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Old June 4th, 2004, 02:39 AM   #1
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Question: Stereo spread vs. zoom

I'm trying to understand the details of stereo recording... Have I understood the following correctly?

M-S stereo:

When using "long" focal lengths (zoom):

Increase the side channel ratio and decrease the mid channel ratio to make the stereo spread wider and to create an illusion of beeing closer to the target (sound coming from the sides moves backward). This would keep the directions of sounds correct. Then increase the overall sound level.

When using wideange:

Increase the mid channel and decrease the side channel to make the stereo spread narrower and to create an illusion of beeing further away from the target. Decrease the sound level if required (probably not).

So, was that correct? How noticeable is the sound direction, after all? Can it conflict with the image in any noticeable way in practice? Speaker setups do vary... Is there any "golden rule" that tells what kind of stereo spread to use with, say f=50mm.

Are there any documents concerning this on the net?
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Old June 4th, 2004, 02:47 AM   #2
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Another question:

Does this M-S matrixing affect the relative loudness of the ambience vs. the center sound?

-somewhat confused again-
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Old June 4th, 2004, 01:06 PM   #3
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Ralf,

Have a look at this for the theory of M/S
http://www.wesdooley.com/aea/library.html

Mathematically, I think that M/S can be thought of as two coincident cardioids. With 100% side channel, they are two back to back cardioids. With 100% mid channel they are one, forward facing cardioid. With 50% side, 50% mid, they are like cardioids at angle of (I think) about 120 degrees.

I suspect the relative balance you want for wide angle and zoom is exactly the reverse of what you are suggesting. You want a narrow angle if your subject is distant, in order not to pick up extraneous lateral sounds. This is the principle of the shotgun mic.
Unfortunately, it doesn't really work for sound. To get good sound, your microphone has to be close to the subject. Sound doesn't zoom.

There is no way you can exactly match the picture image to the sound stage. Fortunately this does not seem to matter with music. It's the same as if you were in a concert hall, and you were looking at different performers with binoculars. You can cope with the sound coming from the wrong direction. But it would be horrible to have the soundstage rotating every time you panned the camera. For dialogue I believe that professionals always record in mono.

Patrick
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