Compact Disc

Compact disc is a round, flat platter on which recorded music, computer data, or other information is stored in the form of digital (numerical) code. The main use of compact discs (CD's) is to store recorded stereophonic music. Discs used for this purpose, called audio CD's, are played on a CD player. The other major type of CD is called CD-ROM (Compact Disc Read-Only Memory). CD-ROM's can store files of text, pictures, sound, and other data, as well as video game programs and other software. These discs are played on a computer's CD-ROM drive or on a special CD-ROM player. Other types of CD's can store high quality images or motion pictures.

CD's are made of hard plastic coated with thin metal, usually aluminun. CD's have only one playing side. A standard CD measures about 4 3/4 inches (120 millimeters) in diameter, with a 1/2-inch (15-millimeter) center hole.

Audio CD's

Audio CD's have three major advantages over cassette tapes: (1) they last longer, (2) they have superior sound quality, and (3) they enable the user to select any part of the recording quickly. Manufacturers have established an industry standard known as the Red Book so that an Audio CD can be played on any CD player. One rule specifies how many minutes of sound each CD can hold. Other features, such as disc dimensions and data format, are also standardized. Audio CD's were first introduced in Japan and Europe in 1982, and in the United States in 1983. By the early 1990's, CD's had largely replaced records and began to outsell music cassettes.

Recording. During recording, a microphone translates sound waves into electric signals. A device called an analog-to-digital converter divides these signals into 44,100 segments for each second of sound. It then converts each segment into a digital code. In this code, each sound segment is expressed as a series of electric pulses representing 1's and 0's.

Manufacturing. A master tape made at a recording studio is sent to a CD manufacturing plant to make a master disc. A laser uses the data on the master tape to cut a spiral track of microscopic pits into the surface of one side of the master disc. The pits represent the digital code. Pits are about 1/42,000 inch (0.6 micrometer) wide. Over 1,000 pits would fit side-by-side into the period at the end of this sentence. Depending on playing time, a pit spiral might contain 3 billion pits. If unwound, this spiral would stretch about 3 1/2 miles (5.6 kilometers).

The master disc is used to make metal copies. These copies are then used as molds to make individual discs. Plastic is fed into a molding machine, melted, and injected into a mold to produce clear plastic discs. These discs are then coated with a metal layer, a protective plastic layer, and a printed label on one side. A sophisticated manufacturing system can produce a CD every five seconds.

Playback. As a CD spins inside a CD player, a laser beam shines through the plastic layer on the underside of the disc and follows the pit spiral. The laser beam reflects off the metal coating. The intensity of the reflection changes as the beam enters and leaves a pit. A device translates the varying reflected light into an electric signal, which is used to produce sound. Because information on the disc is read by a light beam, playing a CD causes no more wear to the recording than reading causes to words printed on paper.

A CD player enables the user to play an entire disc or selected tracks (recordings) on the disc. Fast-forward and reverse controls make it possible to find passages within a track. Other controls let the listener repeat tracks, play tracks in random order, and program specific sequences of tracks. If the player has a device called a CD changer, the user can load several discs at once for an extended period of play. Most changers hold 6 to 10 discs, and some hold 100 or more discs. CD players are available for home, car, or portable use.

CD-ROM's

CD-ROM's (Compact Disc-Read Only Memory) are created and used in much the same way as audio CD's. In the CD-ROM drive, a light-sensitive device produces a digital electronic signal. Special circuits process this signal and send it to a computer or special player. The Yellow Book standard regulates CD-ROM's. A CD-ROM can hold about 650 megabytes (million bytes) of data. One byte equals a letter or some other character. One CD-ROM can hold the text of hundreds of books. The phrase read-only memory means that CD-ROM's store data permanently and cannot be altered by the user.

Many CD-ROM's store mainly textual information and software programs. Other types of CD-ROM's include Photo CD's and Multimedia CD's, which contain a combination of text, graphics, sound, photographs, motion pictures, and other types of media.

The multimedia CD-i (Compact Disc-interactive) format is standardized in the Green Book. With a CD-i, the user's selections determine what happens. A person might tour a video museum on a computer monitor, viewing and listening to the exhibits. Other CD-i's contain encyclopedias, stories, and video games. In addition, a CD-i player can play back audio CD's.

A Photo CD does not have any sound. It can store 100 photographs taken with an ordinary camera. Many photo shops can arrange for photographs to be transferred from negatives or slides to a Photo CD. Both professionals and consumers can use a Photo CD player or certain CD-ROM drives to view the pictures on a television screen or computer monitor.

Other Formats

Video CD's store digital video and audio signals. A 74-minute full-motion, full-color video program with stereo sound can be contained on one disc. The Video CD is standardized in the White Book. A Video CD player or special CD-i player is connected to a television and stereo system to view the program.

The Video CD format uses a data compression method that enables both audio and video to be stored together on a disc. Data compression is the use of special techniques to reduce the number of bytes needed to carry information. It removes inaudible parts of a sound signal, decreasing the amount of audio data that must be stored. Images are coded in ways that minimize the quantity of data needed to record them, without sacrificing picture quality.

Video CD's are produced at CD manufacturing plants. However, special equipment is needed to perform data compression and to play both audio and video master tapes. The Video CD format was introduced in 1994.

DVD's (Digital Versatile Discs or Digital VideoDiscs) are a type of read-only memory compact disc. A DVD is the same size as a CD but has huge storage capabilities. A DVD stores data on one or both sides. Each side of the disc can contain two data layers, one imbedded beneath the other. A DVD can hold up to 17 gigabytes (billion bytes) of information on both sides. The DVD requires a special drive, which can also play CD-ROM's. A DVD drive reads data at a much faster rate than a typical CD-ROM drive. The DVD format appeared in 1996.

CD-R (Compact Disc-Recordable) enables a user to record permanent information. The CD-R format is standardized in the Orange Book. A blank disc can hold as much data as a standardized disc, and it can be recorded with music or other data. However, once recorded, the CD-R disc cannot be erased. The CD-R format uses a recording method known as dye-polymer. An organic dye recording material is sandwiched between the plastic disc and top metal layer. During recording, a high-power laser beam hits the recording layer, storing information in patterns of tiny pits. The disc can be played back on a regular CD player. CD-R's are often used to make small quantities of discs because it is not economical for a CD manufacturing plant to do so.

Minidiscs (MD's) measure only 2 1/2 inches (6.4 centimeters) across and are contained in a flat plastic cartridge. A person can record up to 74 minutes of sound on some MD's and later erase the sound and record on the disk again. Other MD's are for playback only. The MD format is different from the CD format, so MD's require a special recorder/player.

Recordable MD's use a method called magneto-optical recording. During recording, a magnetic layer is momentarily heated by a high-power laser beam. The digital data are magnetically stored in the layer as it quickly cools. The data can be erased by reheating the layer and writing new data. Tests indicate that a magneto-optical MD disc could be erased and recorded over 1 million times and would retain data for over 10 years.

MD units are available for home, car, and portable listening. The sound quality of the MD is comparable to that of the audio CD. The Minidisc format was introduced in the United States in 1992.

Minidisc Data (MD Data) is an adaptation of the audio minidisc. MD Data contains digital code similar to that of a CD-ROM, but the user can record on it as on a floppy disc. The Minidisc Data format was introduced in the United States in 1993.

Contributor: Ken C. Pohlmann, M.S.E.E., Prof. of Music Engineering, Univ. of Miami.

Compact Disc

Audio CD's

CD-ROM's

Other Formats

Master Index