Quadraphonic: Wikis

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4 channels Quadraphonic label

Quadraphonic (or quadrophonic) sound – the most-widely-used early term for what is now called 4.0 stereo – uses four channels in which speakers are positioned at the four corners of the listening space, reproducing signals that are (wholly or in part) independent of one another. Quadraphonic audio was one of the earliest consumer offerings in surround sound. It was a commercial failure due to many technical problems, which were solved too late to save the technology from disaster. The format was more expensive than standard two-channel stereo. It also required extra speakers, and suffered from lack of a standard format for LP records. The rise of home theatre products in the late 1980s and early 1990s brought multi-channel recording formats back to the forefront, although in a completely different form. Quite a few quadraphonic recordings were made before its demise, and some of these recordings were later reissued on CD in Dolby Surround.

Contents

History

1970s Realistic quadraphonic receiver; speakers referred to as "Quatravox" speakers; they share a common ground

Introduced to the American market in September 1970 as the Quad-8 or Quadraphonic 8-Track, "Quad" (as it became known) did not remain restricted to the discrete channel format used in the Quad-8. It appeared in several different and largely incompatible formats on different media. Quadraphonic audio could be obtained from vinyl records, eight track tapes, and reel-to-reel tapes.

Quadraphonic audio on vinyl records was problematic because some systems were based on discrete sound channels (allowing for full separation of the four original recorded channels, albeit with restricted high-frequency response and reduced record life), while others were matrix encoded into two tracks that would also play back in standard, two-channel, stereo on normal audio equipment (so-called 'compatible' quadraphonic).

There were some experiments done with radio broadcasts (e.g. a Cliff Richard concert by the BBC), but they were short-lived. One of the longest-lived radio broadcasts was WQSR-FM "Quad 102 1/2" in Sarasota, Florida. Throughout most of the 1970s this station broadcast a signal which could be tuned as two separate stations with conventional stereo receivers. In addition, San Francisco classical music station KKHI broadcast the San Francisco Opera in 'compatible' (that is, matrix encoded) quadraphonic format during the 1970s.

Discrete formats

Discrete reproduction is the only real Quadraphonic system. As its name suggests, with discrete formats the original four audio channels are passed through a four-channel transmission medium and presented to a four-channel reproduction system and fed to four speakers. This is defined as a 4–4–4 system. To transmit 4 individual audio signals in a compatible manner, there must be four simultaneous linear equations to reproduce the original 4 audio signals at the output. The term compatible indicates that a single channel (mono) system will reproduce all four audio signals in its one speaker. A two-channel (stereo) system will reproduce the Left Front & Left Back audio signals in the Left speaker and the Right Front & Right Back audio signals will reproduce in the Right Speaker. A four-channel (Quadraphonic) system will reproduce the Left Front, Left Back, Right Front, and Right Back audio signals in each of four separate speakers.

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CD-4 (Compatible Discrete 4) / Quadradisc

Compatible Discrete 4 (CD-4) logo
Quadradisc logo
An RCA Quadradisc recording. The color fringes around the reflections are caused by the ultrasonic signal that contains the difference information used to separate the four channels.

Compatible Discrete 4 (CD-4) or Quadradisc (not to be confused with compact disc) was introduced in 1971 as a discrete quadraphonic system created by JVC. Record companies who adopted this format include Arista, Atlantic, Capricorn, Elektra, Fantasy, JVC, Nonesuch, RCA, 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 with the Left recorded on one groove wall and the Right on the other, 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 sub-carrier was recorded on each groove wall. The sub-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 sub-carrier it would be Left Front minus Left Back, and for the Right sub-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 sub-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. Also a direct by-product of CD-4 technology was the Shibata contact stylus. Invented by Dr. Norio Shibata, the Shibata stylus greatly increased the contact area on the groove walls, which in turn reduced wear over time, thus maintaining the original LP's fidelity and frequency range for much longer than normal. This, of course, prolonged the overall playable life of a record – a definite boon for any audiophile or serious record collector. But what was (arguably) even more impressive was that the Shibata stylus was also capable of recovering sizeable portions of program audio that would otherwise be compromised or lost on a damaged or scratched disc (audio that a standard, non-CD-4-compatible stylus would fail to pick up). 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.

