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Word Count: 3,418
CD-ROM Compact Disc Read Only Memory A compact disc format used to hold text graphics and hi-fi stereo sound Its like an audio CD with spiral grooved tracks but uses a different format for recording data The audio CD player cannot play CD-ROMs but CD-ROM players can play audio discs CD-ROMs hold 650MB of data which is equivalent to about 250000 pages of text or 20000 medium-resolution images Sometimes 680MB is used as the capacity depending on whether the total number of bytes 681984000 is divided by 1000000 or 1048576 A CD-ROM drive player reader connects to a controller card which is plugged into one of the computers expansion slots Earlier drives used a proprietary interface and came with their own card requiring a free expansion slot in the computer Today CD-ROMs use SCSI or EIDE and can be installed without taking up an extra slot The first CD-ROM drives transferred data at 150KB per second Speeds doubled to 300KB and continued upward to as much as 32 times the original Access times range from 80 to 120ms Reading a CD-ROM Digital data is carved into the CD-ROM as pits low spots and lands high spots As the laser shines into the moving pits and lands a sensor detects a change in reflection when it encounters a transition from pit to land or land to pit Each transition is a 1 The lack of transitions is 0s There is only one laser in a drive Two are used here to illustrate the difference in reflection CD-ROM Drives In a few short years the Compact Disk - Read Only Memory CD-ROM drive has gone from pricey luxury to inexpensive necessity on the modern PC The CD-ROM has opened up new computing vistas that were never possible
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