Compact Disc

Compact Disc (CD), small disc containing data recorded in digital form, a type of computer medium that resulted from audio technology first developed by the electronics company Philips in the early 1980s. These shiny, almost indestructible discs are now familiar to virtually everyone who enjoys listening to music, or is interested in hi-fi or computers. Nowadays they are used to carry much of the music and software sold to the general public.

A standard CD can hold a tremendous amount of information—about 650 million bytes (megabytes—Mb), which is the equivalent of more than 400 1.44 Mb floppy disks. This means that a single disc can carry more than an hour of high-quality music or an entire feature film. The whole United Kingdom telephone directory—17 million entries—fits on to one CD. Impressive though this may be, there is yet more capacity that could be squeezed on to a CD. About 15 per cent of the available storage on current discs is used to check the integrity of the rest of the data. This error detection and correction storage capacity is important for error-intolerant information such as software, but is not so necessary for audio or video data, both of which are less prone to corruption. In 1999 a CD with a 700 Mb capacity was launched.

The most common type of CD, the audio CD, is increasingly popular as a means of providing the consumer with high-quality recordings of music, while the CD-ROM (compact disc read-only memory) is ideal for distributing information such as encyclopedias and other reference material. CD-ROMs cannot, however, be used to save information as can be done on a floppy disk. This is because each disc is made by burning a sequence of tiny holes into the reflective base material of a blank master disc, which is then made into a mould. These are mounted on a stamp press, heated, and plastic injected into them to make the copied CDs. The pattern of holes etched on the disc is interpreted as digital information when the laser contained in a disc reader scans it. As the disc spins, light from the laser is either reflected or not. This provides a stream of zeros and ones that can be turned into music, text, pictures, or anything else that can be digitally encoded. The way that it works makes the CD-ROM very durable: no physical contact is required to read the information.

The CD-ROM is one of a larger family of CDs. Related compact discs are all based on the same technology and coding, but with different index structures to suit specific applications. Perhaps the best known is the interactive CD-I. The family is growing rapidly and CDs that can be written to (notably, Digital Audio MiniDiscs or MDs) are proving a popular and inexpensive alternative to cassette tapes. While the CD-ROM is made by burning reflective material using a laser, these new CDs are made by heating up a magnetic material which then changes its optical properties.

Despite having huge storage capacity, the compact disc did have some initial drawbacks. Perhaps the main one was the speed at which information could be retrieved. Early disc drives could retrieve data at a rate of 150k bits per second, giving an average access time of several hundred milliseconds, compared to about 10.5 milliseconds for a hard disk. More recent drives have speeds much closer to that of the hard disk.

See also Sound Recording and Reproduction.