Compact Disc
The compact disc (CD) is a digital optical disc data storage format that was co-developed by Philips and Sony to store and play digital audio recordings. In August 1982, the first compact disc was manufactured. It was then released in Octo ...
recordings contain two channels of 44.1-kHz 16-bit linear
PCM
Pulse-code modulation (PCM) is a method used to digitally represent sampled analog signals. It is the standard form of digital audio in computers, compact discs, digital telephony and other digital audio applications. In a PCM stream, the amp ...
audio. However, creators of the CD originally contemplated a four-channel, or
quadraphonic, mode as well.
The proprietary
Red Book specification, as published by
Sony
, commonly stylized as SONY, is a Japanese multinational conglomerate corporation headquartered in Minato, Tokyo, Japan. As a major technology company, it operates as one of the world's largest manufacturers of consumer and professional ...
and
Philips
Koninklijke Philips N.V. (), commonly shortened to Philips, is a Dutch multinational conglomerate corporation that was founded in Eindhoven in 1891. Since 1997, it has been mostly headquartered in Amsterdam, though the Benelux headquarters is ...
, briefly mentions a four-channel mode in its June 1980,
["Red Book" Audio CD specification, June 1980, Sony/Philips] September 1983,
["Red Book" Audio CD specification, September 1983, Sony/Philips] and November 1991
["Red Book" Audio CD specification, November 1991, Sony/Philips] 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."
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, fewer bits per sample, or
compression). Alternatively, the