|Media type||Optical disc|
|Capacity||Typically up to 700 MiB (up to 80 minutes audio)|
|Read mechanism||780 nm wavelength (infrared and red edge) semiconductor laser, 1200 Kib/s (1×)|
|Write mechanism||1200 Kib/s (1×)|
|Developed by||Philips, Sony|
|Usage||Audio and data storage|
Compact disc (CD) is a digital optical disc data storage format. The format was originally developed to store and play only sound recordings (CD-DA), but was later adapted for storage of data (CD-ROM). Several other formats were further derived from these, including write-once audio and data storage (CD-R), rewritable media (CD-RW), Video Compact Disc (VCD), Super Video Compact Disc (SVCD), Photo CD, PictureCD, CD-i, and Enhanced Music CD. Audio CDs and audio CD players have been commercially available since October 1982.
Standard CDs have a diameter of 120 millimetres (4.7 in) and can hold up to about 80 minutes of uncompressed audio or 700 MiB (actually about 703 MiB or 737 MB) of data. The Mini CD has various diameters ranging from 60 to 80 millimetres (2.4 to 3.1 in); they are sometimes used for CD singles, storing up to 24 minutes of audio or delivering device drivers.
At the time of the technology's introduction, it had much greater capacity than computer hard drives common at the time. The reverse is now true, with hard drives far exceeding the capacity of CDs.
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. Compact discs are increasingly being replaced or supplemented by other forms of digital distribution and storage, such as downloading and flash drives, with audio CD sales dropping nearly 50% from their peak in 2000.
- 1 History
- 2 Physical details
- 3 Logical formats
- 4 Manufacture
- 5 Writable compact discs
- 6 Copy protection
- 7 See also
- 8 References
- 9 Further reading
- 10 External links
The Compact Disc is an evolution of LaserDisc technology. Prototypes were developed by Philips and Sony independently from the mid-to-late 1970s. The two companies then collaborated to produce a standard format and related player technology which was made commercially available in 1982.
Digital audio optical disc prototypes
American inventor James T. Russell has been credited with inventing the first system to record digital information on an optical transparent foil which is lighted from behind by a high-power halogen lamp. Russell's patent application was first filed in 1966 and he was granted a patent in 1970. Following litigation, Sony and Philips licensed Russell's patents (then held by a Canadian company, Optical Recording Corp.) in the 1980s.
In 1974, an initiative was taken by L. Ottens, a director of the audio industry group within the Philips Technology Corporation in Eindhoven, the Netherlands. A seven-person project group was formed to develop an optical audio disc with a diameter of 20 cm with a sound quality superior to that of the large and fragile vinyl record. In March 1974, during a meeting of the audio group, two engineers from the Philips research laboratory recommended the use of a digital format on the 20 cm optical disc, because an error-correcting code could be added. It was not until 1977 that the directors of the group decided to establish a laboratory with the mission of creating a small optical digital audio disc and a small player. They chose the term "compact disc" in line with another Philips product, the compact cassette. Rather than the original 20 cm size, the diameter of this compact disc was set at 11.5 cm, the diagonal measurement of a compact cassette.
Meanwhile, Sony first publicly demonstrated an optical digital audio disc in September 1976. In September 1978, the company demonstrated an optical digital audio disc with a 150 minute playing time, 44,056 Hz sampling rate, 16-bit linear resolution, and cross-interleaved error correction code—specifications similar to those later settled upon for the standard Compact Disc format in 1980. Technical details of Sony's digital audio disc were presented during the 62nd AES Convention, held on March 13–16, 1979, in Brussels. Just before that, on March 8, 1979 Philips publicly demonstrated a prototype of an optical digital audio disc at a press conference called "Philips Introduce Compact Disc" in Eindhoven, Netherlands. Thirty years later, on March 6, 2009, Philips received an IEEE Milestone award with the following citation: "On 8 March 1979, N.V. Philips' Gloeilampenfabrieken demonstrated for the international press a Compact Disc Audio Player. The demonstration showed that it is possible by using digital optical recording and playback to reproduce audio signals with superb stereo quality. This research at Philips established the technical standard for digital optical recording systems."
