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Author Topic: MS balance issue  (Read 2387 times)

boz6906

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MS balance issue
« on: April 16, 2014, 01:33:20 pm »

Hello,

I've seen this often discussed on various forums and I'd like to hear your opinions on the subject.

Recently I recorded a septet using my typical ORTF seup with 2x AKG C480.

The client, a well respected conductor/composer asked that I use a M-S arrangement so I added a USM69 in card/bidir as andditional pair.

I don't usually us M-S because of the inaccurate low freq reproduction but it's another M-S issue that I'd like to discuss.

When matrixing the M and S channels, the S channel that is in phase with the M channel shows about 4dB higher than the reversed S channel.

The stereo field sounds good when the in phase side is lowered by 4dB.

Thoughts?
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David Satz

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Re: MS balance issue
« Reply #1 on: April 16, 2014, 07:15:53 pm »

There's nothing wrong with that. It simply depends on the inherent, relative sensitivities of your M and S microphones. No law of physics forces them to be identical, even when they are obtained from different settings within the same stereo mike (almost every multi-pattern microphone's sensitivity varies as a function of its pattern setting).

And even if the two sensitivities happened to be identical, you might well want to boost "S" relative to "M" or vice versa sometimes, depending on microphone placement, the acoustical setting and the parameters of the stereo image that you wanted.

Regarding the low-frequency problem, some of it surely comes from using a dual-diaphragm cardioid for the "M" signal (or the cardioid setting of a dual-diaphragm, multi-pattern microphone such as the USM 69), since that type of microphone loses directivity at low frequencies; its pattern spreads out to become more like a "wide cardioid" as you can readily see in the published polar diagrams. You might want to try using a good single-diaphragm microphone for the "M" channel some time.

Or if the USM 69 is your only practical choice, you might try using its supercardioid setting for the "M" channel. In that setting, the directivity is maintained down to lower frequencies than in the cardioid mode (which, again, is typical of dual-diaphragm microphones).

--best regards
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klaus

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Re: MS balance issue
« Reply #2 on: April 16, 2014, 08:00:37 pm »

Thanks, David, very helpful!
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Klaus Heyne
German Masterworks®
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Kai

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Re: MS balance issue
« Reply #3 on: April 20, 2014, 01:51:51 pm »

I don't usually us M-S because of the inaccurate low freq reproduction...
When an omni is used as M mic the LF is much better with M/S.
The S-channel benefits from using a real figure of eight, not a double-cardiod with right side phase reversed.
The figure of eight will deliver only few LF due to it's natural drop in the frequency response, so the omni dominates here.

One thing people don't realize when comparing directional with omni mics:
The omni records sound pressure (air pressure), the directional mics pressure gradient (air movement).
If you look at a soundwave, the maximum pressure is where the minimum movement is.
This is specially important for standing waves like room resonances with wavelenght of several meters size.
It's one reason why those mics sound different in the LF range, and they might need different placements for sounding good, besides the direct/reverb balance issue.

When matrixing the M and S channels, the S channel that is in phase with the M channel shows about 4dB higher than the reversed S channel.

The stereo field sounds good when the in phase side is lowered by 4dB.
I guess your USM 69 does not deliver the same level for both sides of the capsule that was used for the S-channel.
4dB difference would mean it's closer to hypercardiod then figure of eight.
If you still have the mic you can check that.

Something similar happened to me long time ago when I used a pair of KM56 intended to work as crossed cardiods.
One of the mic's switches delivered a problem, the mic switched itself into omni.
I could fix that later by turning a mixer into a matrix.
By carefully adjusting the inphase and phase levels for each channel and side I could build a normal sounding stereo image.

Regards
Kai
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David Satz

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Re: MS balance issue
« Reply #4 on: April 20, 2014, 07:01:53 pm »

When I saw Kai's response, I realized that I'd misread the original post. My reply above was written as if the 4 dB difference in signal levels had been between the channels in the USM 69. M + S = L while M - S = R and now I see that those resultants are the signals that had the 4 dB difference in level.

Unfortunately I don't know whether peak or average level was meant. Sound coming from the left side in the hall may simply have had 4 dB higher peak levels than the sound coming from right. But a difference of 4 dB as an average for the entire recording, on the other hand, is greater than I would normally expect if the microphone was correctly oriented during the performance. If it was suspended from its cable and twisted slightly clockwise, that would explain a lot, wouldn't it?

So I wouldn't jump too quickly to the conclusion that the microphone is at fault. I suppose it's possible that the front and back halves of the capsule that you used for "S" are no longer well-matched in sensitivity--perhaps you should check that out if only to rule it out. A figure-8 with 4 dB asymmetry isn't quite a hypercardioid (which would technically have 6 dB lower sensitivity in back than in front), but a USM 69 should be within ~2 dB back-to-front in the figure-8 setting, particularly at low and mid frequencies, just as a U 87 should be.

Also, while I'm personally a fan of single-diaphragm condenser microphones, it's precisely the patterns in the range from supercardioid -  hypercardioid - figure-8 where a well-made dual-diaphragm condenser can most closely approximate the performance of a single-diaphragm capsule, in terms of maintaining a uniform polar response across the frequency range. And for M/S recording of an orchestra you definitely want your S-channel microphone to have good low-frequency response, which the USM 69 certainly does have.

So I really don't fault this choice of microphone for this type of recording, although to my ears it can be slightly over-bright in some halls, lacking as it does the built-in high-frequency compensation of the U 67 and U 87 (ca. 4-5 dB at 15 kHz).

--best regards
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