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=== Data access from computers ===
=== Data access from computers ===
Unlike on a [[DVD]] or [[CD-ROM]], there are no "[[computer file|file]]s" 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, [[Microsoft Windows|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.
Unlike on a [[DVD]] or [[CD-ROM]], there are no "[[computer file|file]]s" 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, [[Microsoft Windows|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 [[AIFF]]-extension, which can be copied directly, randomly and individually by track as if it were actual files, in reality macOS preforms it's own as-needed-rips in the background completely transparent to the user. The copied tracks are fully playable and editable on the users computer, and save for the AIFF-container are for all intents and purposes the unaltered PCM data as on the CD it self.


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 [[Audio Interchange File Format|AIFF]], which simply preface the LPCM data with a short [[header (computing)|header]]; [[FLAC]], [[Apple Lossless|ALAC]], and [[Windows Media Audio#Windows Media Audio Lossless|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 compression|lossy]], [[perceptual audio coder|perceptual coding]] formats like [[MP3]], [[Advanced Audio Coding|AAC]], [[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.
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 [[Audio Interchange File Format|AIFF]], which simply preface the LPCM data with a short [[header (computing)|header]]; [[FLAC]], [[Apple Lossless|ALAC]], and [[Windows Media Audio#Windows Media Audio Lossless|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 compression|lossy]], [[perceptual audio coder|perceptual coding]] formats like [[MP3]], [[Advanced Audio Coding|AAC]], [[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.

Revision as of 22:46, 23 October 2020

Compact Disc Digital Audio
Media typeOptical disc
Encoding2 channels of LPCM audio, each signed 16-bit values sampled at 44100 Hz
Capacityup to 74–80 minutes (up to 24 minutes for mini 8 cm CD)
Read mechanismSemiconductor laser (780 nm wavelength)
StandardIEC 60908
Developed bySony & Philips
UsageAudio storage
Released1980
Optical discs
General
Optical media types
Standards
See also

Compact Disc Digital Audio (CDDA or CD-DA), also known as Audio CD, is the standard format for audio compact discs. The standard is defined in the Red Book, one of a series of Rainbow Books (named for their binding colors) that contain the technical specifications for all CD formats.

Standard

The Red Book specifies the physical parameters and properties of the CD, the optical parameters, deviations and error rate, modulation system (eight-to-fourteen modulation, EFM) and error correction facility (cross-interleaved Reed–Solomon coding, CIRC), and the eight subcode channels. These parameters are common to all compact discs and used by all logical formats: audio CD, CD-ROM, etc. The standard also specifies the form of digital audio encoding (2-channel signed 16-bit Linear PCM sampled at 44,100 Hz). Although rarely used, the specification allows for discs to be mastered with a form of emphasis.

The first edition of the Red Book was released in 1980 by Philips and Sony;[1][2] it was adopted by the Digital Audio Disc Committee and ratified by the International Electrotechnical Commission (IEC) Technical Committee 100, as an international standard in 1987 with the reference IEC 60908.[3] The second edition of IEC 60908 was published in 1999[4] and it replaces the first edition, amendment 1 (1992) and the corrigendum to amendment 1. The IEC 60908 however does not contain all the information for extensions that is available in the Red Book, such as the details for CD-Text, CD+G and CD+EG.[5][6]

The standard is not freely available and must be licensed. It is available from Philips and the IEC. As of 2013, Philips outsources licensing of the standard to Adminius, which charges US$100 for the Red Book, plus US$50 each for the Subcode Channels R-W and CD Text Mode annexes.[7]

Audio format

The audio contained in a CD-DA consists of two-channel signed 16-bit Linear PCM sampled at 44,100 Hz.

The sampling rate is adapted from that attained when recording digital audio on a PAL (or NTSC) videotape with a PCM adaptor, an earlier way of storing digital audio.[8] An audio CD can represent frequencies up to 22.05 kHz, the Nyquist frequency of the 44.1 kHz sample rate.

