Quadraphonic sound

4 channels quadraphonic label

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.

History

Quadraphonic sound was one of the earliest consumer offerings in multichannel audio, 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.

Formats

Discrete:

File:Quadradisk logo.png CD-4 / Compatible Discrete 4 / Quadradisc

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, Grunt, JVC, Nonsuch, RCA, Reprise, Warner Brothers. 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 15KHz 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 cartride used had a shibata type stylus and an extended frequency response. Later, linear contact stylii 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.

Channel	Left Front	Right Front	Left Back	Right Back
Normal Frequency Left	1	0	1	0
Normal Frequency Right	0	1	0	1
High-Frequency Left	1	0	-1	0
High-Frequency Right	0	1	0	-1

File:UD4 logo.pngUD-4 / UMX

UD-4 ou UMX - Developed by Nippon/Columbia. Very few items are encoded in this format and it was marketed only in the U.K., Europe and Japan. A regular matrix decoder could be used to playback this recordings, but by adding a special cartridge and a UD-4 demodulator separation and performance of the system increased. UD-4 systems split the audio spectrum into two bands. The lower frequencies were encoded with subcarriers similar to the CD-4 system. The upper band was matrix encoded. the subcarriers for the lower band were placed above the upper band frequencies and were supersonic. UD-4 was not as critical in its setup than CD-4, because the subcarriers did not have to carry the higher frequencies as is true with CD-4. mmm

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 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.

Quad-8 / Quadraphonic 8-Track

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.

Matrix:

File:Sq logo.pngSQ / Surround Quadraphonic / Stereo Quadraphonic

Surround 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, Suprophon, Vangaurd. 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. (Some sources state that "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 early SQ decoders couldn't produce more than 3 db of separation from front to back. By the time "Logic" circuits had been introduced to enhance separation, quad had already been considered a failure. The pinnacle of SQ decoder development was the Tate Directional Enhancement System, which was implemenmted 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. Some of the SQ mixes are still present on CDs, especially on early, non remastered editions.

SQ Encoding Matrix	Left Front	Right Front	Left Back	Right Back
Left Total	1.0	0.0	j0.7	k0.7
Right Total	0.0	1.0	k0.7	j0.7

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

File:Qs logo.png QS / Quadraphonic Stereo

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.

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	k0.38	k0.92

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

EV / Stereo-4

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 / Dynaquad

DY - Developed by Dynaco, also known as Dynaquad. Very few items are encoded in this format.

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. It's reported to be based on the QS Matrix and Ambisonic. Several quadraphonic programmes were made for Radios 3 and 4, while Radio 1 carried quadraphonic session recordings by various bands.

In Ambisonics B-format, known as first-order Ambisonics, sound information is encoded into 4 channels: W (pressure signal), X (front-back information), Y (left-right information) and Z (up-down). Ambisonics UHJ coding can be used to produce stereo compatible Ambisonic records, tapes and broadcasts.

The X,Y and W signals are matrixed into two channels using the following transform:

Ambisonics UHJ coding	X (front-back information)	W (pressure signal)	Y (left-right information)
Left Total	0.093 + j0.255	0.470 + k0.171	+0.328
Right Total	0.093 + k0.255	0.470 + j0.171	-0.328

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

Passive Pseudo Quad

"Passive Pseudo Quad" can be much more realistic than would appear from the name. It has been observed that ambient sounds in a concert, such as applause or even coughs from the audience, are generally received in "opposite phase" by the stereophonic microphones, while sound from the musicians is generally 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 (typically 8+8=16 ohms) between the live feeds to the front speakers. This arrangement was colloquially known as the "Hafler hookup," after audio engineer David Hafler, an early proponent of the idea. The "crosstalk" or loss of stereo separation in the front speakers is less than 2dB while the rear sound level in a typical stereo-recorded live performance is about 7dB below the front, but clearly audible. This "passive" method is arguably as good as any of the expensive "active matrix" electronic decoders which attempt to reconstruct ambient sound from a stereo recording.

Pseudo-surround sound

• Media devices that use more than 2 speakers but only 2 channels. They may use subwoofers, woofers and tweeters.

See also

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

External links

• Quadraphonic discography
• List of quadraphonic 8-tracks
• Dolby surround
• Allpass network shifts 90 degrees over wide frequency range
• Learning about quadraphonics
• QuadraphonicQuad