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Compact Disc subcode

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Subcode or subchannel data (called "control bytes" in the CD-ROM specification) refers to data contained in a compact disc (CD) in addition to digital audio or user data, which is used for control and playback of the CD.

Structure

Subchannel data is multiplexed with the digital audio or user digital data. The data in a CD are arranged in frames. A frame comprises 33 bytes, of which 24 bytes are audio or user data, eight bytes are error correction (CIRC-generated), and one byte is for subcode. Frames are arranged in sectors, which contain 98 frames each. The subcode bytes of the first two frames of a sector are used as two synchronization words. The subcode bytes of the remaining 96 frames of a sector are split into eight 96-bit long subcode channels (also called subchannels or simply channels) by putting together the nth bit of each subcode byte together. Each channel has a bit rate of 7.35 (=44.1/6) kbit/s.

Each subcode bit/subchannel is designated by a letter from P to W, like so:

Channel P Q R S T U V W
Bit 7 6 5 4 3 2 1 0

Channels

Channel P is a simple pause/music flag, which can be used for low-cost search systems. Many 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 through 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 socket for a miniature 8-pin connector on the back labeled "Subcode Out".

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