( CD-ROM Contd:)
Given these microscopic dimensions, a CD-ROM disc offers about 16,000 tracks per inch (TPI). During playback, CDs use a highly focused laser beam and laser detector to sense the presence or absence of pits. Figure: 9.2 illustrate the reading behavior. The laser/detector pair is mounted on a carriage, which follows the spiral track across the CD. A laser is directed at the underside of the CD, where it penetrates more than 1 mm of clear plastic before shining on the reflective surface.
When laser light strikes a land, the light is reflected toward the detector, which, in turn, produces a very strong output signal. As laser light strikes a pit, the light is slightly out of focus. As a result, most of the incoming laser energy is scattered away in all directions, so very little output signal is generated by the detector.
As with floppy and hard drives, the transition from pit to land (and back again) corresponds to binary levels, not the presence or absence of a pit or land. The analog light signal returned by the detector must be converted to logic levels and decoded. A process known as Eight-to-Fourteen Modulation (EFM) is very common with CD-ROMs.
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