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Author Topic: Relationships between spaced omnis and center mic  (Read 12579 times)

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Offline vantheman

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Re: Relationships between spaced omnis and center mic
« Reply #60 on: January 17, 2023, 12:42:36 PM »
Interesting! It makes sense, I just lack the experience to intuit that myself. I’m going to be super close to the source - like between 3 and 6 feet. But what I’ll probably do, since I have access to soundcheck, is record sound check with omnis and analyze, pulling the omnis if necessary.
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Offline Gutbucket

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Re: Relationships between spaced omnis and center mic
« Reply #61 on: January 17, 2023, 01:26:24 PM »
You can probably get away with omnis that close. Might be best to..
Quote
run wide NOS with the cards and know I’ll have a nice spread with space in the middle to mix vocals later, and I could run omnis as a second option and just take the better of the two.
  ..and combine them or not depending on how it sounds.  That's a relatively safe bet.

If really wanting to add a center microphone or coincident pair to the "wide ORTF", substituting supercardioids in place of cardioids to maximize the limited difference between channels that will occur due to the minimal spacing and angle difference between all three microphone positions is likely to help somewhat.  Beyond that, I suspect what would work best when mixing the resulting recording may not be equal level from all microphone positions, but just enough from the center position to help solidify the center of the wide ORTF without getting into interference problems with close to identical levels.Or conversely, if you were using a coincident center pair and considering that the primary source, just enough "wide ORTF" to help that coincident center gain sufficient openness and lushness. In that way you would be using just enough of the extra channel(s) to fix what is lacking in the main pair, more so than using all the microphones as a single array. You are more likely to encounter problems when mixing levels are about equal, although that might not be a problem.  What sounds right is right.

Last spring I made one of the best recordings I've ever done of an instrumental prog-ish rock trio with the band playing a minuscule stage at the back of a tiny kava bar, placing my mic array at the edge of the stage essentially extending over the pedal boards of the guitarist and bassist, the drummer back only 4'  or so.  I used all the mics in my rig, and the (wide) omnis worked great, although I had to retract one of them each time the band got on or off the stage.  I did position the rig the way I prefer when recording a trio with a drummer in the center, which is to setup slightly off-center from the kick drum (avoiding to much woomp), and to point the center mic (a mid/side pair) directly at the snare.
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Offline Gutbucket

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Re: Relationships between spaced omnis and center mic
« Reply #62 on: January 17, 2023, 01:28:26 PM »
I've been wanting to dive a bit deeper on what aspects are effected by the spacing between mics, and your "wide ORTF + center" question made for a good jumping off point for me typing this earlier this morning.

Similar to the addition of some soundboard feed to any taper stereo pair recording, if inclusion of soundboard vocals helps fill the center when using the "wide ORTF" mount, an additional center mic may work by sort of playing the same role, but ideally the spacing should be made greater when introducing that center microphone position at the array itself.  That's because introducing a center microphone or pair to the microphone array raises additional aspects that the inclusion of direct soundboard output does not.  There is more happening than just reinforcement of some sources in the center by mixing in some soundboard:

There are at least three things effected by the near-spacing between microphone positions:
1) Constructive/destructive interference
2) Diffuse field correlation/decorrelation
3) Imaging aspects

These things apply to any number of microphones greater than one, but generally, the well-known two-channel near-spaced microphone configurations have already been optimized in terms of spacing (the spacing for DIN, NOS, ORTF, etc, was determined long ago), and is of somewhat less significance when the channels aren't going to be mixed together electronically, which includes panning microphone outputs to positions other than hard left/right. With three or more microphone positions these things become more significant when some of those channels are going to be mixed together, such as the center microphone being mixed into Left and Right output channels. This is closely related to mono-compatibility, only extended to more than two microphones.

Somewhat more spacing helps with all three of those things.

