In computing, sound reproduction, and video, an optical disc is a
flat disc that stores data in the form of pits (or bumps) along a
spiral groove within its surface. The disc, usually made of
polycarbonate, has a reflective coating often consisting of aluminum.
The data are generally accessed when the reflective surface is
illuminated with intense light, usually in the form of red or blue laser
light—hence the name optical disc. A computer's peripheral device used to read or write an optical disc is called an optical disc drive (ODD).
The technology first became popular in the digital reproduction and distribution of music in the form of compact discs (CDs). Later, as storage capacities grew, the technology was modified to accommodate both film and video programming on what came to be called digital video discs (DVDs). Beyond consumer entertainment applications, the technology is totally pervasive in personal computing and used in both application software distribution and as a data storage and transport medium worldwide.
The popularity and rate of adoption of the optical disc medium has been unparalleled in the history of consumer electronics, as the technology found its place as the superior (digital) alternative to tape-based VHS and cassette tape (analog) technology. A digital copy not only created a perfect replication of the original source but, unlike analog tape, optical discs did not deteriorate with continued use (but are prone to damage by scratches on their surface). The DVD Entertainment Group (a Los Angeles based industry trade organization) cites a group-sponsored 2002 study by Ernst & Young, which reported that since the inception of the DVD format in 1997, software shipments reached more than 790 million units, and 35.5 million hardware players in less than five years.
David Paul Gregg developed an analog optical disc for recording video and patented it in 1961 and 1969 (U.S. Patent 3,430,966). Of special interest is U.S. Patent 4,893,297, first filed in 1968 and issued in 1990, so that it will be a source of royalty income for Pioneer’s DVA until 2007. It encompasses systems such as CD, DVD, and even Blu-ray Disc. Gregg's company, Gauss Electrophysics, was acquired, along with Gregg's patents, by MCA in the early 1960s.
In a parallel manner, and probably inspired by the developments in the U.S., a small group of physicists started their first optical videodisc experiments at Philips Research in Eindhoven, The Netherlands in 1969. In 1975, Philips and MCA decided to join forces. In 1978, much too late, the long waited laserdisc was introduced in Atlanta. MCA delivered the discs and Philips the players. It turned out to be a total technical and commercial failure, and quite soon the Philips/MCA cooperation came to an end. In Japan and the U.S., Pioneer became successful with the videodisc until the advent of DVD.
Philips and Sony formed a consortium in 1979 to develop a digital audio disc, which resulted in the very successful introduction of the compact disc in 1983.
How it works
The technology works by adding pits (or bumps) to the disc surface, usually along a single spiral groove that can cover the entire recorded surface of the disc. The information on the disc is stored sequentially on the spiral track, from the innermost part to the outermost part of the track.
The density of the pits added to the surface determines data capacity. This capacity differentiates between specific disc technologies in use today including CDs, DVD, (using red laser diodes) and the more recent blue laser disc technology called HD-DVD and Blu-ray.
To read the data, the reflective coating on the disc is illuminated with a laser diode, and the pits distort the laser light as it is reflected. As mentioned above, lasers of various colors, particularly red and blue, have been employed in this process. More recent developments in blue laser technology have enabled much higher capacity storage due to the higher frequency of blue light over red laser light technology.
Most optical discs, with the exception of a few (such as black CD-ROMs designed for the original Sony PlayStation), have a characteristic prismatic or iridescent appearance created by the grooves in the reflective layer.
The promotion of standardized optical storage is undertaken by the Optical Storage Technology Association (OSTA). Although optical discs are significantly more durable than earlier audio/visual and data formats, they are susceptible to damage from daily usage and environmental factors. Libraries and archives should enact optical media preservation procedures to ensure continued usability.
First-generation optical discs
Optical discs were initially used for storing music and software. They could not be burned or produced from the personal computer and could only be purchsed at a music store or with a software package. The Laserdisc format stored analog video, but it fought an uphill battle against VHS (mainly due to cost and non-recordability). Other first-generation disc formats are designed to store solely digital data.
