The closely spaced tracks on the readable surface of a Compact Disc cause light to diffract into a full visible colour spectrum
|Media type||Optical disc|
|Capacity||Typically up to 700 MB (up to 80 minutes audio)|
|Read mechanism||780 nm wavelength semiconductor laser|
|Developed by||Philips & Sony|
|Usage||Audio and data storage|
|Optical media types|
A Compact Disc (also known as a CD) is an optical disc used to store digital data, originally developed for storing digital audio. The CD, available on the market since late 1982, remains the standard playback medium for commercial audio recordings to the present day.
Standard CDs have a diameter of 120 mm and can hold up to 80 minutes of audio. There is also the Mini CD, with diameters ranging from 60 to 80 mm; they are sometimes used for CD singles, storing up to 24 minutes of audio.
The technology was later adapted and expanded to include data storage CD-ROM, write-once audio and data storage CD-R, rewritable media CD-RW, Super Audio CD (SACD), Video Compact Discs (VCD), Super Video Compact Discs (SVCD), PhotoCD, PictureCD, CD-i, and Enhanced CD. CD-ROMs and CD-Rs remain widely used technologies in the computer industry. The CD and its extensions have been extremely successful: in 2004, worldwide sales of CD audio, CD-ROM, and CD-R reached about 30 billion discs. By 2007, 200 billion CDs had been sold worldwide.
The compact disc is a successful spin-off of the much less successful Laserdisc technology. In 1979, Sony and Philips Consumer Electronics set up a joint task force of engineers to design a new digital audio disc. The task force, led by prominent members Kees Schouhamer Immink and Toshitada Doi (土井利忠), progressed the research into laser technology and optical discs that had been started by Philips in 1977.
After a year of experimentation and discussion, the taskforce produced the Red Book, the Compact Disc standard. Philips contributed the general manufacturing process, based on video Laserdisc technology. Philips also contributed Eight-to-Fourteen Modulation (EFM), which offers both a long playing time and a high resilience against disc defects such as scratches and fingerprints, while Sony contributed the error-correction method, CIRC. The Compact Disc Story, told by a former member of the taskforce, gives background information on the many technical decisions made, including the choice of the sampling frequency, playing time, and disc diameter. According to Philips, the Compact Disc was thus "invented collectively by a large group of people working as a team."
The first CD that was pressed in Hanover was a recording of Herbert von Karajan conducting the Alpine Symphony by Richard Strauss. In August 1982 the real pressing was ready to begin in the new factory, not far from the place where Emil Berliner had produced his first gramophone record 93 years earlier. (Deutsche Grammophon, Berliner’s company, had by now become a part of PolyGram.) CDs and Sony's CD player CDP-101 reached the market on October 1, 1982 in Japan, and early the following year in the United States and other markets. This event is often seen as the "Big Bang" of the digital audio revolution. The new audio disc was enthusiastically received, especially in the early-adopting classical music and audiophile communities and its handling quality received particular praise. As the price of players sank rapidly, the CD began to gain popularity in the larger popular and rock music markets. The first artist to sell a million copies on CD was Dire Straits, with their 1985 album Brothers in Arms. In 1986 Queen became the first artist to have their entire catalogue converted to the format.
The CD was originally thought of as an evolution of the gramophone record, rather than primarily as a data storage medium. Only later did the concept of an "audio file" arise, and the generalising of this to any data file. From its origins as a music format, Compact Disc has grown to encompass other applications. In June 1985, the CD-ROM (read-only memory) and, in 1990, CD-Recordable were introduced, also developed by Sony and Philips.
 Physical details
A Compact Disc is made from a 1.2 mm thick disc of almost pure polycarbonate plastic and weighs approximately 16 grams. A thin layer of aluminium or, more rarely, gold is applied to the surface to make it reflective, and is protected by a film of lacquer. The lacquer is normally spin coated directly on top of the reflective layer. On top of that surface, the label print is applied. Common printing methods for CDs are screen-printing and offset printing.
