The SoundField SPS422B
Issue 1.7
User Guide
Page 12
USING M/S MODE
M/S or ‘Middle and Side’ is a method of combining the outputs of a forward firing cardioid (or
occasionally an omni) microphone with a laterally mounted figure of eight microphone to provide a
stereo output. A simple sum and difference matrix circuit is required and these are commercially
available as M/S decoders.
Mathematically, there is no difference between decoded M/S and conventional LEFT/RIGHT XY
stereo signals if theoretically perfect microphones are used, but many engineers feel that M/S
produces more natural results with most ‘real-life’ microphones as the centre image is directly on-
axis to the mid microphone. The unique SoundField capsule geometry and matrix circuitry ensures
that both the MID centre mono component and the SIDE signal are resolved accurately with no
phase discrepancies, and that the MID centre signal level remains constant when the width is being
varied.
M/S mode is particularly useful when making live recordings as it allows the stereo image to be
adjusted at the post production stage. The MID signal is recorded onto one track of the stereo
machine and the SIDE signal on the other; it helps to stick to a convention here and it is suggested
to always use the left channel to record the MID signal and the right channel the SIDE signal.
As it stands, this recording will not produce a stereo image when replayed - it must first be decoded
using a ‘sum and difference’ matrix. The two sides of the stereo signal are reconstituted by adding
the SIDE signal to the MID signal to produce one side of the image and substracting the SIDE
signal (using phase inversion) from the MID signal to produce the other side. By adjusting the level
of the SIDE component, the stereo width can be made as wide or as narrow as is desirable and, by
omitting the SIDE component altogether, a pure mono signal is available. Because of the ability to
modify the stereo image in this way, it is recommended that any M/S recordings be made with the
Pattern
and
Width
controls set to their midway positions.
Commercial M/S decoders provide the most accurate means to decode an M/S recording, but in
situations where none are available, it is possible to achieve similar results using a mixing console
equipped with phase invert buttons. This requires the use of three mixer channels and is arranged
as shown below.
MS DECODING USING MIXER CHANNELS
GAIN
GAIN
GAIN
HIGH
HIGH
HIGH
MID
MID
MID
LOW
LOW
LOW
PAN
PAN
PAN
L
L
L
R
R
R
Ø
Ø
Ø
MID SIGNAL
SIDE SIGNAL
ENGANGE PHASE
INVERT SWITCH
PAN POT FOR MID
CHANNEL TO CENTRE
PAN ONE SIDE CHANNEL LEFT
PAN ONE SIDE CHANNEL RIGHT
MOVE FADERS TO ALTER
STEREO IMAGE