Most first-generation disc devices use an infrared laser as a read head. The minimum size of a laser spot is proportional to the wavelength of the laser, making wavelength one factor limiting the information density. Infrared is just beyond the long-wavelength end of the visible light spectrum, so it supports less density than any visible (to humans) color of light. One example of capacity achieved with an infrared laser is 700 MB of net user data for a 12-cm compact disc.
Many factors affect density besides minimum spot size—for example, a multi-layered disc using infrared would hold more data than an otherwise identical disc with a single layer, and other issues—such as whether CAV, CLV, or zoned CAV is used, how data is encoded, and how much margin is left clear at the center and edge—also affect how close a disc can come to taking advantage of the minimum spot size over 100 percent of the disc surface.
- Compact Disc (CD)
- Magneto-optical disc
Second-generation optical discs
Second-generation optical discs were created to store large amounts of data, including TV-quality digital video, software, music, and various other forms of data. These disks were made so that they could be burned from a home computer. Many, though not all of such discs, use a visible light laser (usually red). The shorter wavelength allows a tighter beam, allowing the pits and lands of the disc to be smaller. In the case of the DVD format, this allows 4.7 GB of storage on a standard 12 cm, single-sided, single layer disc; alternately, smaller media such as the MiniDisc and DataPlay formats can have capacity approximately comparable to a much larger standard compact disc.
- DVD and derivatives
- Digital Video Express (DIVX)
- Super Audio CD
- Enhanced Versatile Disc
- Digital Multilayer Disk
- Fluorescent Multilayer Disc
- Phase-change Dual
- Universal Media Disc
Third-generation optical discs
Major third-generation optical discs are currently in development. They are designed for holding high-definition video, games, and other forms of data. They support larger capacities, enabled by the use of short-wavelength visible light lasers (blue-violet for Blu-ray Disc and HD DVD). In practice, effective capacity for multimedia presentations can be drastically improved by using enhanced video data compression algorithms such as MPEG-4.
- Blu-ray Disc
- HD DVD
- Forward Versatile Disc
- Total HD disc
- Versatile Multilayer Disc
- Ultra Density Optical
Next generation optical discs
The following formats are so advanced they can be considered to be ahead of current (third gen) discs. All of the following discs have the potential of over one terabyte of space.
- Tapestry Media
- Holographic Versatile Disc
- Protein-coated disc
- TeraDisc (a 3D optical data storage disc)
Recordable and writable optical discs
There are numerous formats of recordable optical disc on the market, all of which are based on using a laser to change the reflectivity of the recording medium in order to duplicate the effects of the pits and lands created when a commercial optical disc is pressed. Emerging technologies such as holographic data storage and 3D optical data storage aim to use entirely different data storage methods, but these products are in development and are not yet widely available.
The most common form of recordable optical media is write-once organic dye technology, popularized in the form of the CD-R and still used for higher-capacity media such as DVD-R. This uses the laser alone to scorch a transparent organic dye (usually cyanine, phthalocyanine, or azo compound-based) to create "pits" (i.e. dark spots) over a reflective spiral groove. Most such media are designated with an R (recordable) suffix. Such discs are often quite colorful, generally coming in shades of blue or pale yellow or green.
Rewritable, non-magnetic optical media are possible using phase change alloys, which are converted between crystalline and amorphous states (with different reflectivity) using the heat from the drive laser. Such media must be played in specially tuned drives, since the phase-change material has less of a contrast in reflectivity than dye-based media; while most modern drives support such media, many older CD drives cannot recognize the narrower threshold and cannot read such discs. Phase-change discs are designated with RW (ReWriteable). Phase-change discs often appear dark gray.
The earliest form is magneto-optical, which uses a magnetic field in combination with a laser to write to the medium. Though not widely used in consumer equipment, the original NeXT cube used MO media as its standard storage device, and consumer MO technology is available in the form of Sony's MiniDisc. This form of medium is rewriteable.