Track (CD)

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On a compact disc or a DVD, a track is a consecutive set of sectors on the disc containing a block of data. One session may contain one or more tracks of the same or different types. There are several kinds of tracks.

[edit] Audio tracks

Audio track, containing audio in the form of raw PCM samples in 16 bit/44.1 kHz resolution in 2 channels, and a subcode multiplexed with the audio data. In this mode, each sector (called a frame) consists of 2352 bytes of audio data (1176 16-bit samples, or 588 stereo samples, which equals 1/75 second of audio (therefore SMPTE time code equivalent for the audio data consists of hour:minute:sec:frame, where frame ranges from 0 to 74). CIRC error correction is used for the data.

[edit] Sector structure

Each sector consists of a sequence of frames. These frames, when read from the disc, are made of a 24-bit synchronization pattern with the constant sequence 1000-0000-0001-0000-0000-0010, not present anywhere else on the disc, separated by three merging bits, followed by 33 bytes in EFM encoding, each followed by 3 merge bits. This forms a 588 bits long structure (24+3+33*(14+3)) called channel frame. The 33 bytes in channel frame are composed of 24 bytes of user data, 8 bytes of parity, and 1 byte of subcode data.

[edit] Subchannels

Additionally, each sector contains 96 bytes of subchannel data, consisting of 4 packets of 24 bytes each, each containing 1 command byte, 1 instruction byte, 2 parityQ bytes, 16 bytes for data, and 4 parityP bytes. (The parityP and parityQ bits are unrelated to the P and Q channels.) The subchannel bytes are further divided to individual bits, labeled PQRSTUVW, from most to least significant bit, and forming eight parallel bitstreams called channels, subcode channels, or subchannels.

The P channel contains simple pause/music flags and can be used for searching in low cost systems. It is often ignored by contemporary players. 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.
The Q channel may contain various additional information, depending on the track mode:
- positioning information (A-Time, or absolute time)
- Media Catalog Number (MCN), constant per disc
- ISRC code, constant per track
- data flag, telling the player the track is not an audio track and should not be played
- copy flag for serial copy management system,
- four channel audio (rarely used on CDs)
- preemphasis
The R channel to W channel are unused by Red Book compliant CDs and have been used by standard extensions. In CD+G they are used for text and graphics. They are also used to store ITTS data or textual track data in the CD-Text format.