1) The constructive/destructive interference thing relates the spacing between micrphones to wavelength.  The phase difference between direct-arriving signals at more than one microphone shifts with different angles of arrival.  That will produce comb-filtering to some degree, which differs with source position. Comb-filtering can be a most audible thing, most obvious by far while actively changing the spacing or effective spacing (or the delay between channels), especially when the two channels are mixed together. At closer spacings it audibly effects mid or even somewhat higher frequencies where it tends to be especially noticeable.  Wider spacings shift the peaks and valleys downward in frequency. It is very obviously audible when you hear those peaks and valleys shifting around as the spacing is changed, and audible but not nearly as obvious when the spacing remains are static - the peaks and valleys are still there though, acting sort of like and EQ with a wavy curve.  In my experience, this is something a taper needs to home in on empirically, by trying different spacings and figuring out what sounds right to them in terms of tone and frequency balance.  It's certainly the most tweaky aspect, and a relatively small change in spacing will shift the combing significantly, either for the better or worse.  Overall its probably the most important aspect of the three, yet is most difficult to predict beforehand simply by measurement.  I know from experience that the spacings I use myself work, but I cannot offer a prediction for folks using significantly different spacings with regard to this.  The same thing goes on at lower frequencies with wider spaced omnis, except the spacing between comb peaks and valleys is greater.  If you've ever found that sometimes you seem to get less bass rather than more with a pair of spaced omnis, and the bass sounded right in the room, its is likely that the spacing between omnis is causing a destructive valley at the frequency where the bass seems weak, relating to some lateral or off-axis bass mode.  With a different spacing that weak frequency zone would become neutral or emphasized.

2)  Diffuse field correlation/decorrelation is related, but easier to predict as the relationship is more straight forward, essentially about the phase difference between channels being above or below a certain threshold.  Essentially, we want the direct arriving sound from the stage and PA to produce a signal relationship between channels that is mostly phase coherent with clear and predictable phase correlation.  This can occur with spaced microphone positions when the spacing is perpendicular to the wavefront arrival, minimizing the difference in distance from the source to either microphone (even though its these relatively small non-zero differences that contribute to comb-filtering).  At the same time, it helps if the indirect-arriving reverberant sound, dominated by the room and audience sound, has a mostly random phase relationship between channels, making it decorrelated.  That makes that stuff sound diffuse, open, airy, lush, eliminates comb-filtering and perceptually keeps the reverberant room and off-axis audience sound from interfering with the coherent direct sound from the stage and PA.  More spacing = more decorrelation for sources that are increasingly off-center with respect to the array.  Directional pattern and angle can also achieve this, by having the null of one pattern line up with the on-axis direction of the other, but that means pretty wide angles which tend to put the mics off-axis from from the PA, so in most taping situations spacing is the better option to achieve good low diffuse field correlation.

3) Imaging-  This is probably least important but easiest to talk about, and predict (along with correlation).  It is what the Stereo Zoom and virtualization app tools do well.  Some of those, the Schoeps Image Assistant in particular, also predict diffuse correlation, but it's hidden on a different graph than the imaging information which tends to be the primary focus of the apps.  Adding a microphone or coincident pair between an existing stereo pair of microphones will make the stereo recording pickup angle wider.  That may seem somewhat contradictory to it also solidifying the center, and it is to some extent, but its more about how wide the recording sounds, sort of "how much is pulled in" rather than how solid the center seems.  Another aspect of imaging is how accurate the apparent source positioning is - do sources on stage sound like they are well focused and placed in the same position as they were live during the performance?  This is more of a nice to have thing.  Only folks that were there at the live event and pay attention to this kind of thing will know if the imaging accuracy isn't accurate, otherwise it just need to sound good and plausible, and some recordings are more plausible than others.  The previous two aspects are more fundamental and effect enjoyment of a recording far more.  They can even be heard and appreciated with one ear or one speaker, while imaging requires two ears and a proper stereo triangle or headphones.
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Offline Gutbucket

