Four-channel compact disc digital audio
Compact Disc
Compact Disc
The Compact Disc is an optical disc used to store digital data. It was originally developed to store and playback sound recordings exclusively, but later expanded to encompass data storage , write-once audio and data storage , rewritable media , Video Compact Discs , Super Video Compact Discs ,...

 recordings contain two channels of 44.1-kHz 16-bit linear PCM
Pulse-code modulation
Pulse-code modulation is a method used to digitally represent sampled analog signals. It is the standard form for digital audio in computers and various Blu-ray, Compact Disc and DVD formats, as well as other uses such as digital telephone systems...

 audio. However, creators of the CD originally contemplated a four-channel, or quadraphonic
Quadraphonic sound – the most widely used early term for what is now called 4.0 surround sound – uses four channels in which speakers are positioned at the four corners of the listening space, reproducing signals that are independent of one another...

, mode as well.

The proprietary Red Book specification, as published by Sony
, commonly referred to as Sony, is a Japanese multinational conglomerate corporation headquartered in Minato, Tokyo, Japan and the world's fifth largest media conglomerate measured by revenues....

 and Philips
Koninklijke Philips Electronics N.V. , more commonly known as Philips, is a multinational Dutch electronics company....

, briefly mentions a four-channel mode in its June 1980, September 1983, and November 1991 editions. On the first page, it lays out the "Main parameters" of the CD system, including: "Number of channels: 2 and/or 4 simultaneously[*] sampled." The footnote says, "In the case of more than two channels the encoder and decoder diagrams have to be adapted." Some players have adapted this to be half CD Quality, thus a CD Audio disc produced with 22050 (16 bit) and 4 independent channels will play on some players, such as the Emerson
Emerson Radio
Emerson Radio Corporation was founded in 1948. It is one of the United States’ largest volume consumer electronics distributors and has a recognized trademark in continuous use since 1912...


The Red Book also reserved the first bit of the so-called Q subchannel "control field" to signal the presence of four-channel audio, but did not specify a method for using four-channel in the CD system. Had it been later specified, this mode might have included four separate channels of linear PCM audio (requiring some combination of faster rotation, a lower sampling rate, or fewer bits per sample). Alternatively, the "four-channel" bit could have been used merely to indicate the presence of a matrix-encoded recording.

In reality, however, the underspecified "four-channel" mode was dropped from the CD standard when it was adopted by the International Electrotechnical Commission
International Electrotechnical Commission
The International Electrotechnical Commission is a non-profit, non-governmental international standards organization that prepares and publishes International Standards for all electrical, electronic and related technologies – collectively known as "electrotechnology"...

and became IEC 908:1987, and later IEC 60908:1999. (Various national authorities have also adopted the IEC standard. E.g., it is also European Standard EN 60908:1999.)

Neither the 1987 nor the 1999 version of the IEC standard discusses the possibility of four-channel audio. Instead, the IEC document reserves the first bit of the Q subchannel "control field" to a different, although similarly cryptic, purpose—according to clause 17.5 note 2, it is for "Broadcasting use" in "non-audio applications of the Compact Disc."

Since the behavior of the "four-channel" or "Broadcasting use" bit was never specified by either CD standard, no mass-marketed discs have attempted to use the Red Book's four-channel mode, and no players have purported to implement it.

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