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See also: create section with CD ripper

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{{Short description|Data contained in a compact disc in addition to digital audio or user data}}
{{Refimprove|date=September 2014}}

'''Subcode''' or '''subchannel data''' (called ''control bytes'' in the [[CD-ROM]] specification) is 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. The original specification was defined in the [[Red Book (CD standard)|Red Book]] standard for [[Compact Disc Digital Audio]], though further specifications have extended their use (including the [[CD-ROM]], [[CD Text]] and [[CD+G]] specifications).

== Structure ==
Subchannel data is multiplexed with the [[Compact Disc Digital Audio|digital audio]] or [[CD-ROM|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 ([[Cross-interleaved Reed-Solomon coding|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.<ref name="ecma130">{{cite web|url=http://www.ecma-international.org/publications/standards/Ecma-130.htm |title=Data Interchange on Read-only 120 mm Optical Data Disks (CD-ROM) |publisher=ECMA |date=June 1996 |accessdate=2009-04-26}}</ref> Each channel has a bit rate of 7.35 kbit/s.

Each subcode bit/subchannel is designated by a letter from P to W. The following diagram illustrates how the channels are laid out:<ref name="ecma130"/>

{| class="wikitable"
|-
!colspan=9 |Subcode / control byte
|-
!Bit
|8
|7
|6
|5
|4
|3
|2
|1
|-
!Channel
|P
|Q
|R
|S
|T
|U
|V
|W
|-
!Frame 1
| colspan=8 |Sync word 1
|-
!Frame 2
| colspan=8 |Sync word 2
|-
!Frame 3
| rowspan=5 |Channel data
| rowspan=5 |Channel data
| rowspan=5 |Channel data
| rowspan=5 |Channel data
| rowspan=5 |Channel data
| rowspan=5 |Channel data
| rowspan=5 |Channel data
| rowspan=5 |Channel data
|-
!Frame ..
|-
!Frame ..
|-
!Frame ..
|-
!Frame 98
|}

== Channels ==
Both the P and Q channels on a regular audio CD are used for timing information. They assist the CD player in tracking the current location on the disc, and to provide the timing information for the time display on the CD player. The rest are not used in the Red Book specification.

* '''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. It indicates a start of a new track by at least two consecutive seconds (150 sectors) of all 1s, and the last block with all 1s is the first block of the new track.
* '''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.<ref name="ecma130"/>
**''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 compact disc digital audio|four-channel audio]] (applies only to CD-DA). This feature was never used on Compact Discs.{{cn|date=May 2025}}
***'''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.
***'''Digital copy permission 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 [[preemphasis|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 defined in the previous bits.
***'''Q Mode 1''': In this mode, the data bits contain the [[Compact Disc Digital Audio#Data structure|Table of Contents]] of the [[Session (CD)|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)<ref>{{Cite web |title= INFORMATION ON CD FORMATS |url=http://www.csun.edu/science/help/help_docs/CD.htm |access-date=2022-06-26 |website=www.csun.edu}}</ref> 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.
*'''Channels ''R'' through ''W''''' are unused by Red Book compliant CDs and Yellow Book compliant CD-ROMs, and have been used for extensions to the standard:
** [[CD-Text]] is an extension to the [[Red Book (audio CD standard)|Red Book]] standard for audio CDs. It allows for storage of additional information (e.g. album name, song name, and artist) on the ''R'' through ''W'' subcode channels on the disc (either in the lead-in area or in the program or main area).
** The [[CD+G]] or "karaoke" extension also uses the ''R'' through ''W'' subcode channels to store low resolution graphics.
** Several [[Compact Disc and DVD copy protection|copy protection systems]] made use of the fact that some [[Ripping|disk copying utilities]] neglect to copy subcode data due to the obscurity of it. [[CloneCD]] can read subcode data and save it to a [[CloneCD Control File]].

==Jack on CD players==
Some older CD players, such as the [[Pioneer Corporation|Pioneer]] PD-5010, have a socket for an [[Mini-DIN connector#8-pin|eight-pin mini-DIN connector]] on the back labeled "Subcode Out".

[[File:CD Player Subcode Out Socket Pin-Out.svg]]

== See also ==

* [[CD ripper]]

==References==
<references/>

{{Compact disc}}

[[Category:Compact disc]]

Compact Disc subcode - Revision history

imported>Danhash: /* See also */ create section with CD ripper