compact disc, (CD), a molded plastic disc containing digital data that is scanned by a laser beam for the reproduction of recorded sound and other information. Since its commercial introduction in 1982, the audio CD has almost completely replaced the phonograph disc for high-fidelity recorded music. Coinvented by Philips Electronics N.V. and Sony Corporation in 1980, the compact disc has expanded beyond audio recordings into other storage-and-distribution uses, notably for computers (CD-ROM) and entertainment systems (videodisc).
This article briefly describes the physical characteristics and performance of the audio compact disc. For descriptions of machine-readable discs containing multimedia or video data, see the articles CD-ROM and videodisc.
A standard CD is 120 mm (4.75 inches) in diameter and 1.2 mm (0.05 inches) thick. It is composed of a clear polycarbonate plastic substrate, a reflective metallic layer, and a clear protective coating of acrylic plastic. The reflective metallic layer is where audio data is read in the form of minuscule (as short as 0.83 micrometre) depressions (pits) and contrasting flat regions (lands) that are arranged in a spiral track (groove) winding from the disc’s inner hole to its outer edge. The centres of adjacent grooves are spaced 1.6 micrometres apart (see figure). A smaller CD single (80 mm [3.1 inches] in diameter) is also used for audio distribution.
Recording and replication
The production of a CD begins with a digital tape master supplied by the recording studio. The information on this tape is used to modulate a beam of light from a blue laser as it traces a spiral path on the surface of a spinning glass disc. The glass is coated with a photosensitive material that dissolves where it is exposed to laser pulses, forming the pits. This “glass master” is coated with a thin layer of nickel to form a “metal master,” and the metal master in turn is used to produce a number of “mothers.” Each mother serves as the master for several metal “stampers,” onto which molten polycarbonate is injected for molding into clear plastic discs. Each disc is exposed to a stream of vaporized or atomized aluminum, which forms the reflective layer, and is then coated with the protective acrylic layer. The entire production process is carried out under conditions of laboratory-like cleanliness and control.
When a disc is inserted into a CD player, the disc’s track is scanned by a low-intensity infrared laser with a 1-micrometre-diameter focal point. In order for the laser to maintain a constant scanning rate, the disc’s rotation rate decreases from 500 to 200 revolutions per minute as the light beam spirals out from the disc’s centre. (Some CD players use two additional lasers to help control the disc’s rotation and the scanning laser’s focus.) When the light beam strikes a land, it is reflected back to a photodiode, and an electrical pulse is generated. When the light beam strikes a pit, however, no electrical pulse is generated. This is because light reflected from the pit, which has a depth of approximately one-quarter the wavelength of the scanning infrared beam (0.78 micrometre), is out of phase with light reflected from the adjacent separation track, and thus the reflected light is reduced below the level necessary to activate the photodiode. Each “dark” pit on the track is interpreted (based on its length) as a sequence of 0s in binary logic, and each “bright” land is interpreted (again based on its length) as a sequence of 1s. A device known as a digital-to-analog convertor is necessary to translate—and correct for data misread because of minor surface blemishes on the disc or imperfect laser alignment—this binary information into audio signals for playback. The standard CD will hold more than one hour of music.
Analog versus digital sound
In analog sound recording, such as that on phonograph discs, audiocassettes, and standard audiotapes, an analog of the source audio waves is physically produced. Playback then requires an abrasive physical device to literally trace the recorded sound wave. Digital sound recording, such as that on compact discs, videodiscs, and CD-ROMs, instead involves taking multiple discrete measurements of the voltage levels of the continuous source audio waves, a process known as sampling. The most common sampling rate is 44.1 kilohertz (kHz), or 44,100 times per second, which guarantees at least two measurements of any humanly audible sound wave. (The typical sound range audible to a person is 20 Hz to 20 kHz.) The accuracy of the recorded voltage measurements depends critically on the number of binary digits (bits) used to record the measurements. More bits enable finer distinctions to be made in audio voltage levels and, in turn, enable a closer approximation of the original sound wave. The industry standard of 16 bits is sufficient to produce an audibly smooth curve with very little distortion. For even greater fidelity, music studios sometimes use 24-bit encodings for their master tapes. Because the recorded bits are read from the internal reflective layer of the CD by a laser, the disc remains untouched by any physical object and thus does not degrade under normal use.
The number of bits determines the maximum attainable dynamic range. Dynamic range is the ratio of the loudest undistorted sound to the quietest discernible sound, expressed in decibels, that a system is capable of producing. The compact disc’s dynamic range is about 90 decibels, compared with about 70 decibels on the best phonograph discs, thus accounting for the distinct, clear sound obtained from even the cheapest CD players. Nevertheless, some audiophiles maintain that the best phonograph recordings deliver subtle musical overtones that are almost invariably lost in the digitization process.
Other digital discs
Laserdiscs and CD-ROMs
Because any type of information—not just audio—can be digitized, other forms of digital discs have been produced. Among them are the larger (120 to 300 mm [4.7 to 11.8 inches]) laserdisc, or videodisc, which combines digital audio and analog video data, and the CD-ROM. (ROM is an abbreviation for “read-only memory.”) CD-ROM discs are identical in appearance to audio CDs, but they must be read by a computer, and they generally contain up to 680 megabytes of computer programs and data, rather than just audio or video data.