CD-ROM

CD-ROM is a read-only optical media format that uses CDs to store data. A computer with an optical drive can read, but not write, data on a CD-ROM. A single disc can store up to 700 MB of data. Some CD-ROM discs, known as “Enhanced CDs,” store a combination of audio tracks and a data track. CD-ROM discs were often used to distribute software and video games in the 1990s and 2000s before eventually being replaced by higher-capacity DVDs.

CD-ROM discs and audio CDs are physically identical, storing and retrieving data in the same method. Digital data is encoded and stored as a series of pits on a flat reflective layer on the bottom of the disc. The laser in the optical drive reads these pits, then decodes the data. The only difference between the two is how the data is formatted — multiple PCM audio tracks on an audio CD, or a single track containing data on a file system on a CD-ROM.

An optical drive can spin a CD-ROM disc at a significantly higher speed than an audio CD player. A CD spinning at the standard rate for audio playback supports a data transfer rate of approximately 150 KB/s; at this speed, reading an entire CD-ROM full of data takes almost 80 minutes. CD-ROM drives are classified by how many times faster they can spin, with many drives capable of 40x (6,144 KB/s) or more.

CD-ROM

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CD-ROM, type of computer memory in the form of a compact disc that is read by optical means. A CD-ROM drive uses a low-power laser beam to read digitized (binary) data that has been encoded in the form of tiny pits on an optical disk. The drive then feeds the data to a computer for processing.

The standard compact disc was introduced in 1982 for digital audio reproduction. But, because any type of information can be represented digitally, the standard CD was adapted by the computer industry, beginning in the mid-1980s, as a low-cost storage-and-distribution medium for large computer programs, graphics, and databases. With a storage capacity of 680 megabytes, the CD-ROM found rapid commercial acceptance as an alternative to so-called floppy disks (with a maximum capacity of 1.4 megabytes).

Unlike conventional magnetic storage technologies (e.g., tapes, floppy disks, and hard disks), CDs and CD-ROMs are not recordable—hence the tag “read only.” This limitation spurred the development of various recordable magnetic-optical hybrid storage devices; but they generally failed to penetrate beyond the publishing world, where large multimedia files are regularly exchanged, because of incompatibility with standard CD and CD-ROM players. In the early 1990s a new type of CD became available: CD-Recordable, or CD-R. These discs differ from regular CDs in having a light-sensitive organic dye layer which can be “burned” to produce a chemical “dark” spot, analogous to an ordinary CD’s pits, that can be read by existing CD and CD-ROM players. Such CDs are also known as WORM discs, for “Write Once Read Many.” A rewritable version based on excitable crystals and known as CD-RW was introduced in the mid-1990s. Because both CD-R and CD-RW recorders originally required a computer to operate, they had limited acceptance outside of use as computer software and data backup devices.

To handle the proliferation of ever-larger multimedia files (audio, graphic, and video) in computer games, educational software, and electronic encyclopaedias—as well as high-definition movies for television entertainment systems—an expanded storage medium, the digital videodisc ( DVD), was introduced in 1995, and a storage medium with even more capacity, Blu-ray, was introduced in 2002. However, with the increased storage capacity of computers and the easy distribution of large files over the Internet, the use of CD-ROMs declined in the 21st century.

The Editors of Encyclopaedia Britannica This article was most recently revised and updated by Erik Gregersen.