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Re: Relationships between spaced omnis and center mic
« Reply #63 on: January 17, 2023, 01:38:44 PM »
When recording in close proximity to the source there is also consideration of placement closer to some sources than others and the loudness differences between microphone positions based on that alone, which is not a significant thing at greater distances.  In those situations it can be nice to have an array that includes enough spacing such that all sources on stage are more evenly covered, making getting a good balance of all sources easier, rather than tending to spotlight whatever ends up closest to a central very compact recording position.
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Offline goodcooker

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Re: Relationships between spaced omnis and center mic
« Reply #64 on: January 18, 2023, 07:29:33 PM »
I disagree with this statement 100%.

snip <NOS's 12" spacing is optimized for use as a single near-spaced pair without the inclusion of a center mic or coincident pair, but isn't spaced enough to work especially well in combination with a center mic position >snip

NOS with directional mics can very much benefit from a center forward facing single directional mic. I've made a bunch of recordings using this arrangement and it's been great more often than not.
« Last Edit: January 18, 2023, 07:31:07 PM by goodcooker »
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Offline vantheman

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Re: Relationships between spaced omnis and center mic
« Reply #65 on: January 18, 2023, 09:45:06 PM »
When recording in close proximity to the source there is also consideration of placement closer to some sources than others and the loudness differences between microphone positions based on that alone, which is not a significant thing at greater distances.  In those situations it can be nice to have an array that includes enough spacing such that all sources on stage are more evenly covered, making getting a good balance of all sources easier, rather than tending to spotlight whatever ends up closest to a central very compact recording position.

Been thinking about this. Often (edit: always, up to this point) when I run stage lip I don’t have any influence over where amps, monitors, etc, are placed and I just need to embrace the constraints. In this case, I kinda do have some say, but the limited space will probably play a big role on what kind of live mix is possible. Bass placement, to an extent, matters the least I suppose in the live mix, since it would be centered in a final mix. To this end I’ve also wondered whether I should simply high pass at the preamp stage for the band-facing mic array to eliminate low frequency standing waves before the bits are even saved. I’ll have the bass close miked so I can easily center that later. If there’s any logic to this, then what would matter most is the placement of the guitar and drums. Having sat down in front of my stereo the past couple days to listen to some of my favorite trio albums, drums tend to lean a little left or right, and guitar or other soloist a little more firmly left or right.
« Last Edit: January 18, 2023, 09:52:21 PM by vantheman »
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Offline Gutbucket

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Re: Relationships between spaced omnis and center mic
« Reply #66 on: January 19, 2023, 09:51:41 AM »
^ All viable options.  I like to retain something close to what the placement on-stage actually was in the recording, but work the balance however is needed.  Partly because retaining the same general positioning arrangement keeps the reflections and ambient sound positioned correctly, making it easier to preserve a good sense of space and depth.  I've plenty of great recordings where the drums or bass or are over to one side yet the overall soundstage remains balanced, which can be refreshingly interesting because its not the same image geometry every time.  I try to stay open to whatever works in a gestalt sense, and nudging things around a bit without gross repositioning seems to be the most productive way for me to get there without losing depth and naturalness.  In that tiny-stage trio recording I mentioned above, the bass is left, guitar right, and drums centered (as the band was on stage) and it works really well.  I did intentionally setup slightly to the snare side of the kit, primarily to assure an unobstructed direct line from the snare to my center mic pair like I always want when close enough, yet angled the array slightly such that in the recording the drums are fully centered, with snare clear and nicely represented in the center.

I try to get a relatively equal amount of low-frequency energy in both channels of the recording, but perceptually the bass and kick drum might not actually be heard as centered.  What I usually want perceptually centered most of all are main vocals.  When making classical orchestral recordings that feature a singer I've gone both ways with regard to the singer being either perfectly centered, which is not accurate but more expected in a recording, verses slightly to the left of center, reflecting the actual position in performance standing just stage-right of the conductor.