CD data is stored as a series of tiny indentations (pits), encoded in a tightly packed spiral track molded into the top of the polycarbonate layer. The areas between pits are known as "lands". Each pit is approximately 100 nm deep by 500 nm wide, and varies from 850 nm to 3.5 µm in length.
The spacing between the tracks, the pitch, is 1.6 µm. A CD is read by focusing a 780 nm wavelength (near infrared) semiconductor laser through the bottom of the polycarbonate layer. The change in height between pits and lands results in a difference in intensity in the light reflected. By measuring the intensity change with a photodiode, the data can be read from the disc.
The pits and lands themselves do not directly represent the zeros and ones of binary data. Instead, Non-return-to-zero, inverted (NRZI) encoding is used: a change from pit to land or land to pit indicates a one, while no change indicates a zero. This in turn is decoded by reversing the Eight-to-Fourteen Modulation used in mastering the disc, and then reversing the Cross-Interleaved Reed-Solomon Coding, finally revealing the raw data stored on the disc.
While CDs are significantly more durable than earlier audio formats, they are susceptible to damage from daily usage and environmental factors. Pits are much closer to the label side of a disc, so that defects and dirt on the clear side can be out of focus during playback. Discs consequently suffer more damage because of defects such as scratches on the label side, whereas clear-side scratches can be repaired by refilling them with plastic of similar index of refraction, or by careful polishing. Early music CDs were known to suffer from "CD rot" or "laser rot" where the internal reflective layer itself degrades. When this occurs the CD may become unplayable.
 Disc shapes and diameters
The digital data on a CD begins at the center of the disc and proceeds outwards to the edge, which allows adaptation to the different size formats available. Standard CDs are available in two sizes. By far the most common is 120 mm in diameter, with a 74 or 80-minute audio capacity and a 650 or 700 MB data capacity. This diameter has also been adopted by later formats, including Super Audio CD, DVD, HD DVD, and Blu-ray Disc. 80 mm discs ("Mini CDs") were originally designed for CD singles and can hold up to 21 minutes of music or 184 MB of data but never really became popular. Today nearly all singles are released on 120 mm CDs, which is called a Maxi single.
 "Shaped CD"
Novelty shaped CDs are also available in a number of shapes and sizes, and are mostly used for marketing. A common variant is a "business card" CD, a CD-single with portions removed at the top and bottom to more closely resemble the form-factor of a business card.
|Physical size||Audio Capacity||CD-ROM Data Capacity||Note|
|12 cm||74–80 min||650–703 MB||Standard size|
|8 cm||21–24 min||185–210 MB||Mini-CD size|
|~6 min||~55 MB||"Business card" size|
 Logical formats
 Audio CD
The logical format of an audio CD (officially Compact Disc Digital Audio or CD-DA) is described in a document produced in 1980 by the format's joint creators, Sony and Philips. The document is known colloquially as the "Red Book" after the color of its cover. The format is a two-channel 16-bit PCM encoding at a 44.1 kHz sampling rate per channel. Four-channel sound is an allowed option within the Red Book format, but has never been implemented. Monaural audio has no existing standard on a Red Book CD; mono source material is usually presented as two identical channels on a 'stereo' track.
The selection of the sample rate was primarily based on the need to reproduce the audible frequency range of 20 Hz - 20 kHz. The Nyquist–Shannon sampling theorem states that a sampling rate of more than double the maximum frequency of the signal to be recorded is needed, resulting in a 40 kHz rate. The exact sampling rate of 44.1 kHz was inherited from a method of converting digital audio into an analog video signal for storage on U-matic video tape, which was the most affordable way to transfer data from the recording studio to the CD manufacturer at the time the CD specification was being developed. The device that turns an analog audio signal into PCM audio, which in turn is changed into an analog video signal is called a PCM adaptor. This technology could store six samples (three samples per each stereo channel) in a single horizontal line. A standard NTSC video signal has 245 usable lines per field, and 59.94 fields/s, which works out at 44,056 samples/s/stereo channel. Similarly, PAL has 294 lines and 50 fields, which gives 44,100 samples/s/stereo channel. This system could either store 14-bit samples with some error correction, or 16-bit samples with almost no error correction.
