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.

CD-Text

CD-Text is part of the CD+G extension to the Red Book standard for audio CDs. It allows for storage of additional information (e.g. album name, song name, and artist) on a standards-compliant audio CD. The information is stored in the lead-in area of the CD, where there is roughly five kilobytes of space available, or in the R through W subcode channels on the disc, which are not used by strict Red Book CDs. About 24 megabytes of information can be stored there. The text is stored in a format usable by the Interactive Text Transmission System (ITTS). ITTS is also used by Digital Audio Broadcasting or the MiniDisc.

Note that the CD+G or “karaoke” extension also uses the R-W subcode channels to store low resolution graphics.

Jack on CD players

Some older CD players, such as the Pioneer PD-5010, have a jack on the back labeled "Subcode Out".