4-channel reproduction from vinyl in the early 70's.
Early forays into consumer 4 channel reproduction began in earnest in the early 70's. In true arch rival, shambolic and ultimately self-defeating fashion, several incompatible systems became available, each with allegiances scattered amongst the different record companies Throw in the so-called industry “standardization” authorities, and there we have it! A perfect storm for eventual Quad oblivion.
The early approach involved recording on 4 channel tape decks, all the channels/tracks entirely separate or “discrete” as they were known, with no interference from one channel to any other. Then came the turn of 4-channel for vinyl with the many different systems evolving. One of these was based on discrete sound channels (allowing for full separation of the four original recorded channels, albeit with restricted high-frequency response and possibly reduced record life), while the main two other ones were matrix encoded into two tracks, that would also play back on standard, two-channel stereo or mono audio equipment (so-called 'compatible' quadraphonic).
The first of these systems to appear was known as QS or Command Quadraphonic, developed by Sansui Electric. A so-called matrix format, it utilized four sound channels which were "encoded" into two stereo album tracks. On replay, these were "decoded" back into the original four sound channels. The QS system first appeared in the United States in March 1971.
The second Quad vinyl record format, known as SQ, also utilized a matrix-type system of encoding and decoding. It was developed and marketed by Columbia Records and Sony, and entered the US market in April 1971. The SQ format was also used by companies such as EMI who released quite a number of SQ albums. The original system had very limited separation between channels, but this was later greatly improved by the introduction of SQ Full Logic decoding, in 1975.
The third major format for 4-channel vinyl LP's, known as CD-4 or Quadradisc, was devised by the Japanese Victor Corporation (JVC) along with its United States counterpart, RCA. This quadraphonic format was first marketed in the United States in May 1972. A fully discrete system, the previous matrix systems were rejected in favour of a more complex method of 4-channel reproduction, with much better channel separation. A high-frequency "carrier wave"in the record groove contained the necessary information for a "CD-4 demodulator" to retrieve the four sound channels. Record companies who adopted this format included Arista, Atlantic, Capricorn, Elektra, Fantasy, Nonesuch, A&M, Reprise and Warner. This was the only fully discrete quadraphonic LP record system to gain major industry acceptance.
Within the CD-4 system, the quadraphonic audio was divided into left and right channels which were recorded in the vertical plane of the disc groove, as is the case with a normal stereo pressing. 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.
As a by-product, CD-4 was responsible for major improvements in phonograph technology including better cantilever 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 capable of tracking high frequencies, a CD-4 demodulator, a discrete four-channel amplifier, and four full-range loudspeakers. Some manufactures built the CD-4 demodulator into complete four-channel receivers or even into the turntable unit, like B&O.
Quadradiscs/CD-4 records were seldom called quadraphonic. When a record was called quadraphonic, it was almost always an SQ-encoded or an other matrixed record. But when a record was called quadradisc it was always a CD-4 record. The CD-4 record track is broader than a conventional stereo track, so maximum the playing time is lower than a conventional stereo record.