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 (short for 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.
Loading the player...Compact disc
CD’s are made of hard plastic coated with thin metal, usually aluminum. CD’s have only one playing side. A standard CD measures about 43/4 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 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 size and data format, are also standardized. Audio CD’s were introduced in Japan and Europe in 1982, and in the United States in 1983. By the early 1990’s, CD’s had largely replaced long-playing (LP) records and had begun 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, called samples, for each second of sound. It assigns to each sample a corresponding digital code. This code is expressed as a string of 16 electric pulses representing 1’s and 0’s. It represents any one of 65,536 sound values.
Manufacturing.
A master tape made at a recording studio is sent to a CD manufacturing plant to make a master disc. A master disc begins as a glass disc coated with a photoresist, a chemical film that is eaten away when exposed to light. A laser exposes the photoresist in a spiral track, creating microscopic pits that represent the digital data on the tape. Pits are about 1/42,000 inch (0.6 micrometer) wide. Depending on playing time, a pit spiral might contain more than 2 billion pits. If unwound, this spiral would stretch about 3 miles (5 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 create clear discs. These discs are then coated with a metal layer and a protective plastic layer. A label is usually printed directly on this plastic layer. A sophisticated manufacturing system can produce one CD every few seconds.
Playback.
As the CD spins inside a CD player, a laser beam shines through the plastic on the underside of the disc and follows the pit spiral. The metal coating reflects the laser beam. The intensity of the reflection changes as the beam enters and leaves a pit. A device called a digital-to-analog converter translates the 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.
CD players are available for home, car, or portable use. 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. A CD changer enables the user to 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-ROM’s
CD-ROM’s 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. 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 other character. One CD-ROM can hold the text of many 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 computer software. Other types of CD-ROM’s include multimedia CD’s and picture CD’s. Multimedia CD’s contain a combination of text, graphics, sound, photographs, motion pictures, and other types of media. A person using a multimedia CD-ROM might “tour” a museum on a computer monitor, viewing and listening to the exhibits. Other multimedia CD-ROM’s contain encyclopedias, stories, and video games.
A picture CD can store photographs taken with an ordinary camera. Many photo shops can arrange for photographs to be transferred from negatives or slides to a picture CD. Consumers can use CD-ROM drives to view the pictures on a computer monitor. A picture CD stores no sound information.
Other formats
DVD’s
are the same size as CD’s, but they have much greater storage capabilities. A DVD stores data on one or both sides. Each side of the disc can contain two data layers, one embedded beneath the other. A DVD can hold up to 17 gigabytes (billion bytes) of information. According to most DVD manufacturers, the letters DVD have no specific meaning.
The DVD requires a special drive, which can also play CD-ROM’s and audio CD’s. A DVD drive can read data at a much faster rate than a typical CD-ROM drive. The DVD format appeared in 1996. In 2000, manufacturers introduced systems that can record television programs or other data on special DVD’s.
SACD
(Super Audio CD) is a high-resolution audio-CD format. SACD technology divides analog signals into 64 times as many samples per second as do traditional CD’s. A special player is required to play a SACD. The first SACD player became commercially available in 1999. In 2003, manufacturers began producing SACD’s encoded with the same recording in traditional CD format on a separate layer. These discs can be played on both SACD players and traditional CD players.
Video CD’s
store digital video and audio signals. A 74-minute full-motion, full-color video program with stereo sound can be stored on one disc. The White Book standardizes Video CD’s. A Video CD player or DVD player is connected to a television and stereo system to view the program. The Video CD format was introduced in 1994.
CD-R
(Compact Disc-Recordable) is a format that 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 CD-ROM, 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 that would not be economical for a CD manufacturing plant to produce. CD-R’s are also popular with music fans who compile discs with their choice of tracks.
CD-RW
(Compact Disc-ReWritable) is a format that enables a user to record, erase, and re-record such information as music and data. The format is standardized in the Orange Book. A blank disc holds as much data as a standard CD-ROM disc. Only newer, more sensitive players can read a CD-RW disc.
CD-RW format stores information using a method called phase-change recording. The recording layer of a CD-RW disc is made of a material with reversible properties. On a blank disc, this layer is in a crystalline state. During recording, a high-power writing laser emits intense heat on tiny, select areas of the recording layer. These areas change to an amorphous (noncrystallized) state. Unheated areas remain in their crystalline state. Like the pits on an standard CD, the amorphous marks reflect less light than the crystalline areas. A CD-RW is read by reflecting a low-power reading laser off the spiral track on the disc’s surface to a converter that detects the difference between light reflected by the crystalline and amorphous areas. To erase all or part of a disc, a CD-RW drive uses a medium-power laser to heat the recording layer and return it to a crystalline state. Users can write and erase data at least a thousand times.
Minidiscs
(MD’s) measure only 21/2 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 disc 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. MD units are available for home, car, and portable listening.
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. The minidisc format was introduced in 1992.
