Collecting Quadraphonic Records

Stereo Quadraphonic was a matrix quadraphonic system for vinyl. It was introduced by CBS in 1972, and record companies who adopted this format include: Angel, Capitol, CBS, CTI, Columbia, EMI, Epic, Eurodisc, Harvest, HMV, Seraphim, Supraphon and Vanguard.

The system was based on the work of Peter Scheiber, who created the basic mathematical formulae used to matrix four channels into two in 1970. SQ stands for "Stereo Quadraphonic." This system made good sense, as, in the absence of a quad decoder, SQ-encoded records would play as normal stereo records, and CBS stated their desire to maintain excellent compatibility between their SQ-encoded records and standard stereo systems.- from Wikipedia

The early SQ decoders could not produce more than 3dB of separation from front to back. Early "Logic" circuits were introduced to enhance separation to 20dB, but provided poor performance, very noticeable gain-pumping and an unstable 'swaying' soundfield. The SQ system also faced resistance from broadcasters since, while essentially a two-channel system and totally stereo-compatible, it could have substantial mono compatibility problems, which posed serious problems with all televisions and monophonic radios of the era. By the time that the most advanced logic system was introduced for SQ, the Tate Directional Enhancement System "quad" was already considered a failure.

A Prologic II decoder will recover some of the surround sound information present in an SQ mix, as the matrices used are somewhat similar, but directional cues will not be properly located. Some of the SQ mixes are still present on CDs, especially on early, non-remastered editions, on which the original master is, in itself, SQ. (SQ is compatible with two-channel stereo, so there was no need to create a stereo version

Compatible Discrete 4 (CD-4) or Quadradisc (not to be confused with compact disc) was introduced in May 1972 as a discrete quadraphonic system created by JVC and RCA. Record companies who adopted this format include Arista, Atlantic, Capricorn, Elektra, Fantasy, JVC, Nonesuch, A & M, Reprise and Warner.

This was the only fully discrete quadraphonic phonograph record system to gain major industry acceptance.

In the CD-4 system, the quadraphonic audio was divided into left and right channels which were recorded orthogonally in the vertical plane of the disc groove, which is the case with normal stereo. The audio frequencies (20 Hz to 15 kHz), often referred to as the sum channel, would contain the sum of the left front plus left back signals in the left channel and the sum of the right front plus the right back signals in the right channel. In other words, if you looked at the audio frequencies only, you had an ordinary stereo recording. Along with this audio, a separate 30 kHz carrier was recorded on each groove wall. The carrier on each side carried the difference signal for that side. This was the information that enabled a combined signal to be resolved into two separate signals. For the left carrier it would be left front minus left back, and for the right carrier it would be the right front minus the right back. These audio signals were modulated onto the carriers using a special FM-PM-SSBFM (frequency modulation-phase modulation-single sideband frequency modulation) technique. This created an extended carrier frequency range from 18 kHz to 45 kHz for the left and right channels. The algebraic addition and subtraction of the sum and difference signals would then yield compatible and discrete quadraphonic playback. CD-4 was responsible for major improvements in phonograph technology including better compliance, lower distortion levels, pick-up cartridges with a significantly higher frequency range, and new record compounds such as Q-540, which were highly anti-static. A typical CD-4 system would have a turntable with a CD-4 cartridge, a CD-4 demodulator, a discrete four-channel amplifier, and (ideally) four full-range loudspeakers. Some manufactures built the CD-4 demodulator into complete four-channel receivers.

Simply put, CD-4 consists of four recorded signals (LF, LB, RB, RF) and the following coding matrix, similar to FM broadcast stereo multiplexing.

The CD-4 encoding/decoding matrix:

(LF+LB)+(LF-LB)=2LF or left front
(LF+LB)-(LF-LB)=2LB or left back
(RF+RB)+(RF-RB)=2RF or right front
(RF+RB)-(RF-RB)=2RB or right back

Although CD-4 (and quadraphonic audio in general) failed due to late FCC approval of FM quadraphonic broadcasting, the improvements CD-4 engendered spilled over into, and substantially improved, the production of conventional stereo LP records.

You can check out more at Wikipedia Quadraphonic sound