Compact Disc


Compact Disc
a brand of compact disk.

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Introduction
(CD) 

      a molded plastic disc containing digital data that is scanned by a laser beam for the reproduction of recorded sound (musical 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.

Mechanical features

Physical characteristics
 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 (digital sound 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.

Playback
 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

Sampling
      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 (bit)) 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.

Dynamic range
      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 (videodisc), 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.

DVD
 In 1995 Philips and Sony introduced a new type of disc, known as a digital videodisc (DVD), which was able to store up to 4.7 gigabytes of data, such as high-definition digital video files. A DVD has the same dimensions as a standard CD but cannot be read by a standard CD player, although a DVD player can read standard CDs. DVD players use a higher-power red laser (0.65 micrometre) that enables smaller pits (0.4 micrometre) and separation tracks (0.74 micrometre) to be used (see figure—>).

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Universalium. 2010.

Look at other dictionaries:

  • compact disc — com pact disc , compact disk com pact disk (k[o^]m p[a^]kt d[i^]sk ), n. a disk shaped optical data storage medium approximately 4 3/4 in. in diameter, which stores binary data as microscopic nonreflective holes or pits in an otherwise reflective …   The Collaborative International Dictionary of English

  • compact disc — UK US noun [C] ► a CD(Cf. ↑CD): »The authorities revoked the licenses of three compact disc manufacturers today for illegally copying computer software …   Financial and business terms

  • compact disc — n [U and C] CD a small circular piece of hard plastic on which high quality recorded sound or large quantities of information can be stored ▪ The new album is available on vinyl, cassette, or compact disc …   Dictionary of contemporary English

  • compact disc — compact discs N COUNT: also on N Compact discs are small shiny discs that contain music or computer information. The abbreviation CD is also used. The soundtrack of Highlander II will be released on compact disc at Easter. (in AM, also use… …   English dictionary

  • compact disc — or compact disk [käm′pakt΄] n. a small, plastic OPTICAL DISC for storing digital data, recording sounds digitally, etc …   English World dictionary

  • compact disc — (o compact disk) / kɔmpækt disk/, it. / kompakt disk/ locuz. ingl. (propr. disco compatto ), usata in ital. come s.m. (talora abbrev. in compact ). 1. (elettron.) [disco fonografico a lettura ottica mediante raggio laser, che dà un alta qualità… …   Enciclopedia Italiana

  • compact disc — ► NOUN ▪ a small plastic disc on which music or other digital information is stored in the form of a pattern of metal coated pits from which it can be read using laser light reflected off the disc …   English terms dictionary

  • compact disc — (ingl.) m. *Disco compacto. ⊚ Reproductor de discos compactos …   Enciclopedia Universal

  • compact disc — noun count a CD …   Usage of the words and phrases in modern English

  • compact disc — compact (disc), compacto → disco compacto …   Diccionario panhispánico de dudas

  • compact-disc — (izg. kòmpakt dȉsk) m DEFINICIJA v. CD …   Hrvatski jezični portal


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