Stereo quadraphonic

SQ Quadraphonic ("Stereo Quadraphonic") was a matrix 4-channel quadraphonic sound system for vinyl LP records. It was introduced by CBS Records (known in the United States and Canada as Columbia Records) in 1971. Record companies who adopted this format include: Angel, CTI, Columbia (in Europe called CBS Records), EMI, Epic, Eurodisc, Harvest, HMV, Seraphim, Supraphon and Vanguard.

With Matrix formats, the four sound channels (forward left, forward right, back left, back right) are converted (encoded) down to two channels (left, right). These are then passed through a two-channel transmission medium (usually an LP record) before being decoded back to four channels and presented to four speakers.

The SQ encoding is based on the work by Peter Scheiber and further developed by Benjamin Bauer. His basic formula used 90 degree phase shift circuitry to enable enhanced 4-2-4 matrix systems to be developed. This 4:2:4 process could not be accomplished without some information loss. That is to say, the four channels produced at the final stage were not truly identical to those with which the process had begun.

In 4-2-4 matrix four channel stereo, the rear speakers should be of the same or almost same size quality and have the same or almost same frequency range as the front speakers.

The early SQ decoders could not produce more than 3 dB of separation from front to back. Early "Front-rear logic" circuits were introduced to enhance separation to 12 dB and later "Full logic" circuits 20 dB, but both provided poor performance, very noticeable gain-pumping and an unstable 'swaying' sound field.

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