Four channel matrix formats
In the case of matrix formats, the four source channels are converted (encoded) down to two channels, before being decoded back to four channels for replay. This process could not be accomplished without information loss. The four channels produced at the end were not identical to those at the start of the process! In this 4-2-4 matrix four channel arrangement, the rear speakers ought to be of the same or almost the same size as the front speakers.
SQ Quadraphonic or SQ was a matrix 4-channel system for vinyl.. It was introduced by CBS Records in 1971 and record companies who adopted this format included Angel, Columbia (in Europe called CBS Records), EMI, Epic, Eurodisc, Harvest, HMV, Seraphim, Supraphon and Vanguard.
Early SQ decoders could not produce more than 3dB of separation from front to back. The early "Logic" circuits which were introduced to enhance separation to 20dB provided poor performance with very noticeable gain-pumping and an unstable 'swaying' sound stage. The SQ system also faced resistance from broadcasters since, whilst essentially a two-channel system and stereo-compatible, it could have significant mono compatibility issues which could pose serious problems with the televisions and monophonic radios of the era.
CBS stated a desire to maintain excellent compatibility between their SQ-encoded records and standard stereo systems. In practice, there were stereo compatibility problems as a result of the 4-channel encoding/decoding process. When played on two channel stereo equipment, the front 2 channels were normal stereo, but there were some aberrations. The rear channel information narrowed in stereo compared to the front channels, but the real problems occurred with the sounds in the centre. The mid centre point, originally recorded in quadraphonic mode, now only appeared in the left speaker. The point right behind the listener was out of phase in two channel stereo, and missing altogether in one channel mono listening. The left rear and right rear points were 3 dB lower in two channel stereo listening and 6 dB lower in mono listening.
(Source: Kjell Stensson: 4-kanalstekniken ännu i startgroparna, in Stereo Hi Fi Handboken 74, Svenska HiFi Institutet 1973, page 31).
The SQ record track was also broader than a conventional stereo track, so the maximum playing time was lower than a conventional stereo record.
Columbia/CBS Records had a catalogue of four channel records called “Quadraphonic” with a golden frame on the album covers. These records were encoded with the SQ system, but there was also a parallel Quadraphonic catalogue for 8-track tapes, mainly aimed at the car stereo market. These quadraphonic tapes were made in discrete four channel stereo, which gave the whole CBS Quadraphonic project a higher status, bearing in mind the popularity of 8-track at the time.
The Directional Enhancement System, also known as the Tate DES, was an advanced decoder for SQ (although it could be made to work with any matrix system), and this enhanced the directionality of the basic SQ matrix. Unfortunately, it's relatively late arrival didn't help the survival of SQ. It was also rather expensive.
Sometimes the SQ system (and other similar matrix systems) were called “Phase Matrix”, as opposed to QS and similar systems which were called “Regular Matrix”.
QS (Quadraphonic Sound)/ RM (Regular Matrix)
Quadraphonic Sound” (originally called “Quadphonic Synthesizer”, and later referred to as RM (Regular Matrix) was a system based on the same principals as SQ, but developed independently by engineer Isao Itoh of Sansui. It was freely licensed to record companies and adopted by labels such as ABC, Advent, Bluesway, Candide, Command, Decca, Impulse, Longines, MCA, Pye, Turnabout and Vox record companies.
The QS matrix offered the advantages of excellent diagonal separation and stereo compatibility, and although the adjacent speaker separation was only 3 dB, the symmetrical distribution produced more stable quadraphonic images. Sansui's decoders also had good stereo-to-quad capabilities, wrapping the L-R panorama to LB-LF-RF-RB in a horseshoe presentation. Two outboard decoders, the QSD-1 and QSD-2, as well as the QRX series of larger receivers, incorporated this matrix and up-conversion.
In 1973 Sansui introduced the QS Vario Matrix decoder with 20 dB separation in all directions. The Vario Matrix decoder could also play SQ records on Phase Matrix mode with 6 dB separation. Interestingly, Sansui later used front-rear logic in SQ mode.
The QS system could also synthesize four channel sounds from two channel sources. The Vario Matrix decoder could provide four channel sound with a high separation of at least 12 dB. The system was then given the Regular Matrix name when utilized on amplifiers or receivers by manufacturers other than Sansui. Many Japanese trademarks like Pioneer or Kenwood had matrix decoders with two modes - SQ and RM. JVC had two modes on their matrix decoder called Matrix 1 and Matrix 2. This decoder could play both SQ and QS records, but it was a simplified decoder. QS records could also be played by the Marantz Vari-Matrix 4-channel system. European trademarks like Philips or Bang & Olufsen only had decoders for SQ or both SQ and CD-4, but not QS. QS records could give some quadraphonic effect when played on an SQ decoder and were also compatible with yet another 4-channel system, EV or Electro-Voice.
When played on conventional two channel stereo equipment, the front channels are narrower than ordinary stereo channels because of the low separation. But the rear channels are heard from points outside the ordinary stereo spectrum. So the QS record gives a broader stereo picture than conventional two channel stereo. The point behind the listener is out of phase when played in two channel stereo and missing altogether in mono listening. The QS record track was as small as a conventional stereo track, so the maximum playing time was the same as conventional stereo records.
The QS matrix system was employed to create the five-channel Quintaphonic Sound system used for première engagements of the 1975 film, Tommy. The left and right 35mm magnetic soundtracks were QS encoded to create four channels around the cinema audience, while the centre magnetic track was assigned to the speaker behind the screen. The magnetic FX track was unused. This channel layout came close (5.0) to the later 5.1 surround sound layout.
Because SQ means Stereo Quadraphonic, it's easy to incorrectly think that QS means Quadraphonic Stereo. QS is an older system than SQ, and Sansui never called it Quadraphonic Stereo.
The Hafler circuit
A passive “Hafler” circuit mimics the effect of matrix decoding but without using costly electronics. It does this by recovering the ambient sound from a stereo recording. Named after its early proponent audio engineer David Hafler, the circuit exploits the high amount of stereo separation in the front speakers. Using the circuit typically reduces this stereo separation by only about 2 dB.
The rear sound level in a live performance recorded in stereo is reproduced about 7 dB below the front level, but clearly audible. The rear ambient sounds, applause, and coughs from the audience, are sometimes received out of phase by the stereo microphones, whilst sounds from the musicians mostly are in "synchronous phase". Thus, if rear speakers are fed with the difference between the stereo channels, audience noises and reverberation from the auditorium may be heard from behind the listener. This can be most easily achieved by wiring two similar additional rear speakers in series between the live feeds (positive terminals) from the stereo amplifier. Alternatively, one rear speaker can be used on its own.
As an example, in the early to mid 70-s, Ferguson made two channel receivers with a built-in Hafler circuit. Philips had a similar circuit in their two channel receivers. Many receivers from mid price trademarks had such circuits, but often without a volume control for the rear channels. More expensive manufacturers seldom had Hafler circuits or anything similar because it was thought they increased the distortion of the sound. Most of Marantz four channel receivers had a variable matrix called Vari-Matrix (not to be confused with Sansui's QS Vario Matrix), that could simulate four channel stereo from two channel sources in different ways, and the listener could control the effect with a control. The Vari-Matrix could also play all matrix records with good results.
In the early 1970s, the words ambiophony and ambiophonic were synonymous with the words quadraphonic and four channel stereo. But around 1973, the words ambiophony and ambiophonic were used to describe simulated four channel stereo of the Hafler type. In the 2-2-4 simulated four channel stereo, the rear speakers could be smaller, cheaper and have a smaller frequency range than the front speakers