| Four-channel Compact Disc Digital Audio |
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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 audio 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). Alternately, 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 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. SEE ALSO EXTERNAL LINKS
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