page_title: Compact disc
section_title: Physical details
hierarchical_section_title: Compact disc / Physical details
caption_attribution_description: English: This is a photomicrograph of the pits at the inner edge of a CD-ROM taken with an Omax A3590U camera through an Omax A3RDF50 0.50x fixed adapter, Ernst Leitz Wetzlar 1.25x microscope body tube, and 3.5x Leitz Wetzlar objective lens; 2 second exposure under visible fluorescent light.
context_page_description: Compact disc is a digital optical disc data storage format that was co-developed by Philips and Sony and released in 1982. The format was originally developed to store and play only digital audio recordings but was later adapted for storage of data. Several other formats were further derived from these, including write-once audio and data storage, rewritable media, Video CD, Super Video CD, Photo CD, PictureCD, Compact Disc-Interactive, and Enhanced Music CD. The first commercially available audio CD player, the Sony CDP-101, was released October 1982 in Japan.
  Standard CDs have a diameter of 120 millimetres and are designed to hold up to 74 minutes of uncompressed stereo digital audio or about 650 MiB of data. Capacity is routinely extended to 80 minutes and 700 MiB by arranging more data closely on the same sized disc. The Mini CD has various diameters ranging from 60 to 80 millimetres; 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 in 1982, a CD could store much more data than a personal computer hard drive, which would typically hold 10 MB.
context_section_description: 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 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.
  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.
  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. 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 part of the light reflected from its peak is 1/2 wavelength out of phase with the light reflected from the land around it. 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 disk 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. This mechanism allows the laser to read information from the center to the edge of a disc without having to interrupt the spinning of the disc itself.
  The pits and lands do not directly represent the 0's and 1's 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 0's. There must be at least 2, and no more than 10 0's 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 encodi
caption: The pits in a CD are 500 nm wide, between 830 nm and 3,000 nm long and 150 nm deep.
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