In computing and optical disc recording technologies, an optical disc is a flat, usually circular disc which encodes binary data (bits) in the form of pits (binary value of 0 or off, due to lack of reflection when read) and lands (binary value of 1 or on, due to a reflection when read) 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 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 in) in diameter, with 12 cm (4.75 in) being the most common size. A typical disc is about 1.2 mm (0.05 in) thick, while the track pitch (distance from the center of one track to the center of the next) is typically 1.6 µm.

An optical disc is designed to support one of three recording types: read-only (e.g.: 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 tellurium.

Optical 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 (PC). 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 digital audio player (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. This form of optical disc was a very early form of the DVD . It is of special interest that , filed 1989, issued 1990, generated royalty income for Pioneer Corporation's DVA until 2007 —then compassing 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 audio 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 was introduced as Blu-ray Disc. Developed by the Blu-ray Disc Association (BDA), a group of the world's leading consumer electronics, personal computer and media manufacturers (including Apple, Dell, Hitachi, HP, JVC, LG, Mitsubishi, Panasonic, Pioneer, Philips, Samsung, Sharp, Sony, TDK and Thomson). The format was developed to enable recording, rewriting and playback of high-definition video (HD), as well as storing large amounts of data. The format offers more than five times the storage capacity of traditional DVDs and can hold up to 25 GB on a single-layer disc and 50 GB on a dual-layer disc. This extra capacity combined with the use of advanced video and audio codecs will offer consumers an unprecedented HD experience.

While current optical disc technologies such as DVD, DVD±R, DVD±RW, and DVD-RAM rely on a red laser to read and write data, the new format uses a blue-violet laser instead, hence the name Blu-ray. Despite the different type of lasers used, Blu-ray products can easily be made backwards compatible with CDs and DVDs through the use of a BD/DVD/CD-compatible optical pickup unit. The benefit of using a blue-violet laser (405 nm) is that it has a shorter wavelength than a red laser (650 nm), which makes it possible to focus the laser spot with even greater precision. This allows data to be packed more tightly and stored in less space, so it's possible to fit more data on the disc even though it's the same size as a CD/DVD. This together with the change of numerical aperture to 0.85 is what enables Blu-ray Discs to hold 25 GB/50 GB. Recent development by Pioneer has pushed the storage capacity to 500 GB on a single disc by using 20 layers. 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 Laserdisc format stored analog video signals for the distribution of home video, but commercially lost to the VHS videocassette format, 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 digital 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 700 MB of net user data for a 12 cm compact disc.

Other factors that affect data storage density include: more than one layer of data in the disc, method of rotation (Constant linear velocity (CLV), Constant angular velocity (CAV), or zoned-CAV), the composition of lands and pits, and how much clear margin is at the center and the edge.

Compact Disc (CD) and derivatives

* Video CD (VCD)

* Super Video CD

Laserdisc

GD-ROM

Phase-change Dual

Double Density Compact Disc (DDCD)

Magneto-optical disc

MiniDisc

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 4.7 GB storage on a standard 12 cm, single-sided, single-layer disc; alternatively, smaller media, such as the MiniDisc and the DataPlay formats, can have capacity comparable to that of the larger, standard compact 12 cm disc.

Hi-MD

DVD and derivatives

* DVD-Audio

* DualDisc

* Digital Video Express (DIVX)

Super Audio CD

Enhanced Versatile Disc

DataPlay

Universal Media Disc

Ultra Density Optical

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 and focusing optics 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/MPEG-4 AVC and VC-1.

Currently shipping:

* Blu-ray Disc (up to 50 GB)

* China Blue High-definition Disc

In development:

* Forward Versatile Disc

* Digital Multilayer Disk or Fluorescent Multilayer Disc

Abandoned:

* HD DVD

* Versatile Multilayer Disc

Fourth-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:

Holographic Versatile Disc

LS-R

Protein-coated disc

Recordable and writable optical discs

There are numerous formats of optical direct to disk recording devices on the market, all of which are based on using a laser to change the reflectivity of the digital recording medium in order to duplicate the effects of the pits and lands created when a commercial optical disc is pressed. All formats enable reading of computer files as many times as desired by the user, but writing is a different situation. Some formats such as CD-R enable writes to be made only once to each sector on the disk, while other formats CD-RW enable multiple writes to the same sector which is more like a magnetic recording hard disk drive (HDD). In August of 2011, a company named Millenniata announced a format called the M-Disk which, reverting to the original technology of optical disks, creates physical pits in a rock-like layer. The M-Disk is stable up to 932 degrees Fahrenheit, is impervious to humidity issues, and is engineered to maintain its integrity for 1,000 years without degradation.

