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.
Additionally, each sector contains 96 bytes of subchannel data. 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.
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.
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 on a CD-ROM are based on audio tracks, as the CD was originally not designed as a data disc.
The Orange Book specification added the concept of "sessions" to CDs (the original specs for CD-DA and CD-ROM implicitly assume only one "session" per disc). Each session has the three areas that are included in the original structure for CD-DAs and CD-ROMs: a lead-in containing the session's Table of Contents; a program holding individual tracks (the information being stored); and a lead-out to mark then end of the session.
If a disc has multiple sessions, each session has this same structure (lead-in, program area, and lead-out). In these discs, the lead-in areas contain addresses of the previous sessions. The TOC in the lead-in of the latest session is used to access the tracks. Each session must have at least one track. The first lead-out is 6750 sectors (about 13 megabytes) long; each subsequent lead-out is 2250 sectors (4 megabytes) long.
The following table shows the structure of a multi-session CD:
|Session level||Session 1||Session 2||...|
|Track level||Lead in (with TOC)||Track 1||Track 2||...||Lead out||Lead in (with TOC)||Track 1||Track 2||...||Lead out||...|