Quadraphonic
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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.
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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, considered in the audiophile media of the 1980s to be an abject lesson in what happens when content providers wait on hardware manufacturers to give them a reason to produce content, who in turn wait on content providers to give them a reason to produce hardware. 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 format.
Formats
CD-4
Compatible Discrete 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). 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. This system produced additional wear and tear, so JVC introduced "super vinyl", a very durable type of record.
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 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.
SQ
Surround Quadraphonic was a matrix quadraphonic system for vinyl. It was introduced by CBS in 1972. 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.
QS
Quadraphonic Stereo was a system that was conceptually very similar to SQ, but developed independently by engineer Isao Itoh of Sansui. It was freely licensed to record companies but was rarely found on receivers other than Sansui.
Matrix H
Matrix H was a system developed by BBC engineers to carry quadrophonic sound via FM radio in a way that would be compatible with existing mono and stereo receivers. Several quadrophonic programmes were made for Radios 3 and 4, while Radio 1 carried quadrophonic session recordings by various bands.
Pseudo-surround sound
- Media devices that use more than 2 speakers but only 2 channels. They may use subwoofers, woofers and tweeters.
"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 "syncronous 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.