Quadraphonic (or Quadrophonic & sometimes Quadrasonic) sound – the most widely used early term for what is now called 4.0 surround sound – 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 the earliest consumer offering in surround sound. It was a commercial failure due to many technical problems and format incompatibilities. Quadraphonic audio formats were more expensive to produce than standard two-channel stereo. Playback required additional speakers and specially designed decoders and amplifiers.
The rise of home theatre products in the late 1980s and early 1990s brought multi-channel audio recording back into popularity, although in new digitally based formats. Thousands of quadraphonic recordings were made during the 1970s, and some of these recordings have been reissued in modern surround sound formats such as DTS, Dolby Digital, DVD-Audio and Super Audio CD.
In 1967 the rock group Pink Floyd performed the first-ever surround-sound rock concert at “Games for May”, a lavish affair at London’s Queen Elizabeth Hall, where the band debuted its custom-made quadraphonic speaker system. The control device they had made, the Azimuth Co-ordinator, is now displayed at London's Victoria and Albert Museum, as part of their Theatre Collections gallery.
The first medium for 4-channel sound was the quadraphonic reel-to-reel tape, standard in European electronic-music studios by 1953 and introduced to the American market by the Vanguard Recording Society in June 1969. RCA Records followed, in April 1970, with its announcement of Quad-8 or Quadraphonic 8-Track Tape (later shortened to just Q8). These eventually appeared in Sept. 1970.
"Quad" (as it became known) did not remain restricted to the discrete-channel format used in the quadraphonic reel-to-reel or Quad-8. Quadraphonic vinyl albums appeared, which used several different and incompatible modes. The first of these, known as QS or Command Quadraphonic, was developed by Sansui Electric. A so-called matrix format, it utilized four sound channels, which were "encoded" into two stereo album tracks. These were, then, "decoded" back into the original four sound channels. The QS system debuted 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 in Great Britain, who pressed several SQ album releases. The sound separation of the SQ system was greatly improved by the introduction of SQ Full Logic decoding, in 1975.
The third major format for 4-channel vinyl LPs, known as CD-4 or Quadradisc, was devised by the Japanese JVC Corporation along with its United States counterpart, RCA. This quadraphonic format was first marketed in the United States in May 1972. A fully discrete sound mode, it eschewed the previous matrix systems in favor of a more complex -and sonically separate- method of 4-channel reproduction. A high-frequency "carrier wave" instructed a "CD-4 demodulator" on how to separate the four sound channels within the vinyl album.
Audio on vinyl records was problematic, because one of the systems was 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 two 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 radio series, Double Exposure, was briefly syndicated throughout the United States to various FM stations; it was made up of jazz, rock and pop music that had been commercially released in one of the quadraphonic record or tape systems. One of the longest-lived radio broadcasts was WQSR-FM "Quad 102½" 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 as did Chicago station WFMT's live "Chicago Lyric Opera" broadcasts.. KRMH-FM ("Good Karma Radio")(San Marcos/Austin, Texas) broadcast in "Quad Stereo" in the early 1970s from its studios and transmitter near Buda, Texas. Sacramento station KWOD 106.5, named after the format, broadcast briefly beginning in 1977.
||This section may contain original research. (March 2011)|
Discrete reproduction is the only true 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 signals in the Right Speaker.
A full, four-channel (Quadraphonic) system will reproduce the Left Front, Left Back, Right Front, and Right Back audio signals in each of four separate speakers. In discrete four channel stereo the rear speakers should be of the same or almost same size or quality and have the same or almost same frequency range as the front speakers.
CD-4 (Compatible Discrete 4) / Quadradisc
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, when observing the audio frequencies only, the record appeared to have 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
There was also a similar FM radio system called Quadracast. But 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.
The CD-4 record track is broader than a conventional stereo track, so maximum the playing time is lower than a conventional stereo record.
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.
UD-4 / UMX / BMX
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.
The UMX standard contains two subsystems, BMX, a basic 4-2-4 encoded matrix (different from Regular Matrix), and QMX, a 4-4-4 system. UD-4 is the process of modulating QMX onto a record with a process similar to CD-4, but also matrix encoded. The system suffered from incompatibility with regular stereo playback due to phase differences between the left and right channels.
