COINCIDENT OR "INTENSITY" STEREO TECHNIQUES COMPARED

Stereo recording for television in the U.S. has a potentially nasty pitfall waiting for the unwary. The MTS format in the NTSC TV signal is not always mono compatible. If non-coincident stereo recordings are used, they will cancel when received in a mono TV set. The primary coincident techniques are XY, MS, and Blumlein.

XY Cardioids and Hypercardioids:
XY uses a crossed pair of cardioids or hypercardioids. The cardioids are placed at an angle of 90-120 degrees and placed fairly close to the sound source. The angle for hypercardioids is typically more narrow to preserve a solid center image. The increased reach of the hypercardioids allows for a more distant placement.

With any stereo technique, the microphones should have as good a polar response as possible and be closely matched in all aspects.

MS Stereo:
This intensity technique used a cardioid (the M or mid-component) pointed at the source and a figure eight (the S or side-component) oriented laterally. The outputs are combined in a sum and a difference matrix which converts the signals to XY stereo [(M+S) and (M-S)]. The MS technique provides one major advantage in that the ratio of the M and S signals can be changed in the matrix to control the width of the stereo image. The only disadvantage is that the MS signal is confusing to listen to while monitoring. Many modern mixers and recorders have matrix decoders in the monitor circuits to overcome this problem.

Blumlein:
The Blumlein technique employs a crossed pair of figure eights. Blumlein has some reflection problems, and it is not very popular.

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