Audio Developments AD149 - Manual (Page 12)

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The use of hypercardioid capsules offers a good compromise between crossed pairs
of cardioids and crossed pairs of figures-of-eight. The beam of a hypercardioid is
narrower and the 'reach' longer than that of a cardioid, therefore these microphones
can be used at a greater distance from the sound source for the same
direct/reverberant sound ratio but, because of the presence of a rear pickup lobe,
there is an increase in the sense of space. The narrower beam is a slight
disadvantage in that in order to provide a solid central image, the included angle must
be less than that for a pair of cardioid microphones. Hence, this system is not entirely
suitable for wide sound sources.
The M-S technique was rediscovered by Lauridsen and uses a microphone (M) aimed
along the central axis of the sound source and a figure-of-eight microphone (S) at
right angles to this axis (it is assumed that the positive lobe is orientated to the LEFT
for the purposes of these notes). The outputs of the microphones are combined in a
'sum and difference' matrix amplifier to produce conventional left and right stereo
signals. Later summation of the L-R signals to produce a compatible mono signal is
more predictable than when using stereo signals recorded using other formats (=2M);
alternatively, the original M-signal can be used. Thus, by careful choice of type and
position for the M-microphone, the optimum mono signal can be achieved with a
minimum of off-axis colouration.
Of all stereo recording techniques, M-S is the one that offers the engineer the most
control and flexibility, not only during recording but also during post-production. By
varying the ratio of the signals sent to the matrix amplifier, the width of the stereo
image can be adjusted. By changing the pickup pattern of the M-microphone (a
remotely controlled polar response being advantageous in this context), the apparent
distance from the sound source, and hence the amount of ambient information in the
M-signal, can be controlled.
The most popular choice for the M-signal is a cardioid microphone. In this case, after
matrixing the two signals, the resultant left-right stereo is equivalent to an X-Y pair
of hypercardioid microphones. The cardioid is the only M-microphone which does
not encode rear sounds as front sounds - overall pickup from the rear being low.
Angular distortion is low over the pickup angle and there is uniform directional
encoding of reverberation. If an omnidirectional microphone is used for the M-signal,
the X-Y equivalent is a pair of back-to-back cardioid microphones with their common
axis parallel to the axis of the figure-of-eight microphone. This arrangement is useful
when performers can be arranged on both sides of the microphone pair.
If a figure-of-eight microphone is used for the M-signal, the X-Y equivalent is a
crossed pair of figures-of-eight, each being at 45° to the central axis of the sound
source. Of all M-S techniques, this form provides the most even distribution of
reverberant pickup.