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- Morita Akio
compact disc
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What is a compact disc (CD)?
A compact disc (CD) is a molded plastic disc storing digital data, read by a laser beam to reproduce recorded sound and information. Introduced commercially in 1982 for audio, CDs have largely supplanted phonograph records for high-fidelity music.
CDs are also used for computer storage (CD-ROM) and entertainment systems (DVD). A CD player scans the disc's track with a low-intensity infrared laser. The laser beam, when striking a "land," reflects back to a photodiode, creating an electrical pulse; striking a "pit" generates no pulse. Each pit is interpreted as a sequence of 0s, and each land as a sequence of 1s. A digital-to-analog converter translates this binary information into audio signals. The standard CD can hold over an hour of music.
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How does a CD store information?
CDs store information using a laser to read binary data encoded as tiny pits on the disc. A low-intensity infrared laser scans the CD's track. When the laser beam hits a "land," it's reflected back, creating an electrical pulse. But when it strikes a pit, there's no pulse because the reflected light is out of phase.
Each dark pit is read as a string of 0s, and each bright land as a string of 1s. A digital-to-analog converter then translates this binary data into audio signals.
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What does it mean to record data on a CD?
Recording data on a compact disc (CD) involves using a digital tape master from a recording studio to modulate a blue laser beam. This beam traces a spiral path on a spinning glass disc coated with a photosensitive material. The laser pulses create pits by dissolving the exposed material. This "glass master" is then used to create a "metal master," which in turn produces "mothers." Each mother serves as the master for metal "stampers," onto which molten polycarbonate is injected to mold clear plastic discs.
After molding, each disc is coated with vaporized aluminum to form a reflective layer and then with a protective acrylic layer. The entire process occurs under strict, clean conditions. The resulting CDs can store various forms of digital information, including audio, computer programs, and data.
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What equipment is needed to record music or other files onto a CD?
To record music or other files onto a compact disc, the production of a CD starts with a digital tape master provided by the recording studio. The information on this tape modulates a blue laser beam as it traces a spiral path on a spinning glass disc. The glass, coated with a photosensitive material, dissolves where exposed to laser pulses, creating pits. This "glass master" gets coated with nickel to form a "metal master," which, in turn, produces "mothers." Each mother acts as the master for metal "stampers," onto which molten polycarbonate is injected to mold clear plastic discs. Each disc is exposed to vaporized or atomized aluminum, forming the reflective layer, and then coated with a protective acrylic layer.
By the mid-1990s, advancements in computer technology allowed CD recording and replication to bypass the need for a digital tape master. High-quality sound recordings could be directly sent from a microphone or another device to computer programs. These programs store digital files on the computer’s hard disk (or magnetic storage media) before transferring them to a CD.
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How does a laser record and read data on a CD?
A CD's data is stored on a reflective metallic layer as minuscule pits and flat regions called lands, arranged in a spiral track.
During recording, a blue laser modulates according to the information from a digital tape master, creating pits on a spinning glass disc coated with photosensitive material. For playback, a CD player uses a low-intensity infrared laser to scan the disc's track. When the laser beam strikes a land, the light reflects back to a photodiode, generating an electrical pulse. When the beam hits a pit, no pulse is generated because the reflected light is out of phase and too weak to activate the photodiode.
Each dark pit is read as a sequence of 0s, and each bright land as a sequence of 1s. A digital-to-analog converter translates this binary information into audio signals.
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What are the differences between CD-R, CD-RW, and regular CDs in terms of recording?
CD-ROMs, introduced in the mid-1980s, are read-only memory computer disks. Data is encoded as tiny pits and read by a laser beam. Standard CDs are not recordable, which led to the development of CD-R and CD-RW.
CD-Rs have a light-sensitive organic dye layer that can be burned to create dark spots, similar to the pits on a regular CD. They are also known as WORM (Write Once Read Many) discs. CD-RWs, introduced in the mid-1990s, are rewritable and use excitable crystals.
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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 (or record) for high-fidelity recorded music. Coinvented by Philips Electronics NV 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 (DVD).
Mechanical features
Physical characteristics
A standard CD is 120 mm (4.75 inch) in diameter and 1.2 mm (0.05 inch) 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 (0.0016 mm) apart. 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 (see also digital sound recording). 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.
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By the mid-1990s, however, developments in computer technology advanced such that CD recording and replication could avoid the need for a digital tape master. High-quality sound recordings could be sent from the microphone or other device directly to computer programs whose digital files could be stored on the computer’s hard disk (or magnetic storage media) before being transferred to a CD.
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 (see also digital-to-analog conversion). The standard CD will hold more than one hour of music.
