Stereo Quadraphonic

SQ Quadraphonic logo

SQ Quadraphonic ("Stereo Quadraphonic") was a matrix 4-channel quadraphonic sound system for vinyl. It was introduced by CBS Records (in America 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.[1][2] 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.

Usage

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.[3]

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,[4] "quad" was already considered a failure.

Harman Kardon had an SQ decoder that could change the separation so it was as low between the front channels as the separation between front and rear channel.

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.)

Basic SQ Encoding Matrix[5] Left Front Right Front Left Back Right Back
Left Total 1.0 0.0 -j0.7 0.7
Right Total 0.0 1.0 -0.7 j0.7

j = + 90° phase-shift

To provide mono-compatibility a variation on this matrix was proposed:

Modified SQ Encoding Matrix[5] Left Front Right Front Left Back Right Back
Left Total 1.0 0.0 0.7 -j0.7
Right Total 0.0 1.0 -j0.7 0.7

j = + 90° phase-shift

The four channels were encoded and decoded normally in this proposal, but the back-center channel was coded in phase and therefore decoded in front-center.[6]

This system made good sense, as, in the absence of a quad decoder, SQ-encoded records would play almost as normal stereo records, and CBS stated their desire to maintain excellent compatibility between their SQ-encoded records and standard stereo systems. In practice, there were compatibility problems. When played on a two channel stereo equipment, the front channels sounds like ordinary two channel stereo channels. The rear channels are narrower than the front channels. The problem occurs with the sounds in the center. The point right behind the listener is out of phase in two channel stereo and extinct in one channel mono listening. The left rear and right rear points are 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 is broader than a conventional stereo track, so the maximum playing time is lower than a conventional stereo record.

Records

Columbia/CBS Records had a catalog 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 with 8-track tape (mainly aimed for car stereo equipment). But these quadraphonic tapes were made in discrete four channel stereo. That gave the whole CBS Quadraphonic project a higher status.

Variations

Universal SQ

In 1976, Ben Bauer integrated matrix and discrete systems into USQ, or Universal SQ (others had done this with their quad systems too).

It was a hierarchical 4-4-4 discrete matrix that used the SQ matrix as the baseband for discrete quadraphonic FM broadcasts using additional difference signals called "T" and "Q". For a USQ FM broadcast, the additional "T" modulation was placed at 38 kHz in quadrature to the standard stereo difference signal and the "Q" modulation was placed on a carrier at 76 kHz. For standard 2-channel SQ Matrix broadcasts, CBS recommended that an optional pilot-tone be placed at 19 kHz in quadrature to the regular pilot-tone to indicate SQ encoded signals and activate the listeners Logic decoder. CBS argued that the SQ system should be selected as the standard for quadraphonic FM because, in FCC listening tests of the various four channel broadcast proposals, the 4:2:4 SQ system, decoded with a CBS Paramatrix decoder, outperformed 4:3:4 (without logic) as well as all other 4:2:4 (with logic) systems tested, approaching the performance of a discrete master tape within a very slight margin. At the same time, the SQ "fold" to stereo and mono was preferred to the stereo and mono "fold" of 4:4:4, 4:3:4 and all other 4:2:4 encoding systems.

Tate DES (Directional Enhancement System)

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 or kernel system) and enhanced the directionality of the basic SQ matrix.

It first matrixed the four outputs of the SQ decoder to derive additional signals, then compared their envelopes to detect the predominant direction and degree of dominance. A processor section, implemented outside of the Tate IC chips, applied variable attack/decay timing to the control signals and determined the coefficients of the "B" (Blend) matrices needed to enhance the directionality. These were acted upon by true analog multipliers in the Matrix Multiplier IC's, to multiply the incoming matrix by the "B" matrices and produce outputs in which the directionality of all predominant sounds were enhanced. Since the DES could recognize all three directions of the Energy Sphere simultaneously, and enhance the separation, it had a very open and 'discrete' sounding soundfield.

In addition, the enhancement was done with sufficient additional complexity that all non-dominant sounds were kept at their proper levels.

Dolby used the Tate DES IC's in their theater processors until around 1986, when they developed the Pro Logic system. Unfortunately, delays and problems kept the Tate DES IC's from the market until the late 1970s and only two consumer decoders were ever made that employed them; the Audionics Space & Image Composer and the Fosgate Tate II 101A.

The Fosgate used a faster, updated version of the IC, called the Tate II, and additional circuitry that provided for separation enhancement around the full 360° soundfield, using the Haas effect. In order to maintain the highest quality levels, Fosgate used hand-sorted ICs and 1% -tolerance components, and each decoder was hand-optimized. Unlike the earlier Full Wave-matching Logic decoders for SQ, that varied the output levels to enhance directionality, the Tate DES cancelled SQ signal crosstalk as a function of the predominant directionality, keeping non-dominant sounds and reverberation in its proper spatial locations at their correct level. The 101A was later replaced with the 3601. The Fosgates were audiophile units of rather high expense and limited availability.

SQ records could give some quadraphonic effect when played on a QS/RM decoder, although only accurate decoded playback is via a correctly designed SQ decoder. The word Quadraphonic was often - but not always - used as a synonym to the SQ system. Sometimes the SQ system (and other similar matrix systems) were called Phase Matrix in opposite to QS and similar systems, who were called Regular Matrix.

Hardware

Some SQ decoders include:[7]

References

  1. ↑ Scheiber, Peter (December 1969). "Toward a More Accurate Spatial Environment". Journal of the Audio Engineering Society (AES) 17 (6): 690, 691.
  2. ↑ Scheiber, Peter (November 1971). "Analyzing Phase-Amplitude Matrices". Journal of the Audio Engineering Society (AES) 19 (10): 835–839.
  3. ↑ http://www.ka-electronics.com/images/pdf/CBS_Motorola_SQ_Decoders.pdf
  4. ↑ Willcocks, Martin E. G. (March 1974), "Directional Enhancement System for Quadraphonic Decoders". GB 1514162 or US 3944735.
  5. 1 2 http://www.4channelsound.com/encode.htm
  6. ↑ Bauer, Benjamin B.; Budelman, Gerald A.; Gravereaux, Daniel W. (February 1973). "Recording techniques for SQ matrix Quadraphonic discs". Journal of the Audio Engineering Society (AES) 21 (1): 19–26.
  7. ↑ http://www.4channelsound.com/logic.htm
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