Compatible Discrete 4

Compatible Discrete 4, also known as Quadradisc or CD-4 (not to be confused with compact disc) was as a discrete 4-channel quadraphonic system for gramaphone records, commonly called "phonograph" records. The system was created by JVC and RCA in 1971^[1] and introduced in May 1972. Major record companies who adopted this format include Arista, Atlantic, Capricorn, Elektra, Fantasy, JVC, Nonesuch, A & M, Reprise and Warner Bros.^[2]

In discreet quadraphonic systems all 4 channels remain fully independent of each other throughout the entire recording and reproduction chain. There is no intermixing of channels as is done in matrix decoder 4-channel systems such as SQ and QS. This was the only discrete quadraphonic phonograph record system to gain major industry acceptance.

CD-4 technology was also responsible for major improvements in stereo phonograph technology. Improvements included higher compliance, lower distortion levels, phono cartridges with a significantly higher frequency range, and new vinyl compounds for records. Such new compounds included JVC's "Supervinyl", which was more durable than convention materials, and Q-540, which was highly anti-static.

A typical high-performance CD-4 system would include a turntable with a CD-4 compatible phono cartridge, a CD-4 demodulator, a discrete four-channel amplifier, and four identical full-range loudspeakers.^[3] Some audio electronics manufactures included the CD-4 demodulator as a built in feature of other equipment. A four-channel audio receiver unit might contain the demodulator along with FM radio and amplifier circuitry.

Simply put, CD-4 consists of four recorded signals (LF, LB, RB, RF) using a coding matrix similar to FM broadcast stereo multiplexing.

Operation

An RCA Quadradisc recording. The color fringes around the reflections are caused by the ultrasonic signal that contains the difference information used to separate the four channels.

In the CD-4 system, the quadraphonic audio was divided into left and right channels, which were recorded orthogonally in the vertical plane of the disc groove, which is the case with normal stereo. The CD-4 record track is broader than a conventional stereo track, so the playing time is less than a conventional stereo record.

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, when observing the audio frequencies only, the record appeared to have an ordinary stereo recording.

Along with this audio, a separate 30 kHz carrier was recorded on each groove wall. The carrier 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 carrier it would be left front minus left back, and for the right 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 carrier 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.

The CD-4 encoding/decoding matrix:

Quadracast

There was also a similar FM radio system called Quadracast. But CD-4 (and quadraphonic audio in general) failed due to late U.S. Federal Communications Commission approval of FM quadraphonic broadcasting,

References

1. ↑ http://vinylfanatics.com/analoglovers/page11.html
2. ↑ http://www.reddiamondaudio.net/quadraphonic_sound_reproduction.html#CD-4
3. ↑ http://www.4channelsound.com/cd-4.htm

External links

• RCA Quadradisc discography