A BMX decoder could be used to play back UD-4 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 carriers similar to the CD-4 system. UD-4 was less critical in its set-up than CD-4, because the carriers did not have to carry frequencies as high as those found in the CD-4 system.
Q4 / Quadraphonic Reel to Reel
Often judged by audiophiles to be the best of the old Quad formats, this system was based on the 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 7.5 IPS (double the speed of the 8-Track systems), providing even better sound quality.
Quad-8 (Q8) / Quadraphonic 8-Track
Quadraphonic 8-Track was a discrete 4-Channel Tape Cartridge system announced by RCA Records in April 1970 and introduced in Sept. 1970, and first called Quad-8 (later shortened to just Q8). The format was almost identical in appearance to stereo 8-tracks, except for a small sensing 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 backward 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, although most had stopped appearing by the end of 1976.
The vinyl records in Columbia/CBS Records Quadraphonic catalogue were matrix encoded with the SQ system, but the 8-track tapes in the Quadraphonic catalogue was in discrete, four channel stereo.
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. The systems were based on the work of Peter Scheiber, who created the basic mathematical formulae used to matrix four channels into two.
 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.
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.
SQ / Stereo Quadraphonic
SQ Quadraphonic was a matrix 4-channel system for vinyl. It was introduced by CBS 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.
SQ stands for "Stereo Quadraphonic."
The early SQ decoders could not produce more than 3 dB of separation from front to back. Early "Logic" circuits were introduced to enhance separation to 20 dB, 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.
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.
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|
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|
j = + 90° phase-shift
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 mid center point in quadraphonic does only appear in the left speaker. The point right behind the listener is out of phase in two channel stereo and totally extincted 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.
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 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.
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.
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 at their correct level. The 101A was later replaced with the 3601. The Fosgates were audiophile units of rather high expense and limited availability.
The word Quadraphonic was often - but not always - used as a synonym to the SQ system.
SQ records could give some quadraphonic effect when played on a QS/RM decoder.
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.
QS (Quadraphonic Sound)/ RM (Regular Matrix)
Quadraphonic Sound (originally called Quadphonic Synthesizer, and later referred to as RM or Regular Matrix) was a system that was based on the same principles as SQ, but developed independently by engineer Isao Itoh of Sansui, adopted by ABC, Advent, Bluesway, Candide, Command, Decca, Impulse, Longines, MCA, Pye, Turnabout and Vox record companies. It was freely licensed to record companies.
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.
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. Later Sansui used front-rear logic on the SQ mode.) Two outboard decoders, the QSD-1 and -2, as well as the QRX- series of larger receiver-amplifiers, incorporated this matrix and up-conversion. Sansui's QS decoders also had good stereo-to-quad capabilities, wrapping the L-R panorama to LB-LF-RF-RB in a horseshoe topology. (The Vario Matrix decoder could synthesize four channel sound with high separation - at least 12 dB.)
The system was often called RM when used on amplifiers or receivers by other trademarks 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. That decoder could play both SQ and QS records, but it was a simplified decoder. QS records could also be played on Marantz Vari-Matrix system. (European trademarks like Philips or Bang & Olofsen had only 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. QS was also compatible with the original EV system.
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 35 mm 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 now common 5.1 surround sound layout.
|QS Encoding Matrix||Left Front||Right Front||Left Back||Right Back|
j = + 90° phase-shift
When played on a conventional two channel stereo equipment, the front channels are narrower than ordinary two 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 totally broader stereo picture than conventional two channel stereo. The point behind the listener is out of phase when played in two channel stereo and totally extincted in one channel mono listening. In mono the volumes of left and right points behind the listener is reduced with only 3 dB (in SQ 6 dB). (Source: Kjell Stensson: 4-kanalstekniken ännu i startgroparna, in Stereo Hi Fi Handboken 74, Svenska HiFi Institutet 1973, page 30).