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.

UD-4 / UMX

UD-4/UMX logo

UD-4/UMX - Developed by Nippon/Columbia (Denon). This is a hybrid discrete/matrix system. Only 35 to 40 items are encoded in this format, and it was marketed only in the UK, Europe and Japan.

A regular matrix decoder could be used to play back these recordings, but, by adding a special cartridge and a UD-4 demodulator, two supplementary channels could be extracted and used to enhance directional resolution. UD-4 systems first encoded the four original channels into four new channels. Two of these new channels contained the original four channels, matrix encoded. The other two contained only band-limited localization information, and were encoded with sub-carriers similar to the CD-4 system. UD-4 was less critical in its set-up than CD-4, because the sub-carriers did not have to carry frequencies as high as those found in the CD-4 system.[1]

Q4

Often judged by audiophiles to be the best of the old Quad formats, this system was based on a reel-to-reel-type 1/4" tape format, fully discrete and with full bandwidth (unlike the Q8 Cartridge system, which had limited dynamic range). This format was available only in the USA. Playback machines were either dedicated quad machines or 4-track open reel systems, usually running at a speed of 7.5IPS (double the speed of the 8-Track systems), providing even better sound quality.

Quad-8 / Quadraphonic 8-Track

Quadraphonic 8-Track was a discrete 4-Channel Tape Cartridge system introduced by RCA in September 1970, and usually called QUAD-8 (later shortened to just Q8). The format was almost identical in appearance to stereo 8-tracks, except for a small notch in the upper left corner of the cartridge. This signalled a quadraphonic 8-track player to combine the odd tracks as audio channels for Program 1, and the even tracks as channels for Program 2. The format was not entirely compatible with stereo or mono players – although quadraphonic players would play stereo 8-tracks, playing quadraphonic tapes on stereo players results in hearing only one-half the channels at a time. Some stereo (two-channel) 8-track players simulated quadraphonic sound through the upmixing of stereo 8-tracks, but these were not true quadraphonic 8-track players.

The last release in the quadraphonic 8-track format was in 1978.

Matrix formats

With matrix formats, the four channels are converted (encoded) down to two channels. 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. This 4:2:4 process could not be accomplished without information loss. That is to say, the four channels produced at the final stage were not identical to those with which the process had begun.

SQ / Stereo Quadraphonic

SQ logo

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.

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[2], "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.)

Basic SQ Encoding Matrix 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 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 coded and decoded normally in this proposal, but the back-center channel was coded in phase and therefore decoded in front-center.[3]

QS / Quadraphonic Stereo

QS logo

Quadraphonic Stereo (sometimes referred to as RM or "Regular Matrix") was a system that was conceptually very similar to SQ, but developed independently by engineer Isao Itoh of Sansui, adopted by ABC, Advent, Bluesway, Candide, Command, Decca, Impulse, Longines, MCA, Ovation, Pye, Turnabout and Vox record companies. It was freely licensed to record companies, but was rarely found on receivers other than those made by Sansui. The QS matrix has been found to offer the advantages of excellent diagonal separation and stereo compatibility, and although the adjacent speaker separation is only 3 dB, this symmetrical distribution produces 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 topology. Two outboard decoders, the QSD-1 and -2, as well as the QRX- series of larger receiver-amplifiers, incorporated this matrix and up-conversion.

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 mag track was assigned to the speaker behind the screen. The mag FX track was unused. This channel layout came close (5.0) to the later usage in video 5.1 surround sound.

QS Encoding Matrix Left Front Right Front Left Back Right Back
Left Total 0.92 0.38 j0.92 j0.38
Right Total 0.38 0.92 -j0.38 -j0.92

j = + 90° phase-shift

EV / Stereo-4

EV - Developed by Electro Voice, also known as Stereo-4. Despite heavy promotion by RadioShack stores in the USA, very few items were encoded in this format. Stereo-4 decoders were especially good at producing credible 4-channel effects from 2-channel stereo recordings.

