Compact disc subcode
Besides digital audio, a compact disc
Compact Disc
The Compact Disc is an optical disc used to store digital data. It was originally developed to store and playback sound recordings exclusively, but later expanded to encompass data storage , write-once audio and data storage , rewritable media , Video Compact Discs , Super Video Compact Discs ,...

 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
Cross-Interleaved Reed-Solomon Coding
In the compact disc system, cross-interleaved Reed-Solomon code provides error detection and error correction. CIRC adds to every three data bytes one redundant parity byte.-Overview:...

-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 byte
The byte is a unit of digital information in computing and telecommunications that most commonly consists of eight bits. Historically, a byte was the number of bits used to encode a single character of text in a computer and for this reason it is the basic addressable element in many computer...

s (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 bit
A bit is the basic unit of information in computing and telecommunications; it is the amount of information stored by a digital device or other physical system that exists in one of two possible distinct states...

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

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)
International Standard Recording Code
The International Standard Recording Code , defined by ISO 3901, is an international standard code for uniquely identifying sound recordings and music video recordings. IFPI has been appointed by ISO as registration authority for this standard. The ISO technical committee 46, subcommittee 9 is...

. 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
Serial Copy Management System
The Serial Copy Management System or SCMS is a copy protection scheme that was created in response to the digital audio tape invention, in order to prevent DAT recorders from making second-generation or serial copies. SCMS sets a "copy" bit in all copies, which prevents anyone from making further...

 to indicate permission to digitally copy the track.

Four-channel compact disc digital audio
Four-channel compact disc digital audio
Compact Disc recordings contain two channels of 44.1-kHz 16-bit linear PCM audio. However, creators of the CD originally contemplated a four-channel, or quadraphonic, mode as well....

: 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 RW are unused by Red-Book compliant CDs, and have been used for extensions to the standard.


CD-Text is part of the CD+G
CD+G is an extension of the compact disc standard that can present low-resolution graphics alongside the audio data on the disc when played on a compatible device...

 extension to the Red Book
Red Book (audio CD standard)
Red Book is the standard for audio CDs . It is named after one of the Rainbow Books, a series of books that contain the technical specifications for all CD and CD-ROM formats.The first edition of the Red Book was released in 1980 by Philips and Sony; it was adopted by the Digital Audio Disc...

 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
The disc is permanently housed in a cartridge with a sliding door, similar to the casing of a 3.5" floppy disk. This shutter is opened automatically by a mechanism upon insertion. The audio discs can either be recordable or premastered. Recordable MiniDiscs use a magneto-optical system to record...


Note that the CD+G
CD+G is an extension of the compact disc standard that can present low-resolution graphics alongside the audio data on the disc when played on a compatible device...

 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
Pioneer Corporation
is a multinational corporation that specializes in digital entertainment products, based in Kawasaki, Kanagawa, Japan. The company was founded in 1938 in Tokyo as a radio and speaker repair shop...

PD-5010, have a jack on the back labeled "Subcode Out".
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