By Nick Landis on Monday May, 3rd 2010
PQ codes are the first 2 of 8 sub-channel information encoded on a 'Red Book' CD.  Each subcode channel is designated a letter P through W. The P channel gives simple instructions to the player about track locations.  Sometimes it is used by lower cost players due to its simplicity.  More sophisticated CD players will ignore the P channel and go straight to the Q channel for more detailed information.  The Q channel holds all the non-audio information on a typical Audio CD.  Several important player control codes like track number, track time, disc time, serial number, MCN (like a barcode), ISRC, copyright info, etc. . .  CD Text info is also embedded in this channel.  Channels R through W were reserved for expansion of the CD format, but have not come into use.