There was a long debate over the use of 16-bit (Sony) or 14-bit (Philips) quantization, and 44,056 or 44,100 samples/s (Sony) or approximately 44,000 samples/s (Philips). When the Sony/Philips task force designed the Compact Disc, Philips had already developed a 14-bit D/A converter (DAC), but Sony insisted on 16-bit. In the end Sony won, so 16 bits and 44.1 kilosamples per second prevailed. Philips found a way to produce 16-bit quality using its 14-bit DAC by using four times oversampling.[9]

Some CDs are mastered with pre-emphasis, an artificial boost of high audio frequencies. The pre-emphasis improves the apparent signal-to-noise ratio by making better use of the channel's dynamic range. On playback, the player applies a de-emphasis filter to restore the frequency response curve to an overall flat one. Pre-emphasis time constants are 50µs and 15µs (9.49 dB boost at 20 kHz), and a binary flag in the disc subcode instructs the player to apply de-emphasis filtering if appropriate. Playback of such discs in a computer or 'ripping' to wave files typically does not take into account the pre-emphasis, so such files play back with a distorted frequency response.[citation needed]

Storage capacity and playing time

The creators of the CD originally aimed at a playing time of 60 minutes with a disc diameter of 100 mm (Sony) or 115 mm (Philips).[10] Sony vice-president Norio Ohga suggested extending the capacity to 74 minutes to accommodate the recording of Wilhelm Furtwängler conducting Ludwig van Beethoven's Ninth Symphony at the 1951 Bayreuth Festival.[11][12] The additional 14-minute playing time subsequently required changing to a 120 mm disc. Kees Schouhamer Immink, Philips' chief engineer, however, denies this, claiming that the increase was motivated by technical considerations, and that even after the increase in size, the Furtwängler recording would not have fit onto one of the earliest CDs.[9][10]

According to a Sunday Tribune interview,[13] the story is slightly more involved. In 1979, Philips owned PolyGram, one of the world's largest distributors of music. PolyGram had set up a large experimental CD plant in Hannover, Germany, which could produce huge numbers of CDs having a diameter of 115 mm. Sony did not yet have such a facility. If Sony had agreed on the 115-mm disc, Philips would have had a significant competitive edge in the market. The long playing time of Beethoven's Ninth Symphony imposed by Ohga was used to push Philips to accept 120 mm, so that Philips' PolyGram lost its edge on disc fabrication.[13]

The 74-minute playing time of a CD, which is longer than the 22 minutes per side[14][15] typical of long-playing (LP) vinyl albums, was often used to the CD's advantage during the early years when CDs and LPs vied for commercial sales. CDs would often be released with one or more bonus tracks, enticing consumers to buy the CD for the extra material. However, attempts to combine double LPs onto one CD occasionally resulted in the opposite situation in which the CD would instead offer less audio than the LP. One such example was with DJ Jazzy Jeff & The Fresh Prince's double-album He's the DJ, I'm the Rapper, in which initial CD releases of the album had multiple tracks edited down for length in order to fit on a single disc; recent CD reissues package the album across two discs as a result. The emergence of 80-minute CDs allowed for some double albums that were previously edited for length or packaged as double-CDs to be re-released on a single disc, such as 1999 by Prince in the case of the former and Tommy by the Who in the case of the latter.

Playing times beyond 74 minutes are achieved by decreasing track pitch (the distance separating the track as it spirals the disc) in violation of strict Red Book standards. However, most players can still accommodate the more closely spaced data if it is still within Red Book tolerances.[16] Current manufacturing processes allow an audio CD to contain up to 80 minutes (variable from one replication plant to another) without requiring the content creator to sign a waiver releasing the plant owner from responsibility if the CD produced is marginally or entirely unreadable by some playback equipment. In current practice, maximum CD playing time has crept higher by reducing minimum engineering tolerances.

This table shows the progression in the maximum duration of released audio CDs:

Title Artist Label Released Time
Mission of Burma (compilation) Mission of Burma Rykodisc 1988 80:08[17]
Proclamation (bass trombone recital) Douglas Yeo with Black Dyke Band Doyen DOY CD 055 1996 80:17[18]
Tchaikovsky's The Nutcracker Kirov Orchestra cond. Valery Gergiev Philips/Polygram 462 114-2 1998 81:14[citation needed]
Bruckner's Fifth (live) Munich Philharmonic cond. Christian Thielemann Deutsche Grammophon 477 5377 2004 82:34[19]
Chopin & Schumann Etudes Valentina Lisitsa Decca 478 7697 2014 85:16[20]
So80s Presents Alphaville Alphaville Curated By Blank & Jones Soulfood 2014 85:10 and 85:57[21]
Mozart Violin Concertos (Mozart 225 Box Set, CD75) Various Artists Decca / Deutsche Grammophon 478 9864 2016 86:30[22]

Technical specifications

Data encoding

Each audio sample is a signed 16-bit two's complement integer, with sample values ranging from −32768 to +32767. The source audio data is divided into frames, containing twelve 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, as a whole, 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 errors. Therefore, a physical frame on the disc will actually contain information from multiple logical audio frames. This process adds 64 bits of error correction data to each frame. After this, 8 bits of subcode or subchannel 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 generate 33-bytes long frames, called "channel-data" frames. These frames are then modulated through eight-to-fourteen modulation (EFM), where each 8-bit word is replaced with a corresponding 14-bit word designed to reduce the number of transitions between 0 and 1. This reduces the density of 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 × (14 + 3) = 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 only 192 bits music).

