Compact Disc Subcode

Compact Disc Subcode

In addition to digital audio, a compact disc contains digital data called subcode or subchannel data, which is multiplexed with the digital audio. The data in a CD are arranged in frames. A frame comprises 33 bytes, of which 24 bytes are audio (six full stereo samples), eight bytes are error correction (CIRC-generated), and one byte is for subcode. The subcode byte is available for control and display. The eight bits are used as eight different subcoding channels, and given letters designating their usage: P, Q, …, W. Thus each channel has a bit rate of 7.35 (=44.1/6) kbit/s.

In each sector there are 2352 bytes (24×98) of audio content data and 96 bytes of subchannel data.

The 96 bytes of subchannel information in each sector contain four packets of 24 bytes apiece:

1 byte for command, 1 byte for instruction, 2 bytes for parityQ, 16 bytes for data, and 4 bytes parityP.

Each of the 96 subchannel data bytes can be thought of as being divided into eight bits. Each of these bits corresponds to a separate stream of information. These streams are called "channels", and are labeled starting with the letter P, like so:

Channel P is a simple pause/music flag, which can be used for low-cost search systems. Quite a few players ignore it in favor of the Q Channel.

Channel Q is used for control purposes of more sophisticated players. It contains positioning information, the Media Catalog Number (MCN), and International Standard Recording Code (ISRC). 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, and some additional tags:

Data

This track contains data (rather than audio). Can be used for muting in audio CD players.

Copy flag

Used by the Serial Copy Management System to indicate permission to digitally copy the track.

Four-channel compact disc digital audio

The track uses four channel audio. This is very rarely used on Compact Discs.

Pre-emphasis

The audio track was recorded with pre-emphasis. Used very rarely on Compact Discs.

Channels R…W are unused by Red-Book compliant CDs, and have been used for extensions to the standard.