Collaboration and standardization
Later in 1979, Sony and Philips set up a joint task force of engineers to design a new digital audio disc. Led by Kees Schouhamer Immink and Toshitada Doi, the research pushed forward laser and optical disc technology that began independently by the two companies. After a year of experimentation and discussion, the task force produced the Red Book CD-DA standard. First published in 1980, the standard was formally adopted by the IEC as an international standard in 1987, with various amendments becoming part of the standard in 1996.
Philips contributed the general manufacturing process, based on video LaserDisc technology. Philips also contributed eight-to-fourteen modulation (EFM), which offers a certain resilience to 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. The task force consisted of around four to eight persons, though according to Philips, the Compact Disc was "invented collectively by a large group of people working as a team."
First Red Book CDs and players
- The first test pressing was of a recording of Richard Strauss's Eine Alpensinfonie (An Alpine Symphony), played by the Berlin Philharmonic and conducted by Herbert von Karajan, who had been enlisted as an ambassador for the format in 1979.
- The first public demonstration was on the BBC television program Tomorrow's World in 1981, when the Bee Gees' album Living Eyes (1981) was played.
- The first commercial compact disc was produced on 17 August 1982. It was a recording from 1979 of Claudio Arrau performing Chopin waltzes (Philips 400 025-2). Arrau was invited to the Langenhagen plant to press the start button.
- The first popular music CD produced at the new factory was The Visitors (1981) by ABBA.
- The first album to be released on CD was Billy Joel's 52nd Street, which reached the market alongside Sony's CDP-101 CD player on 1 October 1982 in Japan.
The Japanese launch was followed in March 1983 by the introduction of CD players and discs to Europe, and North America (where CBS Records released sixteen titles). 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 gradually came down, 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 its 1985 album Brothers in Arms. The first major artist to have his entire catalogue converted to CD was David Bowie, whose 15 studio albums were made available by RCA Records in February 1985, along with four Greatest Hits albums. In 1988, 400 million CDs were manufactured by 50 pressing plants around the world.
Further development and decline
The CD was planned to be the successor of the gramophone record for playing music, rather than primarily as a data storage medium. From its origins as a musical format, CDs have grown to encompass other applications. In June 1985, the computer-readable CD-ROM (read-only memory) and, in 1990, CD-Recordable were introduced, also developed by both Sony and Philips. Recordable CDs are an alternative to tape for recording music and copying music albums without defects introduced in compression used in other digital recording methods. Other newer video formats such as DVD and Blu-ray use the same physical geometry as CD, and most DVD and Blu-ray players are backward compatible with Audio CD.
By the early 2000s, the CD had largely replaced the audio cassette player as standard equipment in new automobiles, with 2010 being the final model year for any car in the US to have a factory-equipped cassette player. With the increasing popularity of portable digital audio players and solid state music storage, CD players are being phased out of automobiles in favor of minijack auxiliary inputs and connections to USB devices.
Meanwhile, with the advent and popularity of digital audio formats, such as the 256 kbit m4a, sales of CDs began dropping in the 2000s. For example, during the eight-year period ending in 2008, despite overall growth in music sales and one anomalous year of increase, major-label CD sales declined overall by 20%  although independent and DIY music sales may be tracking better according to figures released March 30, 2009 and CDs still continue to sell greatly.
A CD is made from 1.2 millimetres (0.047 in) thick, polycarbonate plastic and weighs 15–20 grams. From the center outward, components are: the center spindle hole (15 mm), the first-transition area (clamping ring), the clamping area (stacking ring), the second-transition area (mirror band), the program (data) area, and the rim. The inner program area occupies a radius from 25 to 58 mm.
