第8章电子信息类专业英语(李白萍).ppt

Unit Eight Auxiliary Storage Passage A CD and CD-ROM 　　A compact disc (CD) is a laser-read (also termed “optically read”) data storage device on which audio, video, or textual material can be stored. Although it has been used primarily to record stereophonic sound and has supplanted the long-playing PHONOGRAPH record as the principal medium for music storage, it offers a huge potential as the medium for the storage of massive amounts of many types of information.[1] 　　Unlike the conventional phonograph record, the CD stores information in digital form. Stereophonic (two-channel) sound signals are digitally sampled at a rate of 44,100 times per second per channel. Each sample is expressed as a binary number value consisting of 16 binary digits, or bits. The sampled digital values, along with error correction data, tracking codes, and cueing data, are recorded on a digital tape, which is used to make compression-molded plastic discs 12 cm (4.72?in) in diameter, each covered by a thin, reflective metallic layer and protected by a clear plastic coating.[2] During playback, a low-powered laser beam reads the digital data through the reflective rear surface of the disk.[1] 　　The CD improves over conventional records and tape recordings with its more uniform and accurate frequency response, a complete absence of background noise, a wider dynamic range (the difference between the softest and loudest recordable musical sounds), and longer wear—since nothing mechanical touches the surface of the disc when it is played. 　　Introduced in 1992, the Sony MiniDisc measures only 6.35?cm?(2.5?in) in diameter, but it can be used to record as well as play up to 74 minutes of music. Its small size is made possible by a system of data reduction that uses psycho-acoustic principles to eliminate data that is not normally heard by the listener-thereby reducing the data density to one-fifth that required for conventional CDs. Mini-Discs can be used only with their own recorder-player. 　　A