There was a long debate over whether to use 14-bit (Philips) or 16-bit (Sony) quantization, and 44,056 or 44,100 samples/s (Sony) or around 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, but Sony insisted on 16-bit. In the end, 16 bits and 44.1 kilosamples per second prevailed. Philips found a way to produce 16-bit quality using their 14-bit DAC by using four times oversampling.
 Storage capacity and playing time
The partners aimed at a playing time of 60 minutes with a disc diameter of 100 mm (Sony) or 115 mm (Philips). Von Karajan suggested extending the capacity to 74 minutes to accommodate a performance of Beethoven’s 9th Symphony at the Bayreuth Festival. 
The extra 14 minute playing time subsequently required changing to a 120 mm disc. Kees 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 on one of the earliest CDs. According to a Sunday Tribune interview, the story is slightly more involved. At that time (1979) Philips owned Polygram, one of the world’s largest distributors of music. Polygram had set up a large experimental CD plant in Hanover, Germany, which could produce huge numbers of CDs having, of course, 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. Sony decided that something had to be done. The long playing time of Beethoven's Ninth imposed by Ohga was used to push Philips to accept 120 mm, so that Philips’ Polygram lost its edge on disc fabrication.
The 74-minute playing time of a CD, which was much longer than the 15 to 20 minutes per side possible with long-playing 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 an opposing situation in which the CD would actually offer fewer tracks than the LP equivalent. An example is the 1987 album Kiss Me, Kiss Me, Kiss Me by The Cure, which states in the CD liner notes: "The track Hey You!!! which appears on the double album and cassette has been omitted so as to facilitate a single compact disc." The 2006 re-release of this album saw the re-inclusion of the missing track. Another example is the original late-1980s Warner Bros. Records reissue of Fleetwood Mac's Tusk album, which substituted the long album version of "Sara" with the shorter single version. Enough complaints were lodged to eventually convince Warner Bros. to remaster the album in the mid-1990s with the original contents intact.
 Main physical parameters
The main parameters of the CD (taken from the September 1983 issue of the audio CD specification) are as follows:
- Scanning velocity: 1.2–1.4 m/s (constant linear velocity) – equivalent to approximately 500 rpm at the inside of the disc, and approximately 200 rpm at the outside edge. (A disc played from beginning to end slows down during playback.)
- Track pitch: 1.6 µm
- Disc diameter 120 mm
- Disc thickness: 1.2 mm
- Inner radius program area: 25 mm
- Outer radius program area: 58 mm
- Center spindle hole diameter: 15 mm
The program area is 86.05 cm² and the length of the recordable spiral is (86.05 cm² / 1.6 µm) = 5.38 km. With a scanning speed of 1.2 m/s, the playing time is 74 minutes, or around 650 MB of data on a CD-ROM. If the disc diameter were only 115 mm, the maximum playing time would have been 68 minutes, i.e., six minutes less. A disc with data packed slightly more densely is tolerated by most players (though some old ones fail). Using a linear velocity of 1.2 m/s and a track pitch of 1.5 µm leads to a playing time of 80 minutes, or a capacity of 700 MB. Even higher capacities on non-standard discs (up to 99 minutes) are available at least as recordables, but generally the tighter the tracks are squeezed the worse the compatibility.
 Data structure
The smallest entity in a CD is called a frame. A frame consists of 33 bytes and contains six complete 16-bit stereo samples (2 bytes × 2 channels × six samples: equals 24 bytes). The other nine bytes consist of eight Cross-Interleaved Reed-Solomon Coding error correction bytes and one subcode byte, used for control and display. Each byte is translated into a 14-bit word using Eight-to-Fourteen Modulation, which alternates with 3-bit merging words. In total there are 33 × (14 + 3) = 561 bits. A 27-bit unique synchronization word is added, so that the number of bits in a frame totals 588 (of which only 192 bits are music).
