On a compact disc or a DVD, a track is a subdivision of its content. Specifically, it 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.

Audio tracks

One song usually comprises one 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.

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.

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 compact disc digital audio (rarely used on CDs)
  • preemphasis
• The R channel through 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.

The audio data are organized in 24-byte blocks called F1 frames,

There are three modes of audio tracks: mode 1, mode 2, and mode 3.

In mode 1 track, the Q channel has different structure for the lead-in and the program area.

References

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