Editing Compact Disc Digital Audio (section)

== Technical specifications ==

=== Data encoding ===
Each audio sample is a [[Signedness|signed]] 16-bit [[two's complement]] [[Integer (computer science)|integer]], which has sample values ranging from −32768 to +32767. The source audio data is divided into frames, containing twelve [[Sampling (signal processing)|samples]] each (six left and six right samples, alternating), for a total of 192 bits (24 bytes) of audio data per frame.

This stream of audio frames is then subjected to CIRC encoding, which segments and rearranges the data and expands it with error correction codes in a way that allows occasional read errors to be detected and corrected. CIRC encoding interleaves the audio frames throughout the disc over several consecutive frames so that the information will be more resistant to [[burst error]]s. Therefore, a physical frame on the disc will actually contain information from multiple logical audio frames. This process adds 64 bits of error correction codes to each frame. After this, 8 bits of [[subcode data]] are added to each of these encoded frames, which is used for control and addressing when playing the CD.

CIRC encoding plus the subcode byte generates 33-byte long frames, called ''channel-data'' frames. These frames are then modulated through [[eight-to-fourteen modulation]] (EFM), where each 8-bit byte is replaced with a corresponding 14-bit word designed to reduce the number of transitions between 0 and 1. This reduces the density of [[Compact disc#Physical details|physical pits]] on the disc and provides an additional degree of error tolerance. Three ''merging'' bits are added before each 14-bit word for disambiguation and synchronization. In total, there are 33&nbsp;× (14&nbsp;+ 3)&nbsp;= 561 bits. A 27-bit word (a 24-bit pattern plus 3 merging bits) is added to the beginning of each frame to assist with synchronization, so the reading device can locate frames easily. With this, a frame ends up containing 588 bits of ''channel data'' which are decoded to 192 bits of digital audio.

The frames of channel data are finally written to disc physically in the form of [[Compact disc#Physical details|pits and lands]], with each pit or land representing a series of zeroes, and with the transition points—the edge of each pit—representing a 1. A ''Red Book''-compatible [[CD-R]] has pit-and-land-shaped spots on a layer of organic dye instead of actual pits and lands; a laser creates the spots by altering the reflective properties of the dye.

Due to the weaker error correction [[CD-ROM#Sector structure|sector structure]] used on audio CDs and [[video CD]]s (''[[CD-ROM#Mode 2 Form 2|Mode 2 Form 2]]'') than on data discs (''Mode 1'' or ''Mode 2 Form 1''), [[C2 error]]s are not correctable and signify data loss.<ref>{{cite web  |title=Fehlerprotoll / Error Check CD  |url=https://just-add-grooves.de/faq-lexikon-cd-vinyl/94-fehlerprotoll-error-check-cd |author1=Alex |website=Just Add Grooves |language=de-de  |access-date=9 August 2020  |archive-date=9 August 2020  |archive-url=https://web.archive.org/web/20200809161953/https://just-add-grooves.de/faq-lexikon-cd-vinyl/94-fehlerprotoll-error-check-cd  |url-status=live  }}</ref><ref name=wiethoff>{{cite web |last1=Wiethoff |first1=André |title=Exact Audio Copy: Audiodaten von optischen Speichermedien extrahieren |url=https://www.hs-rm.de/fileadmin/persons/khofmann/Gastvortraege/Vortragsfolien/20110415-wiethoff.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://www.hs-rm.de/fileadmin/persons/khofmann/Gastvortraege/Vortragsfolien/20110415-wiethoff.pdf |archive-date=2022-10-09 |url-status=live |access-date=9 August 2020 |website=Hochschule Rhein-Main |pages=51–53 |language=de |date=15 April 2011}}</ref> Even with uncorrectable errors, a compact disc player uses [[error concealment]] with the aim of making the damage unhearable.<ref>{{cite web |title=CD |url=https://cs.stanford.edu/people/eroberts/cs201/projects/1997-98/optical-media/cd2.html |website=cs.stanford.edu |publisher=[[Stanford.edu]] |access-date=9 August 2020 |language=en |quote=An added feature of audio CD's is that in the event of damage, the missing data can be interpolated; that is to say, the information follows a predictable pattern that allows the missing value to be guessed at. So if an audio CD is damaged by dirt or a scratch, the missing data can be averaged from a pattern with no noticeable difference to the listener. This is something the next technology in optical digital memory, CD-ROM, cannot do because an executable program's data doesn't follow a natural law. An interpolation-based guess isn't just slightly different; it's completely wrong. Because of this precision, CD-ROM drives for PC's came later and much more expensive than audio. |archive-date=9 August 2020 |archive-url=https://web.archive.org/web/20200809203424/https://cs.stanford.edu/people/eroberts/cs201/projects/1997-98/optical-media/cd2.html |url-status=live }}</ref>

=== 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)
| ...
| ...
|}

=== Bit rate ===
The audio [[bit rate]] for a ''Red Book'' audio CD is 1,411,200 [[bits per second]] (1,411&nbsp;kbit/s) or 176,400 [[bytes per second]]; 2 channels&nbsp;× 44,100 samples per second per channel&nbsp;× 16 bits per sample. Audio data coming in from a CD is contained in sectors, each sector being 2,352 bytes, and with 75 sectors representing 1 second of audio. For comparison, the bit rate of an original speed CD-ROM is 2,048 bytes per sector&nbsp;× 75 sectors per second&nbsp;= 153,600 bytes per second. The remaining 304 bytes in a CD-ROM sector are used for additional data error correction.<!--[[User:Kvng/RTH]]-->

=== Data access from computers ===
Unlike on a [[DVD]] or CD-ROM, there are no "[[computer file|files]]" on a ''Red Book'' audio CD; there is only one continuous stream of [[LPCM]] audio data, and a parallel, smaller set of 8 [[subcode]] data streams. Computer [[operating system]]s, however, may provide access to an audio CD as if it contains files. For example, [[Windows]] represents the CD's Table of Contents as a set of [[Compact Disc Audio track]] (CDA) files, each file containing indexing information, not audio data. By contrast however, [[Finder (software)|Finder]] on [[macOS]] presents the CD's content as an actual set of files, with the [[Audio Interchange File Format|AIFF]]-extension, which can be copied directly, randomly and individually by track as if it were actual files. In reality, macOS performs its own as-needed-rips in the background completely transparent to the user. The copied tracks are fully playable and editable on the user's computer.

In a process called [[ripping]], digital audio extraction software can be used to read CD-DA audio data and store it in files. Common [[audio file format]]s for this purpose include [[WAV]] and AIFF, which simply preface the LPCM data with a short [[header (computing)|header]]; [[FLAC]], [[Apple Lossless|ALAC]], and [[Windows Media Audio Lossless]], which compress the LPCM data in ways that conserve space yet allow it to be restored without any changes; and various [[lossy]], [[perceptual audio coder|perceptual coding]] formats like [[MP3]], [[Advanced Audio Coding|AAC]], and [[Opus (audio format)|Opus]], which modify and compress the audio data in ways that irreversibly change the audio, but that exploit features of human hearing to make the changes difficult to discern.