These 588-bit frames are in turn grouped into sectors. Each sector contains 98 frames, totaling 98 × 24 = 2352 bytes of music. The CD is played at a speed of 75 sectors per second, which results in 176,400 bytes per second. Divided by 2 channels and 2 bytes per sample, this results in a sample rate of 44,100 samples per second.
For CD-ROM data discs, the physical frame and sector sizes are the same. Since error concealment cannot be applied to non-audio data in case the CIRC error correction fails to recover the user data, a third layer of error correction is defined, reducing the payload to 2048 bytes per sector for the Mode-1 CD-ROM format. To increase the data-rate for Video CD, Mode-2 CD-ROM, the third layer has been omitted, increasing the payload to 2336 user-available bytes per sector, only 16 bytes (for synchronization and header data) less than available in Red-Book audio.
For the Red Book stereo audio CD, the time format is commonly measured in minutes, seconds and frames (mm:ss:ff), where one frame corresponds to one sector, or 1/75th of a second of stereo sound. Note that in this context, the term frame is erroneously applied in editing applications and does not denote the physical frame described above. In editing and extracting, the frame is the smallest addressable time interval for an audio CD, meaning that track start and end positions can only be defined in 1/75 second steps.
 Logical structure
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. 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 index 2 and 3 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 carried forward to the DVD as title and chapter, respectively.
 Manufacturing tolerances
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. Thus, in current practice, maximum CD playing time has crept higher by reducing minimum engineering tolerances; by and large, this has not unacceptably reduced reliability.
CD-Text is an extension of the Red Book specification for audio CD that allows for storage of additional text information (e.g., album name, song name, artist) on a standards-compliant audio CD. The information is stored either in the lead-in area of the CD, where there is roughly five kilobytes of space available, or in the subcode channels R to W on the disc, which can store about 31 megabytes.
 CD + Graphics
Compact Disc + Graphics (CD+G) is a special audio compact disc that contains graphics data in addition to the audio data on the disc. The disc can be played on a regular audio CD player, but when played on a special CD+G player, can output a graphics signal (typically, the CD+G player is hooked up to a television set or a computer monitor); these graphics are almost exclusively used to display lyrics on a television set for karaoke performers to sing along with.
 CD + Extended Graphics
Compact Disc + Extended Graphics (CD+EG, also known as CD+XG) is an improved variant of the Compact Disc + Graphics (CD+G) format. Like CD+G, CD+EG utilizes basic CD-ROM features to display text and video information in addition to the music being played. This extra data is stored in subcode channels R-W. Very few, if any, CD+EG discs have been published.
 Super Audio CD
Super Audio CD (SACD) is a read-only optical audio disc format aimed at providing much higher fidelity digital audio reproduction than the Red Book audio CD. Introduced in 1999, it was developed by Sony and Philips Electronics, the same companies that created the Red Book audio CD. SACD was in a format war with DVD-Audio, but neither has yet managed to replace audio CDs.
In contrast to DVD-Audio, the SACD format has the feature of being able to produce hybrid discs: in addition to the SACD audio, these discs contain a standard audio CD layer which is playable in standard CD players, thus making them backward compatible.
Compact Disc MIDI or CD-MIDI is a type of CD where MIDI format is used to store music performance data which upon playback is performed by electronic instruments that synthesize the audio that is heard. Hence unlike Red Book audio CD, these recordings are not audio.
For the first few years of its existence, the Compact Disc was a medium used purely for audio. However, in 1985 the Yellow Book CD-ROM standard was established by Sony and Philips, which defined a non-volatile optical data computer data storage medium using the same physical format as audio compact discs, readable by a computer with a CD-ROM drive.