A thin layer of aluminium or, more rarely, gold is applied to the surface making it reflective. The metal is protected by a film of lacquer normally spin coated directly on the reflective layer. The label is printed on the lacquer layer, usually by screen printing or offset printing.
CD data is represented as tiny indentations known as "pits", encoded in a spiral track moulded 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 distance between the tracks, the pitch, is 1.5 µm.
Scanning velocity is 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 its rotation rate during playback.)
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 650 MB of data on a CD-ROM. 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 narrower track pitch of 1.5 µm increases the playing time to 80 minutes, and data capacity to 700 MB.
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 the way the light is 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 encoding is used: a change from pit to land or land to pit indicates a one, while no change indicates a series of zeros. There must be at least two and no more than ten zeros between each one, which is defined by the length of the pit. 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. These encoding techniques (defined in the Red Book) were originally designed for the CD Digital Audio, but they later became a standard for almost all CD formats (such as CD-ROM).
CDs are susceptible to damage during handling and from environmental exposure. Pits are much closer to the label side of a disc, enabling defects and contaminants on the clear side to be out of focus during playback. Consequently, CDs are more likely to suffer damage on the label side of the disc. Scratches on the clear side can be repaired by refilling them with similar refractive plastic, or by careful polishing. The edges of CDs are sometimes incompletely sealed, allowing gases and liquids to corrode the metal reflective layer and to interfere with the focus of the laser on the pits. The fungus Geotrichum candidum, found in Belize, has been found to consume the polycarbonate plastic and aluminium found in CDs.
Disc shapes and diameters
The digital data on a CD begins at the center of the disc and proceeds toward 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 millimetres (4.7 in) in diameter, with a 74- or 80-minute audio capacity and a 650 or 700 MB (737,280,000 bytes) data capacity. This capacity was reportedly specified by Sony executive Norio Ohga so as to be able to contain the entirety of London Philharmonic Orchestra's recording of Beethoven's Ninth Symphony on one disc. This diameter has been adopted by subsequent 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 24 minutes of music or 210 MB of data but never became popular. Today, nearly every single is released on a 120 mm CD, called a Maxi single.
|Physical size||Audio Capacity||CD-ROM Data Capacity||Definition|
|120 mm||74–80 min||650–700 MB||Standard size|
|80 mm||21–24 min||185–210 MB||Mini-CD size|
|80x54 mm – 80x64 mm||~6 min||10-65 MB||"Business card" size|
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 CD-DA 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 was to be an allowable 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 thus is usually presented as two identical channels in a standard Red Book stereo track (i.e. mirrored mono); an MP3 CD, however, can have audio file formats with mono sound.
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.
Compact Disc + Graphics 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. The CD+G format takes advantage of the channels R through W. These six bits store the graphics information.
CD + 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 high-resolution read-only optical audio disc format that was designed to provide higher fidelity digital audio reproduction than the Red Book. Introduced in 1999, it was developed by Sony and Philips, the same companies that created the Red Book. SACD was in a format war with DVD-Audio, but neither has replaced audio CDs. The SACD standard is referred to the Scarlet Book standard.
Titles in the SACD format can be issued as hybrid discs; these discs contain the SACD audio stream as well as a standard audio CD layer which is playable in standard CD players, thus making them backward compatible.
CD-MIDI is a format used to store music-performance data which upon playback is performed by electronic instruments that synthesize the audio. Hence, unlike the original Red Book CD-DA, these recordings are not digitally sampled audio recordings. The CD-MIDI format is defined as an extension to the original Red Book.
For the first few years of its existence, the CD 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 (VCD)
Video CD (VCD, View CD, and Compact Disc digital video) is a standard digital format for storing video media on a CD. 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 media on standard compact discs. SVCD was intended as a successor to VCD 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, 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 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 CDs 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. This format is not to be confused with Kodak Picture CD, which is a consumer product in CD-ROM format.
