Compact Disc | Definition & Facts | Britannica

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Portable compact disc player.1 of 2
Portable compact disc player.(more)
Study how an infrared laser scans the metallic reflective layer of a disc to read recorded sound2 of 2
Study how an infrared laser scans the metallic reflective layer of a disc to read recorded soundThe laser scanning method employed in compact disc playersAn infrared laser is focused onto the metallic reflective layer of the disc, where a spiral track of “pits” and “lands” represents the zeros and ones of digital signals. The return signal is converted by a photodiode sensor into a digital electric signal, which is converted to analog form for reproduction of the original recorded sound. Optical recording, introduced by Sony Corporation and Philips Electronics N.V. in 1982, allows accurate reproduction of sound over virtually the entire range of human hearing.(more)See all videos for this article

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.

The iPod nano, introduced by Apple CEO Steve Jobs in San Francisco, May 2007. A revolutionary full-featured iPod that holds 1,000 songs and is thinner than a standard #2 pencil. MP3 player, music player, digital music Britannica Quiz Electronics & Gadgets Quiz

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