A Compact Disc (also known as a CD) is an optical disc used to store digital data. It was originally developed to store and playback sound recordings exclusively, but later expanded to encompass data storage CD-ROM, write-once audio and data storage CD-R, rewritable media CD-RW, Video Compact Discs (VCD), Super Video Compact Discs (SVCD), PhotoCD, PictureCD, CD-i, and Enhanced CD. Audio CDs have been commercially available since October 1982.
Standard CDs have a diameter of 120 mm and can hold up to 80 minutes of uncompressed audio (700 MB of data). The Mini CD has various diameters ranging from 60 to 80 mm; they are sometimes used for CD singles, storing up to 24 minutes of audio or delivering device drivers.
CD-ROMs and CD-Rs remain widely used technologies in the computer industry. The CD and its extensions are 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. In 2011, compact discs have been largely replaced by other forms of digital storage such as flash drives, with audio CD sales dropping nearly 50% from their peak in 2000.
The Compact Disc is a spin-off of Laserdisc technology. Sony first publicly demonstrated an optical digital audio disc in September 1976. In September 1978 they demonstrated an optical digital audio disc with a 150 minute playing time, and with specifications of 44,056 Hz sampling rate, 16-bit linear resolution, cross-interleaved error correction code, that were similar to those of the Compact Disc introduced in 1982. Technical details of Sony's digital audio disc were presented during the 62nd AES Convention, held on March 13–16, 1979, in Brussels. 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. On March 6, 2009, Philips received an IEEE Milestone 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."
Later in 1979, Sony and Philips Consumer Electronics (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 Philips and Sony in 1977 and 1975, respectively. 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. The taskforce consisted of around four to eight persons, though according to Philips, the Compact Disc was thus "invented collectively by a large group of people working as a team."
The first test CD was pressed in Langenhagen near Hannover, Germany, by the Polydor Pressing Operations plant. The disc contained a recording of Richard Strauss's Eine Alpensinfonie (in English language, An Alpine Symphony), played by the Berlin Philharmonic and conducted by Herbert von Karajan. The first public demonstration was on the BBC television program Tomorrow's World when The Bee Gees' album Living Eyes (1981) was played. 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. By now, Deutsche Grammophon, Berliner's company and the publisher of the Strauss recording, had become a part of PolyGram. The first CD to be manufactured at the new factory was The Visitors (1981) by ABBA. The first album to be released on CD was Billy Joel's 52nd Street, that reached the market alongside Sony's CD player CDP-101 on October 1, 1982 in Japan. Early the following year on March 2, 1983 CD players and discs (16 titles from CBS Records) were released 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 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.
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. The CD's compact format has largely replaced the audio cassette player in new automobile applications, and 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 have used the same form factor as CDs, and video players can usually play audio CDs as well. With the advent of the MP3 in the 2000s, the sales of CDs has dropped in seven out of the last eight years. In 2008, large label CD sales dropped 20%, although independent and DIY music sales may be tracking better according to figures released March 30, 2009.
A CD is made from 1.2 mm thick (.047 inches), almost-pure polycarbonate plastic and weighs 15–20 grams. From the center outward, components are: the center (spindle) hole, the first-transition area (clamping ring), the clamping area (stacking ring), the second-transition area (mirror band), the information (data) area, and the rim.
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. Common printing methods for CDs are screen-printing and offset printing.
CD data are stored as a series of 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 1.6 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 (actually ridges as seen by the laser) 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 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.
CDs are susceptible to damage from both normal use and 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 disk. Scratches on the clear side can be repaired by refilling them with similar refractive plastic, or by careful polishing.
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 mm in diameter, with a 74- or 80-minute audio capacity and a 650 or 700 MB data capacity. 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.
Novelty CDs are also available in numerous shapes and sizes, and are used chiefly for marketing. A common variant is the "business card" CD, a single with portions removed at the top and bottom making the disk resemble a business card.
|Physical size||Audio Capacity||CD-ROM Data Capacity||Note|
|12 cm||74–99 min||650–870 MB||Standard size|
|8 cm||21–24 min||185–210 MB||Mini-CD size|
|85x54 mm - 86x64 mm||~6 min||10-65 MB||"Business card" size|