3.7.1. Compact Disc
In the case of Compact Discs the ISRCs and other PQ-data are encoded in the disc sub-code (Q channel) in the disc mastering process. For this reason, ISRCs must be encoded for each track in the Pre-Master for CD. The ISRC codes, together with the Digital Copy Prohibited flag, and the relevant point of sale code, such as EAN/UPC should be inserted on the Pre-Master during the pre-mastering process from the original Master.

Channel Q is used for control purposes of more sophisticated players. It has three different modes, but with a common structure for all of them.[1]
        Control bits: The first four bits are used for control, each being a flag for a different feature:
            Four-channel compact disc digital audio flag: indicates that the track uses four-channel audio (applies only to CD-DA). This is very rarely used on Compact Discs.
            Data flag: Indicates that this track contains data (rather than audio). Can be used for muting in audio CD players. Not used in the original CD-DA standard, added in the CD-ROM specifications.
            Copy protection flag: Used by the Serial Copy Management System to indicate permission to digitally copy the track.
            Pre-emphasis flag: The audio track was recorded with pre-emphasis (applies only to CD-DA). Used very rarely on Compact Discs.
        Mode bits: The next four bits indicate the mode of the Q channel, which can vary from 1 to 3, and define the structure and contents of the next bits.
        Data bits: The next 72 bits contain Q-channel data, and their structure depends on the mode define in the previous bits.
            Q Mode 1: In this mode, the data bits contain the Table of Contents of the session (if the Q channel is in the lead-in area), or timing information for the current track (if the Q channel is in the program and lead-out areas of a session).
            Q Mode 2: In this mode, the data bits contain the Media Catalog Number (MCN) of the disc.
            Q Mode 3: In this mode, the data bits contain an International Standard Recording Code (ISRC) for each track (applicable to CD-DA only). The ISRC is used by the media industry, and contains information about the country of origin, the year of publication, owner of the rights, as well as a serial number.
        Cyclic redundancy check bits: The last 16 bits contain an error detection code computed over the previous bits of the channel.