DY / Dynaquad

DY - Developed by Dynaco, also known as Dynaquad. The four speakers were arranged in a diamond (centre-front, centre-left, centre-rear, centre-right). The encoding was unusual in that it did not use 90-degree phase shifters. Very few items are encoded in this format, although it did inspire the "Hafler circuit" described below.

A much simpler form of the Dynaco patent keeps the four speakers in their normal left and right plus front and rear positions. The left and right rear speakers are connected to the two-channel stereo amplifier via a passive matrix circuit, while the front ones stay directly connected to the amplifier. A lot of stereo material, recorded with a central, non-directional microphone (so-called kidney sensitivity diagram) placed in front of the orchestra, possessed suitable difference signals across the stereo signals. When taken across this passive speaker matrix for the rear channels, these produced a quasi-quadraphonic effect at low cost (the patent specifies the use of one fixed 10 ohm resistor and three variable 20 ohm resistors in a star arrangement). Especially for classical music, a fine impression of concert-hall ambience is achieved with such a system.

Dynaco sold this matrix circuit with a large and triple high-wattage potentiometer inside, for a sum equivalent to 70 euros or so in present-day currency. Electronic amateurs could build this circuit much more cheaply – e.g., with a four-position switch (four steps in level of the rear sound from min. to max. level) using fixed resistors of, for example, 20, 10, 5 and 0 (short-circuit) ohms. Because, in practice, only the highest level was of any use, a more basic set-up with only the fixed 10 ohm resistor at close-to-zero cost is possible.

Note that the system requires more or less flat impedance curves for the rear speakers to work properly, which was often the case in the tube-amplifier days. Tube amplifiers had a constant impedance over a wide range, and worked best with high-efficiency speakers. Later on, when transistor amplifiers were used, speakers tended to lose that design feature. (Lower impedance meant higher power output for these amplifiers, compensating for the lower efficiency of such designs.) The system worked best using a transistor-based stereo amplifier, low-efficiency front speakers, and high-efficiency, constant impedance rear speakers.

Matrix H

Matrix H was a system developed by BBC engineers to carry quadraphonic sound via FM radio in a way that would be compatible with existing mono and stereo receivers. Several quadraphonic programmes were made for Radios 3 and 4, including a number of plays and some Promenade Concerts, while Radio 1 carried quadraphonic session recordings by various bands.[4] The "H" has no meaning; they called the first matrix they assessed Matrix A, and then worked through the alphabet[5][6]. The BBC later cooperated with the developers of Ambisonics to produce BBC/NRDC System HJ. This was based on tolerance zones designed to include modified versions of both Matrix H and the prototype two-channel encoding of Ambisonics, known as System 45J. Subsequently, the Nippon-Columbia UMX matrix was brought into the standard, leading to the final UHJ name now associated with Ambisonics.

Hafler circuit

A passive Hafler circuit mimics the effect of "active 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 2dB.

The rear sound level in a live performance recorded in stereo is reproduced about 7dB 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, while 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. (See External links for a circuit diagram.)

See also

References

  1. ^ Cooper, Duane H.; Shiga, Takeo (June 1972). "Discrete-Matrix Multichannel Stereo". Journal of the Audio Engineering Society 20 (5): 346–360.  
  2. ^ Willcocks, Martin E. G. (March 1974), "Directional Enhancement System for Quadraphonic Decoders". GB patent 1514162.
  3. ^ Bauer, Benjamin B.; e.a. (January/February 1973). "Recording techniques for SQ matrix Quadraphonic discs". Journal of the Audio Engineering Society 21 (1): 19–26.  
  4. ^ Ratcliff, P.A.; Meares, D.J. (May 1977). "BBC Matrix H: Compatible system for broadcasting". Wireless World: 41–45.  
  5. ^ Crompton, T.W.J. (November 1974). "The subjective performance of various quadraphonic matrix systems" (PDF). BBC Research Department Report 1974/29. http://downloads.bbc.co.uk/rd/pubs/reports/1974-29.pdf.  
  6. ^ Gaskell, P.S.; Ratliff, P.A. (February 1977). "Quadraphony: Developments in Matrix H decoding" (PDF). BBC Research Department Report 1977/2. http://downloads.bbc.co.uk/rd/pubs/reports/1977-02.pdf.  

External links


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