tailieunhanh - Ebook Tiếng Anh chuyên ngành Điện tử - Viễn thông: Phần 2

Phần 2 của cuốn sách "Tiếng Anh chuyên ngành Điện tử - Viễn thông" gồm các nội dung từ unit 14 đến unit 20. Ngoài ra trong phần này còn cung cấp các bài kiểm tra giữa và cuối khoá học, các bài dịch mẫu cũng như phần giải đáp các bài tập của từng bài trong phần phụ lục giúp cho sinh viên tự đánh giá kết quả học tập của mình và tra cứu những đoạn dịch mẫu khi cần. . | Unit 14 COMPACT DISC A digital recording system known as a compact disc CD system was introduced in 1982. This uses a laser optical mechanism in which a laser beam reads marks on the surface of a specially prepared perspex disk. It gives near-perfect reproduction of sound and the sound quality does not deteriorate with use. This breakthrough completely replaced the grooved phonograph record by the end of the 20 century. The CD differs from the standard phonograph record in two principal ways. The recorded signal on it is in PCM digital format and it is a grooveless contactless system. Instead of a groove a CD has a spiral track made of a series of pits of different lengths. These pits represent the pulse code. The master disc for a CD is an optically flat glass disc coated with a resist. The resist is a chemical that is impervious to another chemical called an etchant which dissolves glass. The master is placed on a turntable. Above it is a recording head containing a laser. The digital signal to be recorded is fed to the laser turning the laser on and off in respond to the binary on-off signal. When the laser is on. it bums away a small amount of the resist on the disc. While the disc turns the recording head moves across the disc leaving a spiral track of elongated burns in the resist surface. After the recording is complete the glass master is placed in the chemical etchant bath. This developing removes glass only W here the resist is burned away. The spiral track now contains a series of small pits of varying length and constant depth. The commercial CD mass produced from the master is made from a clear plastic. It is about inch millimeters thick. The spiral track of pits is on its op surface. This surface has a coating of aluminum so that it reflects light. In playing a CD an extremely fine laser beam is focused on the spiral track from beneath the disc. As the disc turns in the CD player the laser beam is focused on the pits seen from below the disc