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2015年职称英语考试理工类C级概括大意试题练习 Compact Disks 1 If someone says to you your music CDs dont really hold any music on them, and they only have numbers recorded on them, you may not believe it. In fact, he is right in that sound is actually recorded onto the CDs as special numbers - a digital code. The code is pressed onto the CD as bumps on a long spiral track almost five kilometers long. These bumps are an average of 0.5 microns wide 2 A small laser beam shines onto the bumps as the CD turns. The light is reflected back to a receiver that records how the laser light bounces back. This lets the CD player turn the reflected light back into the original code. This means you can hear the original code as music. 3 Digital codes are used with many technologies. E-mail needs these kinds of code numbers. Space probes communicate with their ground station on earth using digital codes. Bar codes are read as digital codes in computer systems. Digital communications with cell phones need digital codes. Weather radios also tune in to specific signals using these codes. 4 There are many types of compact disks. One format is called CD-RWs. They can be recorded on and re-recorded on as you would do with a floppy disk. Another format is the CD-ROM. The technology for recording on these disks is different from other CDs. These CDs have a dye layer that the CD writer can darken or leave clear. The clear and dark spots are the digital code. CD-ROM stands for Compact Disc - Read Only Memory4. This disk is like a super floppy disk that can hold lots of information. One CD-ROM can hold the same amount of data as 500 floppy disks. Information is permanently recorded onto it. Computer games and other programs are considered to be CD-ROMs. | 
