Subcode or subchannel data (called control bytes in the CD-ROM specification) refers to data contained in a compact disc (CD) in addition to digital audio or user data, which is used for control and playback of the CD.
The subcode bytes of the first two frames of a sector are used as two synchronization words.
Each subcode bit/subchannel is designated by a letter from P to W. The following diagram illustrates how the channels are laid out:[1] Both the P and Q channels on a regular audio CD are used for timing information.
They assist the CD player in tracking the current location on the disc, and to provide the timing information for the time display on the CD player.
Some older CD players, such as the Pioneer PD-5010, have a socket for an eight-pin mini-DIN connector on the back labeled "Subcode Out".