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While there are several stereo techniques that can be applied
during miking, for signal processing during production there is only
one technique that is actually useful: M/S. „M“ stands for Middle
(or Mid) and „S“ for Side, which means that signals are separated
from the middle to the sides, instead of from left to right.
M/S can be actually applied during recording: two microphones
with different polar patterns record direct and spatial information.
Besides the microphone technique, M/S can also be used as an
alternative stereo encoding for signal processing, which means
that signals do not necessarily need to be recorded with the M/S
microphone technique to be able to apply M/S encoding after-
wards. In fact, M/S encoding can be generated from L/R encoding
by summing and subtracting signals:
M = L + R, S = L – R
The sum of the left and right signals in the Mid signal corresponds
to the mono signal of the L/R encoding. The Side signal is also cre-
ated from the L/R signal by inverting the polarity of the right chan-
nel. The sum of phase-inverted signals results in the cancellation
of mono information in the signals summed; thus, the Side signal is
made up of the differences between L and R. The detailed formula
may be clearer (the minus sign stands for the phase inversion):
M = L + R, S = L + (-R)
It is also possible to create a L/R signal encoding from an M/S
encoding by summing and subtracting the signals, what is usually
called M/S decoding:
L = M + S, R = M - S
Mathematically, the sum and subtraction of signals guarantees a
lossless conversion from L/R to M/S and back to L/R, which is a
very important aspect for using M/S encoding for signal process-
ing.
M/S Basics
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