The QS record track is as small as a conventional stereo track, so the maximum playing time is the same as conventional stereo records.
As early as 1969 engineer Peter Scheiber developed a matrix system very similar to QS. . He also was instrumental in many improvements to SQ (see above) quality, in collaboration with Martin Willcocks and Jim Fosgate.
Because SQ means Stereo Quadraphonic many people believe that QS means Quadraphonic Stereo. But that is wrong. QS is an older system than SQ and Sansui never called it Quadraphonic Stereo. The QSD-1 Quadphonic Synthesizer (a rack-mount module) was introduced in 1971, soon followed by the smaller, boxy QSD-2.
The concept of Regular Matrix was often used a synonym to QS. But originally the term was used for both QS, EV and other similar matrix system in competition to SQ, which was called Phase Matrix, even though both matrices were phase matrices, just using different parametric values, as is shown in the tables here.
EV / Stereo-4
EV - Developed by Electro Voice, also known as Stereo-4, was developed by Leonard Feldman and Jon Fixler. Despite heavy promotion by RadioShack stores in the USA, very few items were encoded in this format. This was the first commercial matrix system in the market In America it was the first commercial quadraphonic system (for example on Ovation label. Example: Hollins and Starr: Sidewalks Talking 1970).
EV was the first system to use different sets of coefficients for encoding and decoding. Most other systems have decode coefficients that mirror the encode coefficients. Therefore the EV Stereo-4 was something in between a 2-2-4 simulated system and a 4-2-4 matrix system. The original EV system was compatible with the QS system.
Stereo-4 decoders were sometimes used to produce pseudo 4-channel effects from 2-channel stereo recordings. Some records were made in EV system in the beginning of the 1970s. But the system more or less disappeared since the SQ system was introduced by Columbia/CBS Records in America.
In 1973 Electro Voice signed an agreement with Columbia/CBS Records and build a new universal decoder that could decode both SQ and EV records with good results. It could even decode QS records - but not as well. EV later suggested the same coefficients for an encoder, but no records were ever produced with the so-called EV Version 2 System.
The record label Ovation, who initially used the EV matrix, later changed to the Sansui QS matrix.
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° 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 (about 100 US dollars) 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 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. The "H" has no meaning; they called the first matrix they assessed Matrix A, and then worked through the alphabet.
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.
The Matrix H system was the only matrix system with full mono compatibility.
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, 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. This is the type of quad setup used by Seeburg jukeboxes that had quadraphonic sound. (See External links for a circuit diagram.)
In the early and mid-1970s for example Ferguson made two channel receivers with a built-in Hafler circuit. Philips had a similar circuit in their two channel receivers. Many receivers from middle price trademarks had such circuits - but often without volume control for the rear channels. More expensive trademarks seldom had Hafler or similar circuits, because they thought such circuits 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 adapt the sound with a control. The Vari-Matrix could also with good result play all matrix records. Technics by National had a similar matrix decoder with two controls
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. Ambiophonic could also mean the so-called concert hall sound in opposite to a surround sound with instruments all around the listener. The concert hall sound means the listener hears all the instruments from the front, whereas the rear channels are mainly used to give the listener the acoustic effect of sitting in a concert hall. (Ambiophony or ambiophonic sound should not be confused with Ambiophonics or Ambisonics.) (Sources: 1) Kjell Stensson: Efter Stereo HiFi: 'Ambio HiFi' , in Stereo HiFi Handboken 73, Svenska HiFi Institutet 1972, pages 6–16 2) Kjell Stensson: Ambiofoni på enkelt sätt, in Stereo HiFi Handboken 73, Svenska HiFi Institutet 1972, pages 14–15 3) Kjell Stensson: 4-kanalstekniken ännu i startgroparna, In Stereo HiFi Handboken 74, Svenska HiFi Institutet 1973, pages 29–30).
In 2-2-4 simulated four channel stereo the rear speakers could be smaller, cheaper and have a smaller frequency range than the front speakers.
- Azimuth co-ordinator
- Four-channel compact disc digital audio
- Matrix decoder
- Multitrack recording
- Octophonic sound
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