 Video CD
Video CD (aka VCD, View CD, Compact Disc digital video) is a standard digital format for storing video on a Compact Disc. VCDs are playable in dedicated VCD players, most modern DVD-Video players, personal computers, and some video game consoles.
Overall picture quality is intended to be comparable to VHS video. Poorly compressed VCD video can sometimes be lower quality than VHS video, but VCD exhibits block artifacts rather than analog noise, and does not deteriorate further with each use, which may be preferable.
352x240 (or SIF) resolution was chosen because it is half the vertical, and half the horizontal resolution of NTSC video. 352x288 is similarly one quarter PAL/SECAM resolution. This approximates the (overall) resolution of an analog VHS tape, which, although it has double the number of (vertical) scan lines, has a much lower horizontal resolution.
 Super Video CD
Super Video CD (Super Video Compact Disc or SVCD) is a format used for storing video on standard compact discs. SVCD was intended as a successor to Video CD and an alternative to DVD-Video, and falls somewhere between both in terms of technical capability and picture quality.
SVCD has two-thirds the resolution of DVD, and over 2.7 times the resolution of VCD. One CD-R disc can hold up to 60 minutes of standard quality SVCD-format video. While no specific limit on SVCD video length is mandated by the specification, one must lower the video bit rate, and therefore quality, in order to accommodate very long videos. It is usually difficult to fit much more than 100 minutes of video onto one SVCD without incurring significant quality loss, and many hardware players are unable to play video with an instantaneous bit rate lower than 300 to 600 kilobits per second.
 Photo CD
Photo CD is a system designed by Kodak for digitizing and storing photos on a CD. Launched in 1992, the discs were designed to hold nearly 100 high quality images, scanned prints and slides using special proprietary encoding. Photo CD discs are defined in the Beige Book and conform to the CD-ROM XA and CD-i Bridge specifications as well. They are intended to play on CD-i players, Photo CD players and any computer with the suitable software irrespective of the operating system. The images can also be printed out on photographic paper with a special Kodak machine.
 Picture CD
Picture CD is another photo product by Kodak, following on from the earlier Photo CD product. It holds photos from a single roll of color film, stored at 1024×1536 resolution using JPEG compression. The product is aimed at consumers. Software to view and perform simple edits to images is included on the CD.
The Philips "Green Book" specifies the standard for interactive multimedia compact discs designed for CD-i players. This format is unusual because it hides the initial tracks which contains the software and data files used by CD-i players by omitting the tracks from the disc's TOC (table of contents). This causes audio CD players to skip the CD-i data tracks. This is different from the CD-i Ready format, which puts CD-i software and data into the pregap of track 1.
 Enhanced CD
Enhanced CD, also known as CD Extra and CD Plus, is a certification mark of the Recording Industry Association of America for various technologies that combine audio and computer data for use in both compact disc and CD-ROM players.
Replicated CDs are mass-produced initially using a hydraulic press. Small granules of raw polycarbonate plastic are fed into the press while under heat. A screw forces the liquefied plastic into the mold cavity. The mold closes with a metal stamper in contact with the disc surface. The plastic is allowed to cool and harden. Once opened, the disc substrate is removed from the mold by a robotic arm, and a 15 mm diameter center hole (called a stacking ring) is removed. The cycle time, the time it takes to "stamp" one CD, is usually 2–3 seconds.
This method produces the clear plastic blank part of the disc. After a metallic reflecting layer (usually aluminum, but sometimes gold or other metals) is applied to the clear blank substrate, the disc goes under a UV light for drying and it is ready to go to press. To prepare to press a CD, a glass master is made using a high-power laser on a device similar in principle to a CD writer. The glass master is a positive image of the desired CD surface (with the desired microscopic pits and lands). After testing, it is used to make a die by pressing it against a metal disc.