Specifications

+ Base (1×) and (current) maximum speeds by generation	Generation	Base	Max
! (Mbit/s)	! (Mbit/s)	! ×
1st (CD)	1.17	65.62	56×
2nd (DVD)	10.55	210.94	20×
3rd (BD)	36	432	12×

!rowspan="2" align=right align=right align=right align=right align=right align=right align=right align=right align=right align=right align=right align=right align=right align=right align=right align=right align=right align=right align=right align=right align=right align=right align=right align=right align=right align=right align=right align=right align=right align=right align=right align=right align=right align=right align=right align=right

+ Capacity and nomenclature	Designation	Layers(total) !	Diameter !!colspan="2"	Capacity
Gigabyte>GB) !!align=right
BD	SS SL		1 ||	8	7.8
BD	SS DL	align=right	1	2 ||	8	15.6
BD	SS SL	align=right	1	1 ||	12	25
BD	SS DL	align=right	1	2 ||	12	50
CD–ROM 74 min	SS SL	align=right	1	1 ||	12	0.682	0.635
CD–ROM 80 min	SS SL	align=right	1	1 ||	12	0.737	0.687
CD–ROM	SS SL	align=right	1	1 ||	8	0.194	0.180
DDCD–ROM	SS SL	align=right	1	1 ||	12	1.364	1.270
DDCD–ROM	SS SL	align=right	1	1 ||	8	0.387	0.360
DVD–1	SS SL	align=right	1	1 ||	8	1.46	1.36
DVD–2	SS DL	align=right	1	2 ||	8	2.66	2.47
DVD–3	DS SL	align=right	2	2 ||	8	2.92	2.72
DVD–4	DS DL	align=right	2	4 ||	8	5.32	4.95
DVD–5	SS SL	align=right	1	1 ||	12	4.70	4.37
DVD–9	SS DL	align=right	1	2 ||	12	8.54	7.95
DVD–10	DS SL	align=right	2	2 ||	12	9.40	8.74
DVD–14	DS DL/SL	align=right	2	3 ||	12	13.24	12.32
DVD–18	DS DL	align=right	2	4 ||	12	17.08	15.90
DVD–R 1.0	SS SL	align=right	1	1 ||	12	3.95	3.68
DVD–R (2.0), +R, –RW, +RW	SS SL	align=right	1	1 ||	12	4.70	4.37
DVD-R, +R, –RW, +RW	DS SL	align=right	2	2 ||	12	9.40	8.75
DVD–RAM	SS SL	align=right	1	1 ||	8	1.46	1.36
DVD–RAM	DS SL	align=right	2	2 ||	8	2.65	2.47
DVD–RAM 1.0	SS SL	align=right	1	1 ||	12	2.58	2.40
DVD–RAM 2.0	SS SL	align=right	1	1 ||	12	4.70	4.37
DVD–RAM 1.0	DS SL	align=right	2	2 ||	12	5.16	4.80
DVD–RAM 2.0	DS SL	align=right	2	2 ||	12	9.40	8.75
HD DVD	SS SL	align=right	1	1 ||	8	4.70
HD DVD	SS DL	align=right	1	2 ||	8	9.40
HD DVD	DS SL	align=right	2	2 ||	8	9.40
HD DVD	DS DL	align=right	2	4 ||	8	18.80
HD DVD	SS SL	align=right	1	1 ||	12	15.00
HD DVD	SS DL	align=right	1	2 ||	12	30.00
HD DVD	DS SL	align=right	2	2 ||	12	30.00
HD DVD	DS DL	align=right	2	4 ||	12	60.00
HD DVD–RAM	SS SL	align=right	1	1 ||	12	20.00

References

Further reading

Byers, Fred R. (2003). ''Care and Handling of CDs and DVDs — A Guide for Librarians and Archivists''. National Institute of Standards and Technology.

External links

http://www.romeyn.ca/ 50th-anniversary -of-the-optical-disc. by: Jacob Romeyn

Optical Storage Technology Association

Reference guide for optical media by Terence O'Kelly (Memorex Inc.)

Category:Compact Disc Category:DVD Category:Optical disc authoring Category:Optoelectronics Category:Optical computer storage media

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