Editing Compact Disc Digital Audio (section)

=== 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>