Editing Compact Disc Digital Audio (section)

=== Data structure ===
[[File:Basic Illustration and mesures of a CD.svg|thumb|Some of the visible features of an audio CD, including the lead-in, program area, and lead-out. A microscopic spiral of digital information begins near the disc's center and progresses toward the edge.]]

The audio data stream in an audio CD is continuous but has three parts. The main portion, further divided into playable audio tracks, is the ''program area''. This section is preceded by a ''lead-in'' track and followed by a ''lead-out'' track. The lead-in and lead-out tracks encode only silent audio, but all three sections contain [[subcode]] data streams.

The lead-in's subcode contains repeated copies of the disc's table of contents (TOC), which provides an index of the start positions of the tracks in the program area and of the lead-out. The track positions are referenced by absolute [[timecode]], relative to the start of the program area, in MSF format: minutes, seconds, and fractional seconds called ''frames''. Each ''timecode frame'' is one seventy-fifth of a second, and corresponds to a block of 98 ''channel-data frames''—ultimately, a block of 588 pairs of left and right audio samples. Timecode contained in the subchannel data allows the reading device to locate the region of the disc that corresponds to the timecode in the TOC. The TOC on discs is analogous to the [[partition table]] on [[hard drive]]s. Nonstandard or corrupted TOC records are abused as a form of [[CD/DVD copy protection]], in e.g. the [[key2Audio]] scheme.

==== Tracks ====
{{Main|Track (optical disc)#Audio tracks}}
The largest entity on a CD is called a [[Track (optical disc)|track]]. A CD can contain up to 99 tracks (including a data track for [[Mixed Mode CD|mixed mode discs]]). Each track can in turn have up to 100 indexes, though players that still support this feature have become rarer over time. The vast majority of songs are recorded under index 1, with the [[pregap]] being index 0. Sometimes [[hidden track]]s are placed at the end of the last track of the disc, often using index 2 or 3, or using the pregap as index 0 (this latter usage will result in the track playing as the time counter counts down to time 0:00 at the start of the track, index 1.) This is also the case with some discs offering "101 sound effects", with 100 and 101 being indexed as two and three on track 99. The index, if used, is occasionally put on the track listing as a decimal part of the track number, such as 99.2 or 99.3.{{efn|[[Information Society (band)|Information Society]]'s ''[[Hack (album)|Hack]]'' was one of very few CD releases to do this, following a release with an equally obscure [[CD+G]] feature.}} The track and index structure of the CD were carried forward to the DVD format as title and chapter, respectively.

Tracks, in turn, are divided into timecode frames, which are further subdivided into channel-data frames.

==== Frames and timecode frames ====
{{Further|Track (optical disc)#Sector structure}}

The smallest entity in a CD is a channel-data ''frame'', which consists of 33 bytes and contains six complete 16-bit stereo samples: 24 bytes for the audio (two bytes&nbsp;× two channels&nbsp;× six samples&nbsp;= 24 bytes), eight CIRC error-correction bytes, and one [[subcode]] byte. As described in {{slink||Data encoding}}, after the EFM modulation the number of bits in a frame totals 588.

On a ''Red Book'' audio CD, data is addressed using the ''MSF scheme'', with [[timecode]]s expressed in minutes, seconds and another type of ''frames'' (mm:ss:ff), where one frame corresponds to 1/75th of a second of audio: 588 pairs of left and right samples. This timecode frame is distinct from the 33-byte channel-data frame described above, and is used for time display and positioning the reading laser. When editing and extracting CD audio, this timecode frame is the smallest addressable time interval for an audio CD; thus, track boundaries only occur on these frame boundaries. Each of these structures contains 98 channel-data frames, totaling 98&nbsp;× 24&nbsp;= 2,352 bytes of music. The CD is played at a speed of 75 frames per second, 44,100 samples and 176,400 bytes per second.

In the 1990s, [[CD-ROM]] and related [[digital audio extraction]] (DAE) technology introduced the term ''[[CD-ROM#CD-ROM format|sector]]'' to refer to each timecode frame, with each sector being identified by a sequential integer starting at zero, and with tracks aligned on sector boundaries. An audio CD sector corresponds to 2,352 bytes of decoded data. The ''Red Book'' does not refer to sectors, nor does it distinguish the corresponding sections of the disc's data stream except as ''frames'' in the MSF addressing scheme.

The following table shows the relation between tracks, timecode frames (sectors) and channel-data frames:

{| class="wikitable"
|-
! Track level
| colspan = 6 | Track N
|-
! Timecode frame and sector level
| colspan = 3 | Timecode frame and sector 1 (2,352 B of data)
| Timecode frame and sector 2 (2,352 B of data)
| ...
|-
! Channel-data frame level
| Channel-data frame 1 (24 B of data)
| ...
| Channel-data frame 98 (24 B of data)
| ...
| ...
|}