The frames of channel data are finally written to disc physically in the form of pits and lands, with each pit or land representing a series of zeroes, and with the transition points—the edge of each pit—representing 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 sector structure used on audio CDs and video CDs (Mode 2 Form 2) than on data discs (Mode 1 or Mode 2 Form 1), C2 errors are not correctable and signify data loss.[23][24] Even with uncorrectable errors, a compact disc player interpolates the missing information with the aim of making the damage unhearable.[25]

Data structure

This image of a CD-R demonstrates 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 end of the data region and the lead-out can actually be anywhere, depending on how much data is recorded. Data-free areas of the disc and silent portions of the spiral reflect light differently, sometimes allowing track boundaries to be seen.

The audio data stream in an audio CD is continuous, but has three parts. The main portion, which is 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 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 drives. Nonstandard or corrupted TOC records are abused as a form of CD/DVD copy protection, in e.g. the key2Audio scheme.

Tracks

Main article: Track (CD) § Audio tracks

The largest entity on a CD is called a track. A CD can contain up to 99 tracks (including a data track for mixed mode discs). Each track can in turn have up to 100 indexes, though players which handle this feature are rarely found outside of pro audio, particularly radio broadcasting[citation needed]. The vast majority of songs are recorded under index 1, with the pre-gap being index 0. Sometimes hidden tracks are placed at the end of the last track of the disc, often using index 2 or 3. 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. (Information Society's 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 (or sectors), which are further subdivided into channel-data frames.

Frames and timecode frames

Main article: Track (CD) § 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 × two channels × six samples = 24 bytes), eight CIRC error-correction bytes, and one subcode byte. As described in the "Data encoding" section, 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 timecodes expressed in minutes, seconds and another type of frames (mm:ss:ff), where one frame corresponds to 1/75 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 × 24 = 2,352 bytes of music. The CD is played at a speed of 75 frames (or sectors) per second, thus 44,100 samples or 176,400 bytes per second.

In the 1990s, CD-ROM and related Digital Audio Extraction (DAE) technology introduced the term sector to refer to each timecode frame, with each sector being identified by a sequential integer number 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:

Track level Track N
Timecode frame or sector level Timecode frame or sector 1 (2,352 b of data) Timecode frame or 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 kbit/s) or 176,400 bytes per second; 2 channels × 44,100 samples per second per channel × 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 containing 1 second of audio. For comparison, the bit rate of a "1×" CD-ROM is defined as 2,048 bytes per sector × 75 sectors per second = 153,600 bytes per second. The remaining 304 bytes in a sector are used for additional data error correction.

Data access from computers

Unlike on a DVD or CD-ROM, there are no "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 systems, 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 on macOS presents the CD's content as an actual set of files, with the AIFF-extension, which can be copied directly, randomly and individually by track as if it were actual files, in reality macOS preforms it's own as-needed-rips in the background completely transparent to the user. The copied tracks are fully playable and editable on the users computer, and save for the AIFF-container are for all intents and purposes the unaltered PCM data as on the CD it self.

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 formats for this purpose include WAV and AIFF, which simply preface the LPCM data with a short header; FLAC, 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 coding formats like MP3, AAC, 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.

Format variations

Recording publishers have created CDs that violate the Red Book standard. Some do so for the purpose of copy prevention, using systems like Copy Control. Some do so for extra features such as DualDisc, which includes both a CD layer and a DVD layer whereby the CD layer is much thinner, 0.9 mm, than required by the Red Book, which stipulates a nominal 1.2 mm, but at least 1.1 mm. Philips and many other companies have stated that including the Compact Disc Digital Audio logo on such non-conforming discs may constitute trademark infringement.

Super Audio CD was a standard published in 1999 that aimed to provide better audio quality in CDs, but it never became very popular. DVD Audio, an advanced version of the audio CD, emerged in 1999.[26] The format was designed to feature audio of higher fidelity. It applies a higher sampling rate and uses 650 nm lasers.

Main article: Compact Disc and DVD copy protection

There have been moves by the recording industry to make audio CDs (Compact Disc Digital Audio) unplayable on computer CD-ROM drives, to prevent the copying of music. This is done by intentionally introducing errors onto the disc that the embedded circuits on most stand-alone audio players can automatically compensate for, but which may confuse CD-ROM drives. Consumer rights advocates as of October 2001 pushed to require warning labels on compact discs that do not conform to the official Compact Disc Digital Audio standard (often called the Red Book) to inform consumers which discs do not permit full fair use of their content.