A bit like your idea of high-passing the band-facing array at the preamp, in my array the directional mics are supercards which have a sensitivity that begins to drop off below about 200Hz, yet are still sensitive to content all the way down.  This effectively creates a gentle cross-over hand-off to the omnis supplying the meat of the bass, and that seems about right to me.  I don't like a sharp high-pass cutoff as much, but will switch one in if there are low frequency windnoise rumble problems that the windscreens alone are not taking care of, or pickup of vibration through the stand or something, ideally setting the corner frequency of the high-pass only as high as is necessary to take care of that problem.  The omnis make doing that work fine, but I prefer the "right amount" of bass in the directional channels where possible because in my experience the potential for a really great recording is just greater that way.  Sort of a Goldilock middle-way thing.
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Offline Gutbucket

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Re: Relationships between spaced omnis and center mic
« Reply #67 on: January 19, 2023, 10:51:48 AM »
I disagree with this statement 100%.

snip <NOS's 12" spacing is optimized for use as a single near-spaced pair without the inclusion of a center mic or coincident pair, but isn't spaced enough to work especially well in combination with a center mic position >snip

NOS with directional mics can very much benefit from a center forward facing single directional mic. I've made a bunch of recordings using this arrangement and it's been great more often than not.

How about 50%?  ;)

Ok bad choice of words there on my part.  I'm not saying that can't work and produce great recordings, and since many tapers feel NOS on its own is a bit over-wide, a center mic would help correct that.   I really didn't intend to pick on NOS there, just using it as an example of a typical near-spaced configuration after vantheman mentioned using it. 

Let me restate it a better way- Named two channel near-spaced configurations were standardized by the organizations that originated them and went on to became popularly used and preferred because they represent good balances of angle and spacing for a stereo pair of two microphones in common recording situations.  That balance is going to change when introducing a third center microphone.  If that change is what you want and improves things in your situation, great.  That's the right choice, moving things in the right direction.  However, if your intent were to retain the same recording angle that you previously had with two microphones, you'd want to increase the spacing and angle of the original pair to do that.  Even if your intent is to change the recording angle (along with the other changes introduced by the 3rd microphone position, which may well be more important to you) there may be an even more optimal way to do it.

If open to trying it when including the center mic, I think you might like what you hear with somewhat more spacing than NOS between the L/R pair, retaining the same angle.  It's just makes for an even more optimized arrangement of 3-microphones, while most likely retaining what you like from the inclusion of the center mic to begin with.  If you try it let me know your thoughts.  If your intent is to retain standard NOS as a safe backup that won't necessarily require the inclusion of the center microphone, I get that too.  Totally legit choice.

Of course some of this comes down to preference, but some of it is arranging things to optimally pursue that preference.  Taper recording is an odd thing.  There is not really any one right  microphone configuration arrangement to rule them all, but a lot of latitude in what can make a good recording.. and potential for further optimization of what makes a recording a bit better or not.  I'm generally looking for and suggesting trends toward further optimization of what may already be working well that may be usefully applied.
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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 Gutbucket

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Re: Relationships between spaced omnis and center mic
« Reply #68 on: January 19, 2023, 11:06:15 AM »
Goodcooker, thanks for calling that out.  I went back and edited my previous post in light of your comment.

Quote
NOS's 12" spacing is optimized for use as a single near-spaced pair without the inclusion of a center mic or coincident pair, but isn't spaced enough to work especially well in combination with a center mic position. EDIT- (that statement is too strong) NOS's 12" spacing was originally intended and optimized for use as a single near-spaced pair without the inclusion of a center mic or coincident pair. It isn't spaced enough to work really optimally in combination with a center mic position, even though the inclusion of a center mic may very well serve to improve the recording over 2ch NOS alone.  Your call on whether 2-ch NOS or 3 or 4chs consisting of 28" wide L/C/R using a coincident center pair is the way to go. If you are able and its not a hassle to do so, record six channels including the omnis as well.
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)

 

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