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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. The original specification was defined in the Red Book standard for CD Digital Audio, though further specifications have extended their use (including the CD-ROM, CD Text and CD+G specifications).
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. [1] 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: [1]
| Subcode / control byte | ||||||||
|---|---|---|---|---|---|---|---|---|
| Bit | 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 |
| Channel | P | Q | R | S | T | U | V | W |
| Frame 1 | Sync word 1 | |||||||
| Frame 2 | Sync word 2 | |||||||
| Frame 3 | Channel data | Channel data | Channel data | Channel data | Channel data | Channel data | Channel data | Channel data |
| Frame .. | ||||||||
| Frame .. | ||||||||
| Frame .. | ||||||||
| Frame 98 | ||||||||
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.
Some older CD players, such as the Pioneer PD-5010, have a socket for an eight-pin mini-DIN connector on the back labeled "Subcode Out".
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