XY, when done properly, is also mono-compatible, but ORTF isn't.
Imagine a single sound source in front of a pair of mics. If the wave peak=
from the source hits both mics at exactly the same moment you get a
mono-compatible signal, because when you add both signals together (to make=
the mono channel) there is only constructive interference between the two
recorded signals.
Now imagine the source to be to the side. Further, imagine that the XY setu=
p
is side-by-side, not one on top of the other. Now when the sound peak
arrives it will strike one mic before the other. If the spacing is just
right the X+Y signal will have a fully-canceled signal, so the source is no=
t
heard, only the out-of-phase reverberations.
XY can be mono-compatible, but it's very difficult to do for a wide
soundfield and mics larger than a bean. The side-by-side XY setup is prone=
to errors from sources that are to the side, and the above-below XY setup i=
s
prone to errors in sources above and below the mics. MS suffers the same
way, but by putting all the out of phase info in a single channel you at
lease get a clean mono channel out of it. A perfect solution might be three=
recorded channels--four if you want two mono channels at different levels,=
and two channels for the stereo. Now where was that Edirol catalog...?
Bruce Wilson KF7K
http://science.uvsc.edu/wilson
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