Three-channel stereo playback of two tracks derived from three microphones

Playback of two-track stereo source material with a derived center channel offers accurate reproduction of the original stereo geometry, and requires very simple implementation. Essentially this two-track three-channel stereo depends on the principle that if two microphones are properly placed relative to each other and to the plane of the sound source, their combined output is that of a single microphone in the center, which output may be recovered by recombination of the two tracks. When a physical third microphone in the center is employed to feed the two tracks, its recovery from two tracks depends on relative polarity and amplitudes and in one recombination method the center microphone could be cancelled instead of being reproduced. An all-pass network may be used to shift the phase of one track so that on recombination the physical third microphone is always recovered, regardless of the manner in which it was mixed into the two tracks. The all-pass network produces 90° phase shift at only one frequency, but by choice of this frequency and the expectancy of additive polarity of original mixing, the center channel is recovered with excellent acuity, based on tests similar to those of Steinberg and Snow. Experimental recovery of a center output channel from a single center microphone feeding the two tracks, with flanking input signals zero, resulted in a center track output which was substantially indistinguishable from a normal monophonic reproduction.