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Author Topic: M/S ???  (Read 2546 times)

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Offline Gutbucket

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Re: M/S ???
« Reply #15 on: November 27, 2007, 05:32:09 PM »
Just had a thought..

Let's turn this whole thing on it's head.
Could I mic in X/Y and use the M/S function in my preamp or recorder to record the M/S signals? 

Assumming for a moment that the extra matrix steps DO deteriorate the sound appreciably, I could then adjust in post without the need for extra matrixing (if that matters at all ;), it's probably splitting hairs),

More importantly, this could open up some interesting mic techniques.. like choosing polar patterns and mic angles based on other issues than the resulting stereo sound stage.

For instance, if using mics with less than stellar off-axis respose in a 120deg. X/Y setup, I could reduce the angle between mics for more tonal accuracy towards the center, then dial back in more stereo separation by increasing the S signal ratio later to compensate.
 
Unfortunatley I don't think the matrix built-in to preamps and recorders will work 'in-reverse'.
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Offline Will_S

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Re: M/S ???
« Reply #16 on: November 27, 2007, 06:21:04 PM »
Just had a thought..

Let's turn this whole thing on it's head.
Could I mic in X/Y and use the M/S function in my preamp or recorder to record the M/S signals? 

Mathematically, I don't see why this wouldn't work.  If the decoder is outputting the sum of the inputs as the left channel, and the difference between the inputs as the right channel, this should work equally well at creating L and R from M+S and M-S or 2*M and 2*S as L+R and L-R.

Quote
Assumming for a moment that the extra matrix steps DO deteriorate the sound appreciably, I could then adjust in post without the need for extra matrixing (if that matters at all ;), it's probably splitting hairs),

Actually, here's where I can see problems might crop up.

Say you want to use XY cards as your microphones, and you want to use a fairly tight included angle to minimize off-axis frequency response problems.  You will end up with a mid that is much higher in level than the side channel, and you have no means of independently adjusting the gain of mid vs. side since your preamp gain controls are only working on left and right signals.  If you are using a 16 bit recorder, that means less than 16 bits used to capture the master side signal.  Potentially that could lead to problems with noise as you boost the side channel in post (as you probably would want to, given the narrow mic angle you started with).

This also strikes me as a potential problem with micing Mid-Side but recording as L-R, then dematrixing and remixing in post.  If you recorded left and right at 16 bit, the side channel you derive will have less than 16 bit resolution (even if your DAW does its internal processing at 32 bit, it still can't get that level of precision taking the difference between two 16 bit sources) which again might lead to problems when you boost the side channel in post.

Offline Gutbucket

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Re: M/S ???
« Reply #17 on: November 27, 2007, 07:07:31 PM »
Quote
...Assuming for a moment that the extra matrix steps DO deteriorate the sound appreciably, I could then adjust in post without the need for extra matrixing (if that matters at all ;), it's probably splitting hairs),

Actually, here's where I can see problems might crop up.

Say you want to use XY cards as your microphones, and you want to use a fairly tight included angle to minimize off-axis frequency response problems.  You will end up with a mid that is much higher in level than the side channel, and you have no means of independently adjusting the gain of mid vs. side since your preamp gain controls are only working on left and right signals.  If you are using a 16 bit recorder, that means less than 16 bits used to capture the master side signal.  Potentially that could lead to problems with noise as you boost the side channel in post (as you probably would want to, given the narrow mic angle you started with).

That's a concern I'm thinking about too.  Yes the preamp gain would reflect the needs of the X/Y arrayed mics capturing L/R.  However, if I'm using a separate preamp and a recorder with independent gain control for each recording channel, I could then optimally set the recorder's levels for the M & S signals independently, even if that meant the 'correct' M/S ratio was off.  This would mean a better use of recording bits than recording the L/R signal for anything other than a perfectly even 50% matrix ratio. 

Now that could be a problem if there is only one gain stage between mics and all-in-one recorder.  (is this the case for the SD7xx?, assuming it could do the 'reverse' M/S thing?)   

Quote
This also strikes me as a potential problem with micing Mid-Side but recording as L-R, then dematrixing and remixing in post.  If you recorded left and right at 16 bit, the side channel you derive will have less than 16 bit resolution (even if your DAW does its internal processing at 32 bit, it still can't get that level of precision taking the difference between two 16 bit sources) which again might lead to problems when you boost the side channel in post.

If that's a problem it would be an argument that the post procedure does in fact deteriorate the sound, and would apply to doing the matrix>adjust>dematrix thing in post with any source.

Good thoughts.
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Offline Will_S

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Re: M/S ???
« Reply #18 on: November 27, 2007, 08:00:26 PM »
Interesting stuff for sure.

Another thing to think about is that the polar patterns and effective angles you get back will of course be constrained by the mics and angles you choose for your left and right channels (or mid and side).

Personally, I *like* the sound of conincident hypers at ~120° better than I like cards at 90° most of the time.  So I like the fact that that's the baseline pattern I get back from typical card-fig8 mid-side micing with a 1:1 mix of mid and side.  I wouldn't so much like the baseline I'd get back from a pair of cards.  And if I used cards at a reasonable narrow angle to keep the center sound tonally balanced, then for any effctive mic angle I wanted to get back from my M/S decoding I'd end up with fatter pickups on my virtual mics than I would with conventional M-S micing.  [But then maybe I'd really like playing around with a recording originally made with XY hypers...]

or instance, if using mics with less than stellar off-axis respose in a 120deg. X/Y setup, I could reduce the angle between mics for more tonal accuracy towards the center, then dial back in more stereo separation by increasing the S signal ratio later to compensate.

Except that if what you really wanted was the sound of cards at 120° but you recorded with more narrowly angled cards and boosted the side in post, you'd get back something more like fat hypers at 120°.  But this could be a cool way to go if you wanted to run hypers but only had cards at your disposal.

Offline Gutbucket

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Re: M/S ???
« Reply #19 on: November 27, 2007, 08:17:09 PM »
^^^
Like minds think alike.

This is why this stuff never seems to grow old to me.
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Offline Ozpeter

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Re: M/S ???
« Reply #20 on: November 28, 2007, 04:41:50 PM »
I've routinely taken recordings with hardly any stereo separation due to someone having recorded with too narrow and angle with an X/Y pair, and fed the result through Voxengo MSED in the PC - and the improvement is usually dramatic.  In fact, I find that the Rode NT4 usually benefits from this treatment - the normal two dimensional sound can suddenly acquire the extra third dimension of spaciousness and reverberant warmth.

 

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