The die is a negative image of the glass master: several are typically made, depending on the number of pressing mills that are to be making the CD. The die then goes into a press and the physical image is imposed onto the blank CD, leaving a final positive image on the disc. A small amount of lacquer is then applied as a ring around the center of the disc, and a fast spin spreads it evenly over the surface. Edge protection lacquer is also applied before the disk is finished. The disc can then be printed and packed.
 Recordable CD
Recordable compact discs, CD-Rs, are injection molded with a "blank" data spiral. A photosensitive dye is then applied, after which the discs are metalized and lacquer-coated. The write laser of the CD recorder changes the color of the dye to allow the read laser of a standard CD player to see the data, just as it would with a standard stamped disc. The resulting discs can be read by most CD-ROM drives and played in most audio CD players.
CD-R recordings are designed to be permanent. Over time the dye's physical characteristics may change, however, causing read errors and data loss until the reading device cannot recover with error correction methods. The design life is from 20 to 100 years, depending on the quality of the discs, the quality of the writing drive, and storage conditions. However, testing has demonstrated such degradation of some discs in as little as 18 months under normal storage conditions. This failure is known as CD rot. CD-Rs follow the Orange Book standard.
 Recordable Audio CD
The Recordable Audio CD is designed to be used in a consumer audio CD recorder. These consumer audio CD recorders use SCMS (Serial Copy Management System), an early form of digital rights management (DRM), to conform to the AHRA (Audio Home Recording Act). The Recordable Audio CD is typically somewhat more expensive than CD-R due to (a) lower volume and (b) a 3% AHRA royalty used to compensate the music industry for the making of a copy.
 High Capacity Recordable CD
A higher density recording format that can hold about:
- 98.5 minutes of audio on a 12 cm disc (compared to about 80 minutes for Red Book audio).
- 30 minutes of audio on an 8 cm disc (compared to about 24 minutes for Red Book audio).
 ReWritable CD
CD-RW is a re-recordable medium that uses a metallic alloy instead of a dye. The write laser in this case is used to heat and alter the properties (amorphous vs. crystalline) of the alloy, and hence change its reflectivity. A CD-RW does not have as great a difference in reflectivity as a pressed CD or a CD-R, and so many earlier CD audio players cannot read CD-RW discs, although most later CD audio players and stand-alone DVD players can. CD-RWs follow the Orange Book standard.
 High Speed ReWritable CD
Due to technical limitations, the original ReWritable CD could be written no faster than 4x speed. High Speed ReWritable CD has a different design that permits writing at speeds ranging from 4x to 12x.
Original CD-RW drives can only write to original ReWritable CD discs. High Speed CD-RW drives can typically write to both original ReWritable CD discs and High Speed ReWritable CD discs. Both types of CD-RW discs can be read in most CD drives.
Even higher speed CD-RW discs, Ultra Speed (16x to 24x write speed) and Ultra Speed+ (32x write speed), are now available.
 ReWritable Audio CD
The ReWritable Audio CD is designed to be used in a consumer audio CD recorder, which won't (without modification) accept standard CD-RW discs. These consumer audio CD recorders use SCMS (Serial Copy Management System), an early form of digital rights management (DRM), to conform to the United States' AHRA (Audio Home Recording Act). The ReWritable Audio CD is typically somewhat more expensive than CD-RW due to (a) lower volume and (b) a 3% AHRA royalty used to compensate the music industry for the making of a copy.
 Copy protection
The Red Book audio specification, except for a simple 'anti-copy' bit in the subcode, does not include any serious copy protection mechanism. Starting in early 2002, attempts were made by record companies to market "copy-protected" non-standard compact discs, which cannot be ripped (copied) to hard drives or easily converted to MP3s. One major drawback to these copy-protected discs is that most will not play on computer CD-ROM drives, as well as some standalone CD players that use CD-ROM mechanisms. Philips has stated that such discs are not permitted to bear the trademarked Compact Disc Digital Audio logo because they violate the Red Book specification. Numerous copy-protection systems have been countered by readily-available, often free, software.