In
computing and
optical recording, an
optical disc (CD) or
optical disk (WORM) is a flat, usually circular disc which can contain
audio,
video or
data encoded in microscopic pits (or bumps) on a special material (often
aluminium) on one of its flat surfaces. The encoding material sits atop a thicker substrate (usually
polycarbonate) which makes up the bulk of the disc and forms a dust defocusing layer. The encoding pattern follows a continuous, spiral path covering the entire disc surface and extending from the innermost track to the outermost track. The data is stored on the disc with a
laser or stamping machine, and can be accessed when the data path is illuminated with a
laser diode in an
optical disc drive which spins the disc at speeds of about 200
RPM up to 4000 RPM or more depending on the drive type, disc format, and the distance of the read head from the center of the disc (inner tracks are read at a faster disc speed). The pits or bumps distort the reflected laser light, hence most optical discs (except the black discs of the original
PlayStation video game console) characteristically have an
iridescent appearance created by the grooves of the reflective layer. The reverse side of an optical disc usually has a printed label, generally made of paper but sometimes printed or stamped onto the disc itself. This side of the disc contains the actual data and is typically coated with a transparent material, usually
lacquer. Unlike the 3½-inch
floppy disk, most optical discs do not have an integrated protective casing and are therefore susceptible to data transfer problems due to scratches, fingerprints, and other environmental problems.
Optical discs are usually between 7.6 and 30 cm (3 to 12 inches) in diameter, with 12 cm (4.75 inches) being the most common size. A typical disc is about 1.2 mm (0.05 inches) thick, while the track pitch (distance from the center of one track to the center of the next) is typically 1.6
µm (microns).
An optical disc is designed to support one of three recording types: read-only (eg:
CD and
CD-ROM), recordable (write-once, e.g.
CD-R), or re-recordable (rewritable, e.g.
CD-RW). Write-once optical discs commonly have an organic dye recording layer between the substrate and the reflective layer. Rewritable discs typically contain an
alloy recording layer composed of a
phase change material, most often
AgInSbTe, an alloy of
silver,
indium,
antimony and
telluriumOptical discs are most commonly used for storing music (e.g. for use in a
CD player), video (e.g. for use in a
DVD player), or data and programs for
personal computers. The
Optical Storage Technology Association (OSTA) promotes standardized optical storage formats. Although optical discs are more durable than earlier audio-visual and data storage formats, they are susceptible to environmental and daily-use damage. Libraries and archives enact
optical media preservation procedures to ensure continued usability in the computer's optical disc drive or corresponding disc player.
For computer data backup and physical data transfer, optical discs such as CDs and DVDs are gradually being replaced with faster, smaller, and more reliable solid state devices, especially the
USB flash drive. This trend is expected to continue as USB flash drives continue to increase in capacity and drop in price. Similarly, personal portable CD players have been supplanted by portable solid state
MP3 players, and
MP3 music purchased or shared over the internet has significantly reduced the number of audio CDs sold annually.
History
The optical disc was invented in 1958. In 1961 and 1969,
David Paul Gregg registered a patent for the analog optical disc for video recording, . It is of special interest that , filed 1968, issued 1990, generated
royalty income for Pioneer Corporation’s
DVA until 2007 — encompassing the CD,
DVD, and
Blu-ray Disc systems. In the early 1960s, the Music Corporation of America bought Gregg's patents and his company,
Gauss Electrophysics.
Later, in the Netherlands in 1969,
Philips Research
physicists began their first optical videodisc experiments at Eindhoven. In 1975, Philips and MCA began to work together, and in 1978, commercially much too late, they presented their long-awaited
laserdisc in
Atlanta. MCA delivered the discs and Philips the players. However, the presentation was a technical and commercial failure and the Philips/MCA cooperation ended.
In Japan and the U.S.,
Pioneer succeeded with the videodisc until the advent of the DVD. In 1979,
Philips and
Sony, in consortium, successfully developed the
compact disc in 1983.
In the mid-1990s, a consortium of manufacturers developed the second generation of the optical disc, the DVD.
The third generation optical disc was developed in 2000-2006, and the first movies on Blu-ray discs were released in June 2006. Blu-ray eventually prevailed in a
high definition optical disc format war over a competing format, the
HD DVD. A standard Blu-ray disc can hold about 25 GB of data, a DVD about 4.7 GB, and a CD about 700 MB.
First-generation
Initially, optical discs were used to store music and computer software. The
laser disc format stored analog video signals, but, commercially, lost to the
VHS videotape cassette, due mainly to its high cost and non-re-recordability; other first-generation disc formats were designed only to store digital data and were not initially capable of use as a video medium.
Most first-generation disc devices had an infrared laser reading head. The minimum size of the laser spot is proportional to its wavelength, thus wavelength is a limiting factor against great information density, too little data can be stored so. The infrared range is beyond the long-wavelength end of the visible light spectrum, so, supports less density than any visible light colour. One example of high-density data storage capacity, achieved with an infrared laser, is 700MB of net user data for a 12 cm compact disc.
NOTE: other factors affecting data storage density are, for example, a multi-layered infrared disc would hold more data than an identical single-layer disc; whether CAV, CLV, or zoned-CAV; how the data are encoded; how much clear margin at the center and the edge
Second-generation
Second-generation optical discs were for storing great amounts of data, including broadcast-quality digital video. Such discs usually are read with a visible-light laser (usually red); the shorter wavelength and greater
numerical aperture allow a narrower light beam, permitting smaller pits and lands in the disc. In the DVD format, this allows 4.7GB storage on a standard 12 cm, single-sided, single-layer disc; alternately, smaller media, such as the
MiniDisc and the
DataPlay formats, can have capacity comparable to that of the larger, standard compact 12 cm disc.
Third-generation
Third-generation optical discs are in development, meant for distributing
high-definition video and support greater data storage capacities, accomplished with short-wavelength visible-light lasers and greater numerical apertures. The Blu-ray disc uses blue-violet lasers of greater aperture, for use with discs with smaller pits and lands, thereby greater data storage capacity per layer.
In practice, the effective multimedia presentation capacity is improved with enhanced video
data compression codecs such as
H.264, and
VC-1.
Next generation
The following formats go beyond the current third-generation discs and have the potential to hold more than one terabyte (1
TB) of data:
Recordable and writable optical discs
Specifications
