How a CD works
Compact disc (CD) players have increasingly replaced the phonograph and the cassette tape. They offer a clear, crisp sound, faint background noise, and have the additional advantage of longer life. In addition to their audio content, some compact discs contain digital graphics that can be displayed on a television screen.
To understand how a CD works one must understand what a CD consists of. A CD is about 1.2 millimeters thick. Most of the CD consists of an injection-molded piece of clear polycarbonate plastic. During manufacturing, a high precision laser beam is used to burn microscopic pits arranged as a very long spiral track of data in the thin layer of a disk. Once the clear piece of polycarbonate is formed, a thin, reflective aluminum layer is placed onto the top of the disc. Then a thin acrylic layer is sprayed over the aluminum to protect it. Finally, a label may be printed onto the acrylic.
Compact discs are never in physical contact with any pickup devise. Instead, pits embedded in the surface of the disc are read by a laser beam of light. A CD player contains a low power laser and high precision lenses and mirrors. A servomotor positions the lenses and mirrors to attract on the disc. The laser directs a narrow beam of light onto tracks of the spinning disc. Along a track, sections with pits scatter the light differently from ridges or sections without pits. The sequence of the sections represents the sound information. The laser starts reading the disc from the inside and ends on the outside. While a CD is spinning, a laser beam shines on the pits and ridges. When the beam strikes a ridge the beam is reflected onto a photoelectric cell or photodetector and gives off a current. When the laser beam shines on a pit, only half of the light hits the surface. The other half goes into the high part of the pit and the photodetector does not produce current. Therefore the photodetector receives a series of light pulses according to where the pits and ridges are in the disc. The photodetector sends the pulses to a digital to analogue converter (DAC). A DAC converts the series of pulses back to binary coding, and then to decimal values. Finally, the original analogue signal can be rebuilt and converted into sound.
The same technology that is used for sound can be used for storing other types of information as well including computer programs, pictures, and animation. The compact disc is one of the most useful forms of storing various types of data.
The Compact Disc (CD) Player. Microsoft Encarta 98 Encyclopedia. 1997.
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