8

Chapter 7 Imaging 93
What Is MS?
Relegated to obscurity for a good long while, MS stereo (middle-side as opposed to
left-right) has recently enjoyed a renaissance of sorts.
Making a Middle Side Recording
Two microphones are positioned as closely together as possible (usually on a stand or
hung from the studio ceiling). One is a cardioid (or omnidirectional) microphone which
directly faces the sound source that you want to record—in a straight alignment. The
other is a bidirectional microphone, with its axes pointing to the left and right (of the
sound source) at 90˚ angles.
 The cardioid microphone records the middle signal to the left side of a stereo track.
 The bidirectional microphone records the side signal to the right side of a stereo track.
MS recordings made in this way can be decoded by the Direction Mixer.
Why Make MS Recordings?
The advantage that MS recordings have over XY recordings (two cardioid microphones
that are directed to a point halfway to the left and right of the sound source) is that the
stereo middle is actually located on the on-axis (main recording direction) of the
cardioid microphone. This means that slight fluctuations in frequency response that
occur off the on-axis—as is the case with every microphone—are less troublesome.
In principle, MS and LR signals are equivalent, and can be converted at any time. When
“–” signifies a phase inversion, then the following applies:
M = L+R
S = L–R
In addition, L can also be derived from the sum of—and R, from the difference
between—M and S.
Here’s some interesting trivia for you: Radio (FM) broadcasts feature M and S stereo.
The MS signal is actually converted to a signal suitable for the left and right speakers by
the receiver.