Quadraphonic sound: Difference between revisions

Quadraphonic sound uses four channels in which speakers are positioned at all four corners of the listening space, reproducing signals that are independent of each other.

Quadraphonic sound was one of the earliest consumer offerings in surround sound, introduced to the American market in the early 1970s. Quad was not one format but myriad different and largely incompatible formats on different media: quadraphonic could be obtained from vinyl records, eight tracks, and reel-to-reel. Further complicating quadraphonic was the fact that some systems were discrete, while others were matrix. There were even some experiments with radio broadcasts, for example a Cliff Richard concert by the BBC, although they were short-lived.

"Quad", in its original form, was a commercial failure, the LP formats were plagued with technical problems, most of which were solved too late to save quad. It also was more expensive, and required extra speakers, which became a decorating problem. It also suffered from lack of a standard format for LP records. However, quite a few recordings were made before its demise. It was only the rise of home theater products in the late 1980s and early 1990s that brought multi-channel recording formats back to the forefront, albeit in a completely different and perhaps unexpected form.

As its name suggests, with discrete formats the four channels are passed through a four-channel transmission medium and presented to four speakers.

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 (as a subsidiary of RCA). Record companies who adopted this format include: Arista; Atlantic; Capricorn; Elektra; Fantasy; JVC; Nonesuch; RCA; Reprise and Warner. This format was less popular than others because of incompatibility, poor longevity, and strict setup requirements. The quadraphonic music was encoded with sum and difference signals (encoded in the 18 to 30 kHz range) on the standard stereo grooves of vinyl which also had the undesirable side-effect of limiting the top end response to 15 kHz at the most. To play back the record, a special high-frequency cartridge and stylus was required, in addition to a CD-4 demodulator and the usual quadraphonic receiver or amplifier. This system produced additional wear and tear, so JVC introduced "super vinyl", a very durable type of record. The cartridge used had a shibata type stylus and an extended frequency response. Later, linear contact styli were developed that improved the performance of CD-4 systems. However, this development came too late to save CD-4 from extinction. CD-4 records could be played as stereo records if care was taken to use a shibata (or linear contact) stylus to protect the subcarrier modulations.

Although the format itself failed, the improvements it engendered in "super vinyl" and mastering techniques spilled over into, and substantially improved, the production of conventional stereo LP records.

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 playback these recordings but, by adding a special cartridge and a UD-4 demodulator, an additional two supplementary channels could be extracted and used to enhance directional resolution. UD-4 systems first encoded the four channels into four new channels. Two of these contained the the original four channels, matrix encoded. The other two contained only band-limited localisation information, and were encoded with subcarriers similar to the CD-4 system. UD-4 was less critical in its setup than CD-4 because the subcarriers did not have to carry as higher frequencies as those in the CD-4 system.^[1]

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 only available 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), giving even better sound quality.

Quadraphonic 8-Track was a discrete system introduced by RCA in late 1970. 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 signaled 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 8-track players touted simulated quadraphonic sound (through upmixing stereo 8-tracks) but were not quadraphonic 8-track players. The last release in the quadraphonic 8-track format was in 1978.

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 at the end were not identical to the four you started out with.

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, Vanguard. The system is based on the work of Peter Scheiber, who created the basic mathematical formulas used to matrix four channels into two in 1970. "SQ" is an acronym for "Stereo Quadraphonic." This makes sense since without a quad decoder SQ encoded records play as a normal stereo record and CBS stated their desire to maintain excellent compatibility of their SQ encoded records with standard stereo systems. Additionally - and perhaps most importantly - these type of records along with the QS format, allowed the full bandwidth from 20 Hz to 20 kHz to be used, giving a much more "open" & detailed top end.

The early days of SQ were marred by the fact that the first basic SQ decoders couldn't 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. By the time that the most advanced Logic System was introduced for SQ, the Tate Directional Enhancement System^[2] invented by Martin Willcocks, "quad" was already considered a failure. Initially the Tate DES chips were in short supply as the original run were committed for use in movie theaters in the first generation of Dolby matrix surround sound system. The Tate DES is what created the surround sound in theaters for the initial first run release of Star Wars. The pinnacle of SQ decoder development was the Tate Directional Enhancement System, which was implemented in decoders produced by Audionics of Oregon and Fosgate. These units are sought by SQ collectors for their superior performance.

A Prologic II decoder will recover some of the surround information present on 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, such as Pink Floyd's The Dark Side Of The Moon.

j = + 90º phase-shift , k = - 90º phase-shift

Quadraphonic Stereo 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 Sansui. The QS matrix is found to offer the advantage of excellent diagonal separation and, though the adjacent speaker separation is only 3dB, this symmetrical distribution produces more stable quadraphonic images.

The QS matrix system was employed to create the five-channel Quintaphonic Sound system used for premiere engagements of the 1975 movie Tommy. The left and right 35mm magnetic soundtracks were QS-encoded to create four channels around the theater; the center mag track was assigned to the speaker behind the screen. The mag FX track was unused.

j = + 90º phase-shift , k = - 90º phase-shift

EV - Developed by Electro Voice, also known as Stereo-4. Despite heavy promotion by Radio Shack 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 - 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.

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.^[3] The "H" has no meaning; they called the first matrix they assessed Matrix A, and then worked up 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 UD-4 matrix was brought in to the standard, leading to the final UHJ name now associated with Ambisonics.

A passive Hafler circuit mimics the effect of "active matrix" decoding but without using costly electronics by recovering the ambient sound from a stereo recording. Named after its early proponent audio engineer David Hafler, the Hafler circuit approach exploits the high amount of stereo separation in the front speakers (also called crosstalk), and 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 about 7 dB below the front, but clearly audible.

The rear ambient sounds such as applause, and even coughs from the audience, are generally received in "opposite phase" by the stereo microphones, while sound from the musicians is in "synchronous phase". Thus, if rear speakers are fed with the difference between the stereo channels, audience noises and echoes from the auditorium can 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 from the stereo amplifier. Alternatively, one speaker can be used on its own. (See External links for a circuit diagram.)

• Ambisonics
• Azimuth co-ordinator
• Four Channel Compact Disc Digital Audio
• Multitrack recording
• Surround sound

• Quadraphonic discography
• List of quadraphonic 8-tracks
• Qdraphonic - website honoring (and exploring) quadraphonic sound
• Learning about quadraphonics
• QuadraphonicQuad
• Collectible Quadraphonic Tapes
• Quadraphonic website by Wendy Carlos (formerly Walter Carlos)
• Hafler circuit
• Dolby surround