PQ Subcode

Two channels of subcode information contained in typical audio compact discs. Anyone who edits audio for audio CD creation has had some encounter (no matter how well disguised in the user interface of the equipment being used) with PQ subcodes. Moving track start times in a CD editing system changes the PQ subcode information. The “P” subcode is a simple music track separator flag. Its status is normally a binary “0” during the music and in the lead in track, but changes to “1” at the start of each selection. CD players can use it as a way to find track start points. The “Q” Subcode is used for more sophisticated control purposes; it contains data such as track number and time. There are other subcodes (R, S, T, U, V, W) defined in the Red Book CD spec for use with things like text and graphics, but they aren’t commonly used.