The compact disc (CD) is a digital optical disc data storage format co-developed by Philips and Sony to store and play digital audio recordings. It employs the Compact Disc Digital Audio (CD-DA) standard and is capable of holding uncompressed stereo audio. First released in Japan in October 1982, the CD was the second optical disc format to reach the market, following the larger LaserDisc (LD). In later years, the technology was adapted for computer data storage as CD-ROM and subsequently expanded into various writable and multimedia formats. By 2007, over 200 billion CDs (including audio CDs, CD-ROMs, and CD-Rs) had been sold worldwide.
Standard CDs have a diameter of 120 mm (4.7 in) and typically hold up to 74 minutes of audio or approximately 650 MiB (681,574,400 bytes) of data. This was later regularly extended to 80 minutes or 700 MiB (734,003,200 bytes) by reducing the spacing between data tracks, with some discs unofficially reaching up to 99 minutes or 870 MiB (912,261,120 bytes) which falls outside established specifications. Smaller variants, such as the Mini CD, range from 60 to 80 mm (2.4 to 3.1 in) in diameter and have been used for CD singles or distributing device drivers and software.
The CD gained widespread popularity in the late 1980s and early 1990s. By 1991, it had surpassed the phonograph record and the cassette tape in sales in the United States, becoming the dominant physical audio format. By 2000, CDs accounted for 92.3% of the U.S. music market share. The CD is widely regarded as the final dominant format of the album era, before the rise of MP3, digital downloads, and streaming platforms in the mid-2000s led to its decline.
Beyond audio playback, the compact disc was adapted for general-purpose data storage under the CD-ROM format, which initially offered more capacity than contemporary personal computer hard disk drives. Additional derived formats include write-once discs (CD-R), rewritable media (CD-RW), and multimedia applications such as Video CD (VCD), Super Video CD (SVCD), Photo CD, Picture CD, Compact Disc Interactive (CD-i), Enhanced Music CD, and Super Audio CD (SACD), the latter of which can include a standard CD-DA layer for backward compatibility.
A CD is made from 1.2-millimetre (0.047 in) thick, polycarbonate plastic, and weighs 14–33 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 aluminum 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 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 distance between the windings (the pitch) is 1.6 μm (measured center-to-center, not between the edges).
When playing an audio CD, a motor within the CD player spins the disc to a scanning velocity of 1.2–1.4 m/s (constant linear velocity, CLV)—equivalent to approximately 500 RPM at the inside of the disc, and approximately 200 RPM at the outside edge. The track on the CD begins at the inside and spirals outward, so 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 MiB 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 MiB. Even denser tracks are possible, with semi-standard 90 minute/800 MiB discs having 1.33 μm, and 99 minute/870 MiB having 1.26 μm, but compatibility suffers as density increases.
A CD is read by focusing a 780 nm wavelength (near infrared) semiconductor laser (early players used He–Ne 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. Because the pits are indented into the top layer of the disc and are read through the transparent polycarbonate base, the pits form bumps when read. The laser hits the disc, casting a circle of light wider than the modulated spiral track reflecting partially from the lands and partially from the top of any bumps where they are present. As the laser passes over a pit (bump), its height means that the round trip path of the light reflected from its peak is 1/2 wavelength out of phase with the light reflected from the land around it. This is because the height of a bump is around 1/4 of the wavelength of the light used, so the light falls 1/4 out of phase before reflection and another 1/4 wavelength out of phase after reflection. This causes partial cancellation of the laser's reflection from the surface. By measuring the reflected intensity change with a photodiode, a modulated signal is read back from the disc.
To accommodate the spiral pattern of data, the laser is placed on a mobile mechanism within the disc tray of any CD player. This mechanism typically takes the form of a sled that moves along a rail. The sled can be driven by a worm gear or linear motor. Where a worm gear is used, a second shorter-throw linear motor, in the form of a coil and magnet, makes fine position adjustments to track eccentricities in the disc at high speed. Some CD drives (particularly those manufactured by Philips during the 1980s and early 1990s) use a swing arm similar to that seen on a gramophone.
The pits and lands do not directly represent the 0s and 1s of binary data. Instead, non-return-to-zero, inverted encoding is used: a change from either pit to land or land to pit indicates a 1, while no change indicates a series of 0s. There must be at least two, and no more than ten 0s between each 1, 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 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 enter the CD and corrode the metal reflective layer and/or interfere with the focus of the laser on the pits, a condition known as disc rot. The fungus Geotrichum candidum has been found—under conditions of high heat and humidity—to consume the polycarbonate plastic and aluminium found in CDs.
The data integrity of compact discs can be measured using surface error scanning, which can measure the rates of different types of data errors, known as C1, C2, CU and extended (finer-grain) error measurements known as E11, E12, E21, E22, E31 and E32, of which higher rates indicate a possibly damaged or unclean data surface, low media quality, deteriorating media and recordable media written to by a malfunctioning CD writer.