Quadraphonic 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 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 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.
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) A single channel (mono) system will reproduce all four audio signals in its one speaker.
(b) 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.
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 with the left recorded on one groove wall and the right on the other, 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, if you looked at the audio frequencies only, you had an ordinary stereo recording. Along with this audio, a separate 30 kHz subcarrier was recorded on each groove wall. The subcarrier 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 subcarrier it would be left front minus left back, and for the right subcarrier 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 subcarrier 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:
Although 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.
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 sub-carriers similar to the CD-4 system. UD-4 was less critical in its set-up than CD-4, because the sub-carriers did not have to carry frequencies as high as those found in the CD-4 system.
The last release in the quadraphonic 8-track format was in 1978.
The system was based on the work of Peter Scheiber, who created the basic mathematical formulae used to matrix four channels into two in 1970. SQ stands for "Stereo Quadraphonic." This system made good sense, as, in the absence of a quad decoder, SQ-encoded records would play as normal stereo records, and CBS stated their desire to maintain excellent compatibility between their SQ-encoded records and standard stereo systems.
The early SQ decoders could not produce more than 3dB of separation from front to back. Early "Logic" circuits were introduced to enhance separation to 20dB, 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.
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.)
{| class="wikitable" style="text-align:center" ! Basic SQ Encoding Matrix !! Left Front !! Right Front !! Left Back !! Right Back |- ! Left Total | 1.0 || 0.0 || -j0.7 || 0.7 |- ! Right Total | 0.0 || 1.0 || -0.7 || j0.7 |} j = + 90° phase-shift
To provide mono-compatibility a variation on this matrix was proposed:
{| class="wikitable" style="text-align:center" ! Modified SQ Encoding Matrix !! Left Front !! Right Front !! Left Back !! Right Back |- ! Left Total | 1.0 || 0.0 || 0.7 || -j0.7 |- ! Right Total | 0.0 || 1.0 || -j0.7 || 0.7 |} j = + 90° phase-shift
The four channels were coded and decoded normally in this proposal, but the back-center channel was coded in phase and therefore decoded in front-center.
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 35mm 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 later usage in video 5.1 surround sound.
{| class="wikitable" style="text-align:center" ! 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 || -j0.38 || -j0.92 |} j = + 90° phase-shift
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 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.
The rear sound level in a live performance recorded in stereo is reproduced about 7dB 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. (See External links for a circuit diagram.)
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