Track (CD)
On an optical disc
Optical disc
In computing and optical disc recording technologies, an optical disc is a flat, usually circular disc which encodes binary data in the form of pits and lands on a special material on one of its flat surfaces...

, a track (CD) or title (DVD
A DVD is an optical disc storage media format, invented and developed by Philips, Sony, Toshiba, and Panasonic in 1995. DVDs offer higher storage capacity than Compact Discs while having the same dimensions....

) is a subdivision of its content. Specifically, it is a consecutive set of sectors
Cylinder-head-sector, also known as CHS, was an early method for giving addresses to each physical block of data on a hard disk drive. In the case of floppy drives, for which the same exact diskette medium can be truly low-level formatted to different capacities, this is still true.Though CHS...

 on the disc containing a block of data. One session
Session (CD)
Data on a compact disc or a DVD are laid out in sessions. Each session consists of a lead-in, containing the session's table of contents, the program area in which the individual tracks are located, and the lead-out....

 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
In music, a song is a composition for voice or voices, performed by singing.A song may be accompanied by musical instruments, or it may be unaccompanied, as in the case of a cappella songs...

 usually comprises one audio track, containing audio in the form of raw PCM
Pulse-code modulation
Pulse-code modulation is a method used to digitally represent sampled analog signals. It is the standard form for digital audio in computers and various Blu-ray, Compact Disc and DVD formats, as well as other uses such as digital telephone systems...

 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
SMPTE time code
SMPTE timecode is a set of cooperating standards to label individual frames of video or film with a time code defined by the Society of Motion Picture and Television Engineers in the SMPTE 12M specification...

 equivalent for the audio data consists of hour:minute:sec:frame, where frame ranges from 0 to 74). CIRC
Cross-Interleaved Reed-Solomon Coding
In the compact disc system, cross-interleaved Reed-Solomon code provides error detection and error correction. CIRC adds to every three data bytes one redundant parity byte.-Overview:...

 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
Eight-to-Fourteen Modulation
Eight-to-fourteen modulation is a data encoding technique – formally, a channel code – used by compact discs and pre-Hi-MD MiniDiscs. EFMPlus is a related code, used in DVDs and SACDs. EFM and EFMPlus were both invented by Kees A...

 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
Compact disc subcode
Besides digital audio, a compact disc contains digital data called subcode or subchannel data, which is multiplexed with the digital audio. The data in a CD are arranged in frames. A frame comprises 33 bytes, of which 24 bytes are audio , eight bytes are error correction , and one byte is for subcode...

, 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
      International Standard Recording Code
      The International Standard Recording Code , defined by ISO 3901, is an international standard code for uniquely identifying sound recordings and music video recordings. IFPI has been appointed by ISO as registration authority for this standard. The ISO technical committee 46, subcommittee 9 is...

       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
      Serial Copy Management System
      The Serial Copy Management System or SCMS is a copy protection scheme that was created in response to the digital audio tape invention, in order to prevent DAT recorders from making second-generation or serial copies. SCMS sets a "copy" bit in all copies, which prevents anyone from making further...

    • Four-channel compact disc digital audio
      Four-channel compact disc digital audio
      Compact Disc recordings contain two channels of 44.1-kHz 16-bit linear PCM audio. However, creators of the CD originally contemplated a four-channel, or quadraphonic, mode as well....

      (rarely used on CDs)
    • preemphasis
      In processing electronic audio signals, pre-emphasis refers to a system process designed to increase the magnitude of some frequencies with respect to the magnitude of other frequencies in order to improve the overall signal-to-noise ratio by minimizing the adverse effects of such phenomena as...

  • The R channel through W channel are unused by Red Book
    Red Book (audio CD standard)
    Red Book is the standard for audio CDs . It is named after one of the Rainbow Books, a series of books that contain the technical specifications for all CD and CD-ROM formats.The first edition of the Red Book was released in 1980 by Philips and Sony; it was adopted by the Digital Audio Disc...

    -compliant CDs and have been used by standard extensions. In CD+G
    CD+G is an extension of the compact disc standard that can present low-resolution graphics alongside the audio data on the disc when played on a compatible device...

     they are used for text and graphics. They are also used to store ITTS data or textual track data in the CD-Text
    CD-Text is an extension of the Red Book Compact Disc specifications standard for audio CDs. It allows for storage of additional information on a standards-compliant audio CD...


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.


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
Radio is the transmission of signals through free space by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space...

Broadcasting is the distribution of audio and video content to a dispersed audience via any audio visual medium. Receiving parties may include the general public or a relatively large subset of thereof...

. Every track at least has index 1, and often has a pre-gap which is index 0. Additional songs, such as "hidden track
Hidden track
In the field of recorded music, a hidden track is a piece of music that has been placed on a CD, audio cassette, vinyl record or other recorded medium in such a way as to avoid detection by the casual listener...

s", 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
An episode is a part of a dramatic work such as a serial television or radio program. An episode is a part of a sequence of a body of work, akin to a chapter of a book. The term sometimes applies to works based on other forms of mass media as well, as in Star Wars...

 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
Chapter (books)
A chapter is one of the main divisions of a piece of writing of relative length, such as a book. Chapters can be numbered in the case of such writings as law code or they can be titled. For example, the first chapters of some well-known novels are titled:*"The Boy Who Lived" – Harry Potter...

 in a book
A book is a set or collection of written, printed, illustrated, or blank sheets, made of hot lava, paper, parchment, or other materials, usually fastened together to hinge at one side. A single sheet within a book is called a leaf or leaflet, and each side of a leaf is called a page...

. This was inherited from its predecessor the LaserDisc
LaserDisc was a home video format and the first commercial optical disc storage medium. Initially licensed, sold, and marketed as MCA DiscoVision in North America in 1978, the technology was previously referred to interally as Optical Videodisc System, Reflective Optical Videodisc, Laser Optical...

, which contained only one title divided into chapters.

Data tracks

Data tracks on a CD-ROM
A CD-ROM is a pre-pressed compact disc that contains data accessible to, but not writable by, a computer for data storage and music playback. The 1985 “Yellow Book” standard developed by Sony and Philips adapted the format to hold any form of binary data....

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