Four-channel sound fulfills the promise of providing a new creative medium to the artist and augmented space perspective to the listener. Investigation of methods of recording four channels on a two-channel disc record has led to the development of a stereophonic-quadraphonic (SQ) matrix system which, through a suitable encoding matrix, can be used to produce a stereophonic record that can be played on a conventional stereophonic phonograph retaining spatial identification with the original four-channel sound; and, further, offering a capability for decoding back to four channels by means of an inverse matrix. Three newly enunciated psychoacoustic principles have played a role in the development of the SQ matrix. By working along with these principles, it has been possible to design the SQ matrix so as to diminish the audibility of side-effect signals. An electronic logic which senses the existence of the side-effect signals and adjusts the gains of the appropriate channels may be used to further enhance the quadraphonic performance, resulting in virtual identity of SQ reproduction with the original four-channel master tape.
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