As well as the main data channel
on a CD
, there are 8 additional subcode
channels labelled P
interleaved with the main channel.
The P channel indicates the start and end of each track and is intended for use by simple audio players without complete Q decoding.
The Q channel supports all the standard goodies one expects from CDs -- minutes, seconds and frames, TOC details (in the lead-in), track type, catalogue number, etc.
Channels R through W are for graphics (CD-G) and/or text (CD Text) most commonly found on karaoke CDs.
Syncing subcode data back up with the main channel is one of the more common problems encountered when attempting to copy a CD -- see Why you can't bit copy a CD.