Optical Disk
Optical discs are the state of the art in computer memory and storage, using lasers to record and retrieve information. Like magnetic discs, they can store many different kinds of information--sound, text, and pictures (both still and animated)--on the same disc. However, optical discs are far superior to their magnetic counterparts because they hold more data and, because the laser cannot damage the disc, they are more durable.
IBM began the first experiments with optical technology in the mid-1960s but made little progress until the semiconductor was developed. Sony and Philips saw the potential that the technology offered, and the two joined forces later in the decade, hoping to find a viable application for it. The result, in the 1970s, was the videodisc, which use a laser stylus to play back analog (as opposed to digital) information recorded in spiral tracks on a plastic disc. This spiral-track method is similar to the one used to make phonograph records. Videodiscs' picture quality is superior to that of videocassettes but their use is limited in this country; they enjoy wider popularity in Japan.
In the mid-1980s Sony and Philips unveiled yet another joint venture, the prerecorded compact disc, or CD. Unlike the videodisc, a CD is recorded digitally, which not only produces a better sound, it uses space on the disc more efficiently (hence the term "compact"), allowing more data (in this case, music) to be stored. Within only few years, CDs dominated the record music industry, turning LPs into relics.
CDs can also be used to store visual data in a form called CD-ROM (read-only memory). When used in this way they are read by CD players in a computer to reproduce text, graphics, and sound from the same disc. Because of their high density, CD-ROM discs can store incredible amounts of information; they have become the standard format for large published works like software documentation and encyclopedias.
Whether for music or text, CDs and CD-ROM discs are produced the same way. Digital data (the binary language of ones and zeroes common to all computers) are encoded onto a master disc. A laser burns small holes, or pits, into a thin metallic film sandwiched between a plastic substrate and a protective plastic or glass film. These pits are a rough binary equivalent of ones. Smooth areas of the disc untouched by the laser, called land, are analogous to zeroes. Thus data is translated as a series of reflections (land) and nonreflections (pits). Once transferred to the disc, they are not erasable. This master disc can be used to produce identical copies of itself.
When a CD player or computer reads data from an optical disc, it uses a photodetector or diode to catch low-power laser light reflected off the disc's surface. The light's intensity at any given point is determined by the arrangement of pits and land on the disc. Information received by the diode is digitized and then converted to music, text, or images.
CD-ROMs were a vast improvement over magnetic media in terms of sheer data capacity, but they could only read; users could not write new information onto the optical disc. To meet this need, IBM and other computer manufacturers came up with WORM (write once read many) drives and optical discs. These can both read from and write onto a computer's optical disc. Data is added using a high-power laser in the same way that CD and CD-ROM discs are encoded. Once entered, data storage is permanent; the discs cannot be erased.
WORM drives can be used in place of or in addition to magnetic discs in a personal computer. They are also manufactured in two-drive systems called jukeboxes that hold 100 discs and a potential terabyte (1 trillion bytes) of information. This is especially impressive once you realize that an entire encyclopedia set can be stored on 20% of one standard optical disc.
The next obvious evolution of optical storage technology was the erasable optical disc, and that, too, has hit the market. The most successful system to date uses magneto-optic, or magnetically assisted, recording. Unlike CD and WORM systems, data spots are heated, not burned, into the disc medium, and then magnetized. The magnetization corresponds to a zero or a one depending on its direction.
The first commercial expression of this technology, the Tahiti I disc drive, was developed by the Maxtor Corporation in 1988. Its access time was a then-breathtaking 43 milliseconds, half the rate of contemporary hard magnetic disc drives. Even more amazing was the new read/write head, which was half the weight of conventional models.
Another type of erasable optical storage uses crystalline materials whose structure changes to an amorphous state at a certain light wavelength. This has been less successful than magneto-optical systems because the crystals eventually lose their ability to change.
Yet another erasable optical disk system hit the computer world like a thunderbolt in 1988 when the Tandy Corporation announced its THOR-CD. This hybrid system can write, retrieve, and erase data from audio OR computer systems. Although the particulars are still a trade secret, the system probably uses two lasers, one to write and one to erase data. The next set of innovations in optical disk technology were introduced in the the mid-1990s, a new and improved CD-ROM was introduced called the Digital Versatile Disk, or DVD. DVDs offered approximately seventimes more storage than a CD-ROM.
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