spaced mic positions adjacent to it. In my case the side-facing L/R and rear-facing Ls/Rs supercards, and in simpler OMT setups the similar spacing between the single front-facing and back-facing mics.
--- Quote --- Within MS, as I said previously, I've liked the addition of a pressure omni for the lows, and that could be high passed. I'm curious whether that will at all prove useful with spaced omni's, I'm thinking maybe not.
--- End quote ---
The addition of a single pressure omni coincident with a M/S rig changes things in less complex ways than the addition of a center single omni to ORTF. Essentially, beyond extending sensitivity to lower frequencies, it shifts the Mid channel pickup pattern more toward omni. With all 3 mics coincident in space, there isn't the complex 3-way phase relationship ORTF + a center omni produces by occupying 3 non-coincident near-spaced positions.
--- Quote from: EmRR ---
--- Quote from: Gutbucket ---Likewise I'm currently curious how a true shotgun (albeit a short one) as Mid compares against the supercard Mids I'm using
--- End quote ---
I will get back to this question. I’ve run the KMR 81i twice, once in MS and once with two in a PAS array along with a MS pair and a board feed. The PAS was kinda weird in the imaging, as dsatz has frequently pointed out. It does have that certain Neumann lower midrange quality we like so much. In both cases, the highs are both clear (on-axis) AND muffled (off-axis), I can hear the interference tube for better or worse. I take his point that the off-axis stuff will have a much smoother transition with a supercard in the upper mids and highs, and in the lower registers sound mostly the same. With the MS version I have a full multitrack to combine it with, the PAS version worked OK as supplemental 'focus' fill with the MS doing most of the work, plus I like that particular mic's low mid contribution.
--- End quote ---
This is a good discussion. My hope is that the audibly bad off-axis behavior of an the interference-tube center microphone will be low enough in level to be masked by the contribution of the other microphones in the array. The other microphones are on-axis covering the center-mic's off-axis directions and they have good off-axis behavior themselves. If that making effect holds true, the additional forward-axis clarity from the higher directivity of a gun-mic should make it beneficial over a less tightly focused supercard with superior off-axis behavior in the special center position. Of course it also depends on how heavily one needs to lean on the center channel in the mix - off-axis interference-tube artifacts will be boosted by the same amount as the good on-axis signal as center channel level is increased. That kind of mix decision is subjective, and is honestly one of the more important mix variables. Sometimes center channel needs to be boosted significantly to achieve sufficient clarity and psychoacoustic proximity. Doing so intentionally biases the array more strongly toward the forward-axis. The ability to do that can really benefit the recording when that's needed, even if it collapses the soundstage width somewhat (and may compromise sector image linking in surround), and is not a true representation of how it would have sounded to a listener standing at the recording location. < But I don't care at all about that kind of documentary recording anyway. I want to make the best sounding recording I can which convinces the listener of its "realness" by how good it sounds. Working the illusion.
--- Quote ---I feel like I'd use the shotgun MS setup in a pinch in a big distant room again, if I knew I wouldn't have much footprint or options, I don't see myself revisiting a close spaced pair again anytime soon, maybe widely spaced in a larger multitrack situation.
--- End quote ---
I've thought about a pair of shotguns versus a single one as Mid or as the C in L/C/R. I do think a PAS pair could be useful, however I'd set them up as a coincident pair rather than near-spaced and tweak stereo width afterward. I would do so partly as the logical final step towards increasing PAS directivity for good clarity (each gun on axis with a stack, rather than one centered between stacks) and partly to optimize pickup of whatever stereo information is present in the PA mix, assuming there is any. Upon reflection, these are actually two sides of the same coin. Stereo width adjustment of the pair in post manipulates the sum and difference information between the two channels. By increasing stereo width (increasing the ratio of Side vs Mid) we are emphasizing the difference between those channels. And if that difference is more or less solely determined by the difference in source content each is on-axis with, rather than time-of-arrival phase-differences at LF extending to phase-randomization at HF from the non-coincident near-spacing, it would seem such a width adjustment would better target just the differences in direct sound between the two sides of the PA, in comparison with a single forward facing gun + stereoization of it with a bi-directional oriented 90 degrees off-axis. Essentially, this is a special-case of the old comparison of the virtues of X/Y verses M/S coincident stereo and the advantage one may have over the other when both can be setup with one goal in mind then ratio adjusted in post to best effect with another goal in mind. Hope that makes sense.
I don't really want to run X/Y shotguns (to awkward and bulky), and have no shotguns anyway so I probably will never get around to trying this. But I would have loved to have tried taping Dan Healy's lush stereo Dead mixes this way in combination with a pair of wide omnis, as sort of the inverse of the then common to that era technique of a near-spaced shotgun pair + single omni.
--- Quote ---I gotta wonder if there is value in using only one, pointed directly at the closest stack rather than at the center of the stage, then mixing it in the center of another stereo array. It's usually vocal register stuff I'm missing, and dead on the stage adds more drums than anything, which is NOT the deficit in the other arrays.
--- End quote ---
Yes you can! I've done something very similar when taping non PA amplified acts up close, to specifically focus on something I wish to highlight and dominate the center of the playback image. For instance I'll point the center mic at the snare or more toward the primary player to increase pickup of that source (the center mic gets angled more toward one side), even though the rest of the mic array may be angled somewhat differently to get even Left/Right pickup of the entire band and a correct front/back relationship between band and audience. This can be quite useful up close. For PA taping, if close enough to the PA that there is significant differentiation between pointing a center shotgun at a stack (more vocals) versus the between the stacks (more drums) it should work the same. In that way, the center microphone can be thought of more as a primary emphasis thing rather than a hard direction with respect to the other mic channels when recording. Likewise its level (and EQ) control emphasis of proximity and clarity in the mix. It is only its panning in the mix where perceived direction comes into play. In the stereo mix you can pan the center microphone slightly to one side or the other of center to balance things as necessary. This aspect is really cool and one of the really useful flexibility in mixing things.
EmRR:
--- Quote from: Gutbucket on September 18, 2019, 07:08:53 PM ---My hope is that the audibly bad off-axis behavior of an interference-tube center microphone will be low enough in level to be masked by the contribution of the other microphones in the array. The other microphones are on-axis covering the center-mic's off-axis directions and they have good off-axis behavior themselves. If that masking effect holds true, the additional forward-axis clarity from the higher directivity of a gun-mic should make it beneficial over a less tightly focused supercard with superior off-axis behavior in the special center position.
--- End quote ---
I'd say that seems totally true. In the context of the shotgun alone as Mid of a MS array with minimal side contribution to the mix, I'm hearing the interference tube. Not necessarily bad, just a different quality, less seamless image from center to sides.
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