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Author Topic: One channel is louder: my error or not?  (Read 1913 times)

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Offline prepschoolalumniblues

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One channel is louder: my error or not?
« on: September 15, 2019, 12:57:17 PM »
Last night, I taped four channels for the first time (XY center cardioids, split omnis). This was a very casual gig with an acapella group who ran their own sound into a pair of basic PA speakers. (The soloist had a microphone; the rest was unamplified.)  I’ve been playing with my tracks in Audacity today and I’ve noticed that my ears pick up on what the waveforms suggest: in both of my microphone pairs, the left channel is a bit louder than the right channel.

I’m trying to figure out if I messed something up with my microphone setup, if my microphones aren’t balanced (if that’s the right word?) or if the group just didn’t have their PA set up evenly across the two sides of the stage. Any suggestions for how to troubleshoot this? This was a low-stakes practice run for me with my new gear, but I’d like to avoid it in the future if I can figure out what’s wrong.

Applying gain to the right channels seems to kind of help, although in my omni pair I suddenly hear a lot more of the locusts chirping in my right ear (outdoor show). In other words, maybe it’s just the amplified sound through the speakers that is unbalanced between the two channels — which would suggest a performer problem and not recorder problem.
Mics: Line Audio CM4 (sc); Line Audio OM1 (o); Audio-Technica AT853Rx (c, sc, o); Audio-Technica ATU853 (c, o); Oktava MK-012 (hc, c, o)

Decks: Sound Devices MixPre-3, Marantz PMD661 (Oade warm mod), Marantz PMD620MKII, Marantz PMD-706 (Oade warm and concert mods)

Offline Gutbucket

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Re: One channel is louder: my error or not?
« Reply #1 on: September 16, 2019, 06:58:19 PM »
Good ears.

It can stem from a number of causes, some of which you can adjust for before making the recording and some of which you will need to adjust for afterwards.  Regardless, the ultimate answer is to adjust to whatever balance you deem most appropriate by ear.  Do so for each stereo source in isolation prior to mixing multiple sources together.  You may also want to tweak the overall balance after combining sources, but if each was well balance prior to mixing any final overall adjustment should be minor if needed at all.

Numbers of causes: Could be from differences in sensitivity or response of the two mics forming the pair, differences in gain or input trim through the preamp/recorder, environmental factors (louder stuff off to one side), setup factors (mic array pointed off axis), etc. 

If it's due to mic sensitivity differences you may be able to determine that offset ahead of time and adjust for it prior to recording.  You'll still want to listen for balance and tweak it as necessary, but this will at least start off closer to correct.  So if you know one omni is 2dB more sensitive or whatever than the other you can accommodate for that if you make sure to always run the more sensitive mic through the preamp/input channel with that much less gain applied.. and not accidentally mix them up (which would double the level difference rather than correct for it).  But this isn't really necessary and more of a nice to have kind of thing.  You should listen for and adjust balance of each pair anyway, so don't sweat a slight mic sensitivity mis-match unless correcting for it later reveals noise or response differences in one channel verses the other after they are balanced.

To best accommodate for environmental/setup directional imbalance issues which are obvious during setup, aim the microphone array not at the visual center of the stage, but at the apparent acoustic center from which the sound seems to emanate when you close your eyes and listen without any visual reference.  The actual acoustic center can be considerably offset from than the visual center, especially if recording from an off center location.  In that case you will mostly often find yourself rotating the array away from the visual center and towards the closer side of the stage / closer PA speaker.  This is not something intuitive unless you listen with your eyes closed.  It's pretty amazing the degree to which our eyesight overrides our hearing.  Most (sighted) tapers intuitively want to point the array towards the center of the stage or the opposite side of the stage to try and compensate for being off to one side.  That doesn't work and only serves to aggravate the acoustic mis-balance. 

If an mis-balance only becomes obvious after the music starts and you have easy access to the recording position, you can quietly loosen one telescopic section of the stand and rotate the mic array to point directly at the apparent acoustic center while listening by ear.  Do so smoothly to avoid transmitting structure-born noise through the stand which could be audible on the recording.  Consider this kind of quick initial directional adjustment similar to tweaking levels at the start and make it a part of your routine.  Obviously you can't do this if the mics are up on stage or remote from you.

« Last Edit: September 16, 2019, 07:49:34 PM by Gutbucket »
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Offline Gutbucket

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Re: One channel is louder: my error or not?
« Reply #2 on: September 16, 2019, 07:34:05 PM »
Further-

A very powerful aspect of the recording you just made is several additional degrees of advanced balance manipulation you now have control over in making the best sounding mix.  As mentioned, start your mixing process by balancing each source in isolation. 

