Posted by: "macmang4125"
> I think what they are saying is that conventionally converting MS to A
> & B is done by
> M (A+B) - S (A-B) = 2B
> M (A+B) + S (A-B) = 2A
> The above calculation does depend on there being a direct correlation
> between the M and S signals in level terms.
> So increasing the level of S will affect the final stereo image. So if
> you want to save a stereo recording as an archive where the S is level
> boosted. Then anyone in the future who is unaware that the S is
> boosted who then decodes the signal back to A & B stereo will end up
> with a result different than what was intended.
There have been reports that reencoding to M/S and then back again to
A/B is not as lossless as it should be in theory. How the actual
software involved handles differences that vary moment by moment is
probably part of this. I don't think the particular ratio of the mix
will create more problem.
The two mics don't record the same soundfields. Neither sees all of A or
B, but different aspects of each. So you don't get 2A & 2B, but a
composite that's not generally more than 1A & 1B. And there does not
have to be the same signal levels for both M & S. The M/S decoding
theory does not require it. In fact the levels are never static but
constantly varying. And if one watches the metering there is often
little correlation between the variance of the two channels. A clear
indication the two mics are picking up different soundfields.
As I noted when I decode in the computer generally the S is set less
than the M. This mostly because I usually have a central caller or
callers that are louder than the surrounding ambiance. There is more
sound coming from the front than from the sides. Of course part of the
reason the S is less is because I recorded it with more gain in the
field. But not all is due to that, some is the soundfield I'm recording.
It would be a error to mix the S in at a greater ratio than it was in
the soundfield you recorded. I do hear a lot of mixes where that error
has occurred. Mixing more S in widens the soundfield so people are
tempted. But past a certain point the middle begins to fail.
If I was recording a ambiance that was pretty even in all directions
then the optimum mix would probably be even. Though even then we are
probably going to get into the figure 8 seeing a larger total area than
the mid does. And if the sensitivity of the M & S are identical or not.
And delve into exactly what the polar patterns are like.
The differences between the M & S are what eventually, through the
decoding process, gives space and volume to the stereo playback. Without
any differences in level in the details I believe what it falls back to
is mono. Good thing that does not happen.
Walt
|