The technical format of an audio compact disc (Compact Disc Digital Audio—CDDA) is laid down in a document produced in 1980 by the format's joint creators, Sony and Philips. The document is known colloquially as the "Red Book" after the colour of its cover. The format is a two-channel 16-bit PCM encoding at a 44.1 kHz sampling rate. Four-channel sound is an allowed option within the Red Book format, but has never been implemented.
