Track (CD)

On an optical disc, a track (CD) or title (DVD) is a subdivision of its content. Specifically, it is a consecutive set of sectors (called "timecode frames" on audio tracks) 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, and there is also a sub-track index for finding points within a track.

Audio tracks are defined in the Red Book specification for CD Digital Audio (which was the first CD specification). One song or movement 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.

Each sector (or "timecode frame") consists of a sequence of channel 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.

The 98 channel frames which make up the 2352 (98 * 24) byte sectors (or frames) contain 98 bytes of subchannel data, of which 96 bytes are usable. 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. These are used to control addressing and playback of the CD.

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