One additional balance axis is the overall balance between sources- the level of wide-spaced omnis versus the level of the X/Y cardioids.  Psychoacoustically this is sort of a forward/backward balance adjustment, but its also a center/sides type balance adjustment.  My suggestion is to bring up the omnis first, then play with the level of the X/Y pair.  This may be as much as you need and as far as you want to go to start.

But there is more.. Yet another balance aspect is finding the most appropriate X/Y stereo width.  This is influenced by whatever the omni spacing was and whatever the X/Y angle was.  But afterwards when mixing you can tweak X/Y stereo width to get the best blend and balance between the wide omnis and X/Y cards across the center.  Use a stereo-width adjustment tool to do that (or a L/R>Mid/Side>L/R chain along with a modification of the Mid/Side ratio, which is what a stereo-width adjustment tool is actually doing).  Typically you'll want less X/Y stereo width when mixing with the omnis than you would if listening to the X/Y pair on its own.  My suggestion is to get a good initial balance between the omnis and X/Y pair, then play with X/Y width.  Start by dialing it all the way down to mono or all Mid.  That's equivalent to having a single subcardioid microphone pointing directly forward instead of an X/Y pair.  Then listen while adjusting X/Y stereo width.  You'll hear it when it becomes too wide.  Go back and forth a bit and you'll find the sweet setting where the X/Y stereo blends optimally into the omni stereo and you get a nice solid yet wide image all the way across soundstage.

If working that way, you can take this one step further still by setting up your X/Y pair so the mics are Pointed At Stacks rather than at 90 degrees or whatever.  This keeps them focused on the PA as much as possible for the clearest sound, yet in most cases will make for a rather narrow X/Y angle.  But since we are readjusting stereo width of the X/Y pair to suit when mixing anyway, we can widen the image a bit instead of narrowing it if that ends up working best in the mix.  For the same reason you can push the omnis wider than you would want to if they were used on their own.  Essentially this is optimizing the mic setup more for best direct sound / indirect sound capture rather than for best imaging while recording, then tweaking for best imaging in post once you have a more optimal capture.

BTW, if at any time you question your own hearing balance when mixing, just invert Left/Right stereo to Right/Left momentarily and determine if the balanced image stays balanced (or if a left-heavy image switches to the same degree of right-heaviness along with the inversion).  If using headphones this is easy- simply flip them around on your head and wear them facing backwards for a moment.  If using monitor speakers you'll need to flip sides somewhere in the signal path (DAW output bus, monitor amp, etc).
« Last Edit: September 16, 2019, 07:55:12 PM by Gutbucket »
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Offline Gutbucket

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Re: One channel is louder: my error or not?
« Reply #3 on: September 16, 2019, 07:47:10 PM »
What is "optimal" essentially boils down to your own judgement call.  Mix it however you feel it works best.  Some balance aspects may be contradictory.  For instance, environmental sounds could be right heavy (like your locusts), while the the performer balance is left heavy.  Sometimes in that case you can work a cross-balance between pairs to try and even things out- shift balance of the omni pair toward the left to better balance the locusts (or room 'verb or audience reaction), while shifting balance in the X/Y pair toward the right to re-balance the performers, with the end result being each tweak sort of counter-balances the other.  You usually  can't push that to extremes, but even a slight adjustment can help things snap into place.  At this point you are really getting to the point where you can make a recording sound better balanced than it did live.
« Last Edit: September 16, 2019, 07:57:13 PM by Gutbucket »
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Re: One channel is louder: my error or not?
« Reply #4 on: September 17, 2019, 11:52:36 AM »
many times it can be frequency dependent, bass reflections especially can push one side louder.
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Offline rumbleseat

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Re: One channel is louder: my error or not?
« Reply #5 on: October 20, 2019, 09:14:11 AM »
Hi!

I just finished working on my Bonnaroo recordings (https://taperssection.com/index.php?topic=192327.0) and wanted to share an insight.  Most of these raw recordings sounded terribly off-center – almost unlistenable on headphones.  I’d run into this issue in the past and have tried to correct it with a combination of boosting one channel and EQ’ing the channels separately.  This time, it was unfixable.

Earlier in this thread, Gutbucket gave the solution to fixing this issue in the field when he said “rotate the array away from the visual center and towards the closer side of the stage / closer PA speaker”.  But what about fixing it in post?  It turns out that adding a small delay in one of the channels miraculously moves the soundstage to the center!

The two other factors that can cause recordings to sound off-center (as mentioned earlier in this thread) are EQ and balance.  I adjust EQ, balance, and timing as follows:

First, split the channels and adjust the EQ of one channel to match the other, especially in the low frequency region.  Voxengo’s free spectrum analyzer, SPAN is very useful routed as “dual mono” with “right underlay”. https://www.voxengo.com/product/span/.
Then carefully adjust the channel balance so they match in volume.
To adjust the timing, Voxengo’s free Sound Delay VST plugin works well in real-time https://www.voxengo.com/product/sounddelay/.  Again, select “dual mono” routing (channels do not have to be split). Then spin the “Audio Delay x0.1” dial.  This dial gives you up to “a foot” of delay (1 msec) and that will probably be sufficient (unless you have really widely spaced mics…) If adding delay makes things worse, switch from Left to Right.

