Compatible Discrete 4

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.

Compatible Discrete 4 (CD-4) or Quadradisc (not to be confused with compact disc) was introduced in May 1972 as a discrete quadraphonic system created by JVC and RCA in 1971. Record companies who adopted this format include Arista, Atlantic, Capricorn, Elektra, Fantasy, JVC, Nonesuch, A & M, Reprise and Warner.

This was the only fully discrete (all channels recorded separately) quadraphonic phonograph record system to gain major industry acceptance.

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) using a coding matrix similar to FM broadcast stereo multiplexing.

Operation

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:

Quadraphonic channel		Sum channel		Difference channel
Left front	=	LF + LB	+	LF − LB
Left back	=	LF + LB	−	LF − LB
Right front	=	RF + RB	+	RF − RB
Right back	=	RF + RB	−	RF − RB

Quadracast

There was also a similar FM radio system called Quadracast. But 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.

External links

• Mark Anderson. Quadraphonic discography
• QuadrophonicQuad
• SurroundSound - Quad and surround music discussion group
• Quadraphonic website by Wendy Carlos
• 1970s Surround Sound Audio
• RCA Quadradisc discography