The Philips Green Book specifies a standard for interactive multimedia compact discs designed for CD-i players (1993). CD-i discs can contain audio tracks which can be played on regular CD players, but CD-i discs are not compatible with most CD-ROM drives and software. The CD-i Ready specification was later created to improve compatibility with audio CD players, and the CD-i Bridge specification was added to create CD-i compatible discs than can be accessed by regular CD-ROM drives.
Enhanced Music CD (CD+)
Enhanced Music CD, also known as CD Extra and CD Plus, is a format which combines audio tracks and data tracks on the same disc by putting audio tracks in a first session and data in a second session. It was developed by Philips and Sony and it is defined in the Blue Book.
Vinyl Disc is the hybrid of a standard Audio CD and the vinyl record. The vinyl layer on the disc's label side can hold approximately three minutes of music.
Replicated CDs are mass-produced initially using a hydraulic press. Small granules of heated raw polycarbonate plastic are fed into the press. 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 created. The time it takes to "stamp" one CD, is usually 2 to 3 seconds.
This method produces the clear plastic blank part of the disc. After a metallic reflecting layer (usually aluminium, but sometimes gold or other metal) is applied to the clear blank substrate, the disc goes under a UV light for curing and it is ready to go to press. To prepare to press a CD, a glass master is made, using a high-powered laser on a device similar 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: typically, several are made, depending on the number of pressing mills that are to make the CD. The die then goes into a press and the physical image is transferred to the blank CD, leaving a final positive image on the disc. A small amount of lacquer is applied as a ring around the center of the disc, and rapid spinning spreads it evenly over the surface. Edge protection lacquer is applied before the disc is finished. The disc can then be printed and packed.
Manufactured CDs that are sold in stores are sealed via a process called "polywrapping" or shrink wrapping.
The most expensive part of a CD is the jewel case. In 1995, material costs were 30 cents for the jewel case and 10 to 15 cents for the CD. Wholesale cost of CDs was $0.75 to $1.15, which retailed for $16.98. On average the store received 35 percent of the retail price, the record company 27 percent, the artist 16 percent, the manufacturer 13 percent, and the distributor 9 percent. When 8-track tapes, cassette tapes, and CDs were introduced, each was marketed at a higher price than the format they succeeded, even though the cost to produce the media was reduced. This was done because the apparent value increased. This continued from vinyl to CDs but was broken when Apple marketed MP3s for $0.99, and albums for $9.99. The incremental cost, though, to produce an MP3 is infinitely small.
Writable compact discs
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-Rs follow the Orange Book standard.
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 disc rot, for which there are several, mostly environmental, reasons.
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 lower production volume and a 3% AHRA royalty used to compensate the music industry for the making of a copy.
High-capacity recordable CD is a higher density recording format that can hold 90 or 99 minutes of audio on a 12 cm (5 in) disc (compared to about 80 minutes for Red Book audio), or 30 minutes of audio on an 8 cm (3 in) disc (compared to about 24 minutes for Red Book audio). The higher capacity is incompatible with some recorders and recording software.
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.
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 the Serial Copy Management System (SCMS), an early form of digital rights management (DRM), to conform to the United States' Audio Home Recording Act (AHRA). 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.
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 CDs. High Speed CD-RW drives can typically write to both original ReWritable CDs and High Speed ReWritable CDs. Both types of CD-RW discs can be read in most CD drives. Higher speed CD-RW discs, Ultra Speed (16x to 24x write speed) and Ultra Speed+ (32x write speed), are now available.
The Red Book audio specification, except for a simple 'anti-copy' statement in the subcode, does not include any copy protection mechanism. Known at least as early as 2001, attempts were made by record companies to market "copy-protected" non-standard compact discs, which cannot be ripped, or copied, to hard drives or easily converted to MP3s. One major drawback to these copy-protected discs is that most will not play on either computer CD-ROM drives, or 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 specifications. Numerous copy-protection systems have been countered by readily available, often free, software.
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