Editing Quadraphonic sound (section)

==== CD-4 or Quadradisc ====

[[File:QuadraDisk.jpg|thumbnail|An RCA Quadradisc record]]

The third major format for four-channel vinyl LPs, known as ''[[Compatible Discrete 4|CD-4]]'' or ''[[Compatible Discrete 4|Quadradisc]]'', was devised by the Japanese [[JVC]] Corporation along with its US counterpart [[RCA Records]].

This quadraphonic format was first marketed in the United States in May 1972. A fully discrete system, it eschewed matrix technologies in favor of a method similar to the encoding of [[FM broadcasting#Stereo FM|stereo FM]] broadcasts.  With stereo records, the system uses 2 main left and right audio channels, and this is what allows CD-4 to maintain compatibility with conventional stereo playback. CD-4 also adds 2 additional ''difference'' audio channels to the main channels. The difference signals are encoded in [[Ultrasound|ultrasonic]] carrier frequencies in the range of 30&nbsp;kHz, which is above the audible range. CD-4 requires a specialized [[Magnetic cartridge|phono cartridge]] with a [[Shibata stylus]] to read these additional high frequencies. The combined signals are then sent to a special demodulator for four-channel decoding. The demodulator converts the ultrasonic signals back into the audible range and uses the difference channels to separate rear audio information from the main channels. Because the CD-4 system maintains four independent signals throughout the process it can accurately reconstruct the intended four-channel sound field.