In 2005, Sony BMG Music Entertainment was criticized when a copy protection mechanism known as Extended Copy Protection (XCP) used on some of their audio CDs automatically and surreptitiously installed copy-prevention software on computers (see Sony BMG copy protection rootkit scandal). Such discs are not legally allowed to be called CDs or Compact Discs because they break the Red Book standard governing CDs, and Amazon.com for example describes them as "copy protected discs" rather than "compact discs" or "CDs".

See also

References

  1. ^ "How the CD was developed". BBC News. 17 August 2007. Retrieved 17 August 2007.
  2. ^ "Philips Compact Disc". Philips Historical Products. Archived from the original on 25 April 2016. Retrieved 24 January 2016.
  3. ^ IEC 60908:1987 Compact disc digital audio system
  4. ^ IEC 60908:1999 Audio recording - Compact disc digital audio system (PDF)
  5. ^ Approved Compact Disc Logo configurations
  6. ^ Specs for Freeware Developers Archived 1 May 2012 at the Wayback Machine
  7. ^ "CD Products". lscdweb.com. Retrieved 24 May 2013.
  8. ^ 2-35] Why 44.1KHz? Why not 48KHz?
  9. ^ a b K. Schouhamer Immink (1998). "Compact Disc Story". Journal of the Audio Engineering Society. 46 (5): 458–460. Retrieved 6 February 2018.
  10. ^ a b K. Schouhamer Immink (2007). "Shannon, Beethoven, and the Compact Disc". IEEE Information Theory Society Newsletter. 57: 42–46. Retrieved 6 February 2018.
  11. ^ Philips. "Beethoven's Ninth Symphony of Greater Importance than Technology". Retrieved 9 February 2007.[permanent dead link‍]
  12. ^ AES. "AES Oral History Project: Kees A.Schouhamer Immink". Retrieved 29 July 2008.
  13. ^ a b Cassidy, Fergus (23 October 2005). "Great Lengths". Sunday Tribune. Archived from the original (reprint) on 12 October 2007. Retrieved 7 January 2017.
  14. ^ Hoffmann, Frank; Ferstler, Howard (2005). Encyclopedia of Recorded Sound. CRC Press. p. 1289. ISBN 978-0-415-93835-8.
  15. ^ Goldmark, Peter. Maverick inventor; My Turbulent Years at CBS. New York: Saturday Review Press, 1973.
  16. ^ Andy McFadden (9 January 2010). "CD-Recordable FAQ". Retrieved 30 December 2010.
  17. ^ "Mission of Burma 1988 Rykodisc compilation information". discogs.com. Retrieved 18 January 2011. This Rykodisc release was the first compact disc to contain 80 minutes of music; 78 minutes had previously been the longest length possible to encode on a CD.
  18. ^ "Complete Program Notes for Proclamation". Retrieved 13 March 2016.
  19. ^ "BRUCKNER: Symphony No. 5 in B flat major (original version) — Munich Philharmonic/Christian Thielemann — DGG - Audiophile Audition". Audiophile Audition. Archived from the original on 16 May 2013.
  20. ^ "Valentina Lisitsa - Études - Amazon.com Music". www.amazon.com. Retrieved 1 January 2017.
  21. ^ https://www.discogs.com/Alphaville-Curated-By-Blank-Jones-So80s-Soeighties-Presents-Alphaville/master/780651. {{cite web}}: Missing or empty |title= (help)
  22. ^ McElhearn, Kirk (12 May 2017). "CDs Longer than 80 Minutes Are Becoming More Common". Kirkville. Retrieved 12 May 2017.
  23. ^ "Fehlerprotoll / Error Check CD" (in German).
  24. ^ Wiethoff, Diplominformatiker, André (15 April 2011). "Exact Audio Copy – Audiodaten von optischen Speichermedien extrahieren" (PDF) (in German). Vortrag am 15.4.2011 an der Hochschule Rhein-Main. pp. 51–53. Retrieved 9 August 2020. {{cite web}}: Check |last1= value (help)CS1 maint: location (link)
  25. ^ "CD". cs.stanford.edu. Stanford.edu. Retrieved 9 August 2020. 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.
  26. ^ Taylor, Jim. "DVD FAQ". DVD Demystified. Archived from the original on 22 August 2009. Retrieved 21 August 2012.
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