quadraphonic adj : of or relating to quadraphony; "his quadraphonic sound system" [syn: quadriphonic, quadrasonic, quadrisonic]

English

Etymology

quad + phonic

Adjective

quadraphonic

1. In the context of "of a sound system": employing four independent channels or speakers

See also

• stereophonic

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 each other. Quadrophonic audio was one of the earliest consumer offerings in surround sound. It was a commercial failure due to many technical problems that were solved too late to save the technology from disaster. The format was more expensive than stereo, required extra speakers, and suffered from lack of a standard format for LP records. The rise of home theater 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 quadrophonic recordings were made before its demise, and some of these recordings were later reissued on CD in Dolby Surround.

History

Introduced to the American market in September 1970 as the Quad-8 or Quadrophonic 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. Quadrophonic audio could be obtained from vinyl records, eight track tapes, and reel-to-reel tapes.

Quadrophonic 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 play back in stereo on two-channel audio equipment (so-called 'compatible' quadrophonic).

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 broadcasters was WQSR-FM "Quad 102 1/2" in Sarasota, Florida. Through 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) quadrophonic format during the 1970s.

Formats

Discrete:

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

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 quadrophonic system created by JVC (a 50% owned subsidiary of Matsushita Electric Corp. Japan). Record companies who adopted this format include: Arista; Atlantic; Capricorn; Elektra; Fantasy; JVC; Nonesuch; RCA; Reprise and Warner.

This was the only fully discrete Quadrophonic Phonograph record system to gain major industry acceptance.

In the CD-4 system, the Quadrophonic 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 (20Hz to 15KHz), 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 only looked at the audio frequencies you had an ordinary stereo recording. Along with this audio, a separate 30KHz sub-carrier was recorded on each groove wall. The sub-carrier on each side carries the difference signal for that side. This is the information that enables 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 18KHz to 45KHz for the left and right channels. The algebraic addition and subtraction of the sum and difference signals would then yield compatible and discrete Quadrophonic playback. CD-4 was responsible for major improvements in phonograph technology including better compliance, lower distortion levels, pickup cartridges with a significantly higher frequency range, and new record compounds such as Q-540 which were highly anti-static. Also a direct byproduct of CD-4 technology was the Shibata line contact stylus. Invented by Dr. Norio Shibata, the Shibata stylus greatly reduced the contact mass of the stylus to the record groove which reduced wear over time and maintained the original LP's fidelity and frequency range for much longer than any standard contact stylus available at the time could. This, of course, prolonged the overall playable life of a record; a definite plus for any audiophile or serious record collector. But what was (arguably) even more impressive was that the Shibata stylus was also capable of recovering sizable portions of program audio that would otherwise be compromised (i.e. 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. This coding is needed because the high frequency "difference" signals have limited bandwidth and so are not suitable as normal audio channels. However, when a limited bandwidth is used as a difference signal, the resulting four channels of music are full bandwidth but there is a very slight limitation in how each side (or sum) is resolved into front and rear sounds.

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 Quadrophonic audio in general) failed due to late FCC approval of FM Quadrophonic 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 - 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 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 frequencies as high as those in the CD-4 system.

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 / Quadrophonic 8-Track

Quadrophonic 8-Track was a discrete 4-Channel Tape Cartridge system introduced by RCA in Sept. 1970 and 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 signaled a quadrophonic 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 quadrophonic players would play stereo 8-tracks, playing quadrophonic tapes on stereo players results in hearing only one-half the channels at a time. Some stereo 8-track players touted simulated quadrophonic sound (through upmixing stereo 8-tracks) but were not quadrophonic 8-track players.

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

Matrix:

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 with which one began.

SQ / Stereo Quadrophonic

Stereo Quadrophonic was a matrix quadrophonic 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 is based on the work of Peter Scheiber, who created the basic mathematical formulas used to matrix four channels into two in 1970. SQ stands for "Stereo Quadrophonic." 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 competing QS format, allowed the full bandwidth from 20 Hz to 20 kHz to be used, giving a much more "open" and detailed top end.

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 2-channel system, and totally stereo-compatible, it could have substantial mono compatibility problems - which posed serious issues with monophonic radios & all televisions of the era.

By the time that the most advanced Logic System was introduced for SQ, the Tate Directional Enhancement System 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 for the first release of Star Wars. They were also used 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, on which the original master is in itself SQ (SQ is stereo compatible, so there was no need to create different versions of it)

j = + 90° phase-shift , k = - 180° phase-shift

QS / Quadrophonic Stereo

Quadrophonic 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 Sansui. The QS matrix is found to offer the advantage of excellent diagonal separation, full mono compatibility, and though the adjacent speaker separation is only 3 dB, this symmetrical distribution produces more stable quadrophonic 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 35 mm 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 = + 90° phase-shift , k = - 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 (center-front, center-left, center-rear, center-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. Only the left and right rear speakers are connected to the two channel stereo amplifier via a passive matrix circuit, the front ones stay directly connected to the amplifier. A lot of stereo material, recorded with a central, non-directional microfone (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-quadrafonic effect at low cost (in the patent: one fixed resistor 10 Ohm and three variable 20 Ohms in a star arrangement). Especially in classical music a fine impression of concert hall ambience is achieved on most of this material.

Dynaco sold this matrix circuit with a large and triple high-wattage potentiometer inside for presentday 70€ or so. Electronic amateurs could build this much cheaper e.g. with a 4 position switch (four steps in level of the rear sound from min. to max. level) using fixed resistors of e.g. 20, 10, 5 and 0 (short) Ohms. Since in practice only the highest level was of any use, basic setup 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 are constant impedance across a wide range and worked best with such high efficiency speakers. Later on when transistor amplifiers were used, speakers tended to lose that design feature(lower impedance meant higher power with these amplifiers, compensating for the lower efficiency of such designs). The system works best using a transistor 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 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, including a number of plays and some Promenade Concerts, while Radio 1 carried quadrophonic session recordings by various bands. 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 UMX matrix was brought in to 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 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, 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 (positive terminals) from the stereo amplifier. Alternatively, one speaker can be used on its own. (See External links for a circuit diagram.)

References

See also

• Ambisonics
• Azimuth co-ordinator
• Four-channel compact disc digital audio
• Multitrack recording
• Surround sound

External links

• Quadrophonic discography
• List of quadrophonic 8-tracks
• Quadrophonic - website honoring (and exploring) quadrophonic sound
• Learning about quadrophonics
• QuadrophonicQuad
• Collectible Quadrophonic Tapes
• Quadrophonic website by Wendy Carlos
• Hafler circuit
• Dolby surround