If you can run simultaneous VST plugins (Audacity can’t…), then Voxengo’s Correlometer https://www.voxengo.com/product/correlometer/ is useful (aim for positive correlation across the frequency range as you dial in the delay).  MAAT’s 2BusControl https://www.maat.digital/2buscontrol/ is also handy as it has a button to “flip” left and right – your soundstage should stay centered.  Use the “mono” button to reference your center.

All of these plugins are free, and they’re the ones I stumbled upon and got working.  I’m sure there are dozens of others that will get you to the same endpoint.

Looking back, this was the first time I used AKG C568EB short shotguns pointed just outside the stacks and apparently, these mics made the off-center situation worse since the stereo information in each channel is not as “smeared” as it would be with the standard cardioids that I’ve used in the past.

Examples?  Attached below are samples of the raw file and finished recording from Courtney Barnett’s performance.
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Offline hoserama

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Re: One channel is louder: my error or not?
« Reply #6 on: October 20, 2019, 11:25:45 AM »
I use waves S1 imager to rebalance the stereo field. After that, I usually mono out the low-end (150hz and below), provided there's no real phasing issues going on. Works for most audience recordings I've handled.
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Offline kuba e

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Re: One channel is louder: my error or not?
« Reply #7 on: October 20, 2019, 05:29:49 PM »
First, split the channels and adjust the EQ of one channel to match the other, especially in the low frequency region.  Voxengo’s free spectrum analyzer, SPAN is very useful routed as “dual mono” with “right underlay”. https://www.voxengo.com/product/span/.

Thank you Rumbleseat. I have been using SPAN, but I haven't known about this setting. Underlay right channel is very nice feature. It is great for eq matching.

It turns out that adding a small delay in one of the channels miraculously moves the soundstage to the center!

Yes, the playback stereo image is created by level and time differences. And you can correct stereo image by changing the levels and the delay. I asked about the same in the past. There are some threads where it is all very nicely explained. If you can't find it, I'll look for it.

If you can run simultaneous VST plugins (Audacity can’t…), then Voxengo’s Correlometer https://www.voxengo.com/product/correlometer/ is useful (aim for positive correlation across the frequency range as you dial in the delay).  MAAT’s 2BusControl https://www.maat.digital/2buscontrol/ is also handy as it has a button to “flip” left and right – your soundstage should stay centered.  Use the “mono” button to reference your center.

Nice idea too. Gutbucket wrote about correlation in the past. I hope I remember this well. If I wrote something wrong or unclear, please correct me. The correlation coefficient of the right and left channel expresses a certain quality of spaciousness of the recording. The "spaciousness" of the recording is influenced by the recording ambience and also by the microphones angle and spacing:

Recording with coincident (xy) directional mics has zero "spaciousness". The stereo playback image is created by level difference. It is clear and sharp image. The correlation coefficient is always positive. The amount of correlation depends on the angle between the microphones.
Edit: I forgot fig. 8, hypercardiod and other patterns with negative lobe. These patterns can have negative correlation. Stereo playback image is created by level difference and opposite phase. Theoretical example is the pair of fig. 8 rotated 180 ° relative to each other. The right and left signals are the same, but in the opposite phase. The correlation is -1.

Recording with spaced omni mics has a "spaciousness". The stereo playback image is created by time differences. It is not so clear and sharp like xy. The correlation is depending how great is the spacing. The correlation is positive for the lowest frequencies. But the high frequencies are alterning between positive and negative values. The greater the spacing, the more it hits low frequencies. It is the relationship of the different sounds paths to the left and right microphones and the wavelengths of that sounds. 

Spaced pair of directional mics (e.g. din, ortf) is combination of this two cases. Some tapers are also using array of microphones. The basic array is great spaced omni with pair xy in the center.  The spaced omnis are bringing "spaciousnes" even in the low frequencies. And xy is creating sharp stereo image. If you're interested at that, look at Gutbucket's OMT.
« Last Edit: October 21, 2019, 04:55:41 AM by kuba e »

Offline rumbleseat

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Re: One channel is louder: my error or not?
« Reply #8 on: October 22, 2019, 08:25:40 AM »
Thanks for your input kuba e and hoserama!
I took a look at the S1 Imager video and manual and it looks very nice.

Off topic - Stumbled upon John Eargle's "The Microphone Book" while I was browsing.  Chock full of interesting info!
https://soma.sbcc.edu/users/davega/FILMPRO_181_AUDIO_I/FILMPRO_181_04_Reference_Notes/FILMPRO_181_Microphones/The%20Microphone%20Handbook.pdf
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