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" 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

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.

ector 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.

ubchannels

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.

Index

Each CD track has an index, however it is rare to find a CD player that displays or can access this feature, except occasionally in pro audio equipment, usually for radio broadcasting. Every track at least has index 1, and often has a pre-gap which is index 0. Additional songs, such as "hidden tracks", usually have index 2 or 3.

Video tracks

On a DVD, each track is called a title, because it is intended to hold a single movie title, or episode of a TV series. Extra content and bonus features on a DVD are also on separate tracks or titles. The sub-track index is called a chapter, like a chapter in a book. This was inherited from its predecessor the LaserDisc, which contained only one title divided into chapters.

Data tracks

Data tracks on a CD-ROM are based on audio tracks, as the CD was originally not designed as a data disc.

References