Circuits for three-channel stereophonic playback derived from two sound tracks

Derivation of a third playback channel from two stereo sound tracks may be accomplished by several means. The center channel may be derived by recombination prior to power amplification; acoustically, after amplification, by using two center speakers; and in a variety of phase relationships, including the limiting case of equal signals (monophonic) in which either sum or difference combination may be chosen by polarity selection.