Definitions for "Subchannels"
Audio CDs have 8 subchannels of non-audio data interleaved with the audio data, called the P, Q, R, S, T, U, V, and W channels. You can think of them as small, separate streams of data running alongside the audio, which can be read by a player at the same time as the audio, if the player is "smart" enough to interpret them. For example, CD+Graphics discs (karaoke) store rudimentary graphics and text in the subchannels, but you need a special player to read and display this information. The P and Q channels are used to tell an audio player how to play back an audio disc. The Q channel contains the index markers. In the pause (gap) before a track begins, the index marker is set to 0 (zero). When a track begins, the index marker changes to 1. (If a track contains subindexes, these are incremented by 1 sequentially during the track. In this case the Q channel might contain 1, 2, 3, 4, etc.) When the track ends, the Q channel index marker goes back to 0, then re-starts at 1 when the next track begins.
Eight subchannels of non-audio data on a CD (lettered P–W). Channels P and Q contain instructions for an audio player, telling it how to play the disc. On a CD+G disc, one of the subchannels would be used for storing song lyrics.