David Satz wrote:

If the M microphone emphasizes certain sound components (e.g. the high
frequencies) while the S microphone doesn't, then in the eventual L/R
stereo output of the matrix, those components will tend to gather at
the image's center more than the midrange sound does. The apparent
left-right position of any given instrument or voice would thus be
blurred. Conversely, if the S microphone boosts the highs while the
M microphone doesn't, high frequency energy would tend to be shifted
more toward the left and right than the midrange sound--increasingly
so with individual voices or instruments that are already off-center.

Yes. But even if they are matched blurring will occur
because frequency response is angle dependant or inversely,
the polar plot is frequency dependant. The consequence is
that the virtual mics given by the vector sum of the real
mics have axes that move about with frequency.

It will take me another little while to say where I don't agree; the
main issue is the fact that most microphones (except for the best
small-diaphragm figure-8s and certain rather noisy omnis) don't have
the same frequency response at all angles of incidence. So all our
"virtual microphones" have rather complex, irregular response--that's
another reason I tend to favor a super- or hypercardioid for "M".

I hadn't read ahead. You clearly understand the point I
made above.


However, I'll also point out that the analog of using a non-flat S mike
is processing the S channel independently of the M channel--and there is
a long tradition of doing exactly that. One of the attractions of M/S
(whether the recording is made originally that way, or whether another
type of X/Y recording is matrixed to M/S) is that the M and S channels
can be equalized independently or processed in other ways independently,
then rematrixed to L/R stereo.

You want to make sure that any such manipulation that is
frequency dependant be done with linear phase filters and
that the channel not processed always be delayed by the
group delay of the filter to keep the two channels time
coherent. Changing the relative phase of M and S outputs
also has a large effect on source placement within the image.

All of this having to do with frequency dependant polar
responses and the consequence on imaging applies as well to
the XY configuration. At one extreme, look at the
frequencies where the polar plot gets close to round and all
those frequencies will be moved to the center of the image.


Bob
--

"Things should be described as simply as possible, but no
simpler."

A. Einstein