Audio Developments AD144 User Manual

Page 35

M-S NOTES

The M-S techniques proposed by Alan Blumlein in the early nineteen-thirties have
recently been (re)discovered. These techniques fall into two parts: those used during
original recordings, and those used in post-production. The way to change the width
of the image from an X-Y stereo microphone (without physically moving it), or from an
existing L-R recording, is to convert to the M-S domain in order that the M/S
relationship can be altered. Because the human brain is incapable of unscrambling
signals in the M-S domain, all M-S signals must be returned to the L-R domain. These
conversion procedures can be achieved electronically or by the use of a specially
designed transformer, and are called encoding - or is it decoding? In order to
overcome any possible ambiguity, we refer to the encoding and decoding
processes as matrixing - since they involve a matrix amplifier. This matrix
amplifier will accept an L-R input signal and convert it to an M-S output; or an M-S
input signal and convert it to an L-R output - hence the confusion.

The matrix is a sum and difference amplifier which adds, in phase, the two input
signals to produce a left signal [M+S=L]; it also adds the two input signals in
anti-phase to produce a right signal [M+(-S)=R]. Similarly [L+R=M] and [L-R=S].

Increasing the S-signal widens the L-R stereo image; decreasing the S-signal makes
the stereo image narrower.

Changing the phase of S-signal swaps over the left and right outputs of the matrix.
Thus if an M-S microphone is set up with incorrect orientation of the S-microphone,
changing the phase of the S-microphone channel will rectify the situation - without
having to disturb the microphone.

Mixing in the M-S domain is a legitimate technique and simultaneous mixing in the M-
S and L-R domains is possible with AD144. (X-Y microphones will first have to be
matrixed to the M-S domain.) Any M-S microphone may be treated as being in the
M-S domain at the pre- and post-fader auxiliary points. M-signals may be mixed
on AUX 1/3 and S-signals on AUX 2/4. Separate matrix amplifiers may be switched
into the A/B output DEC (5) and C/D output DEC (18). The capabilities and flexibility
of the monitor modules are increased by an order of magnitude by the inclusion of a

OUT

+
+

MATRIX AMP