Q. What is Compact Disk Read Only Memory?

This technology has evolved out of entertainment electronics market where cassette tapes and long playing records are being replaced by CDs. Term CD used for audio records stands for Compact Disk. Disks used for data storage in digital computers are called as CD-ROM whose diameter is 5.25 inches. It can store around 650MB. Information in CD-ROM is written by creating pits on disk surface by shining a laser beam. As disk rotates laser beam traces out a continuous spiral. Focused beam makes a circular pit of around 0.8-micrometer diameter wherever a 1 is to be written and no pits (also known as a land) if a 0 is to be written. Figure below displays CD-ROM and DVD-ROM.

CD-ROM with pre-recorded information is read by CD-ROM reader which employs a laser beam for reading. It is rotated by a motor at a speed of 360 RPM. A laser head moves in and out to specified position. As disk rotates the head senses pits and land. This is converted to 1s and 0s by electronic interface and sent to computer. Disk speed of CD-ROM is denoted by notation nx where n is an integer denoting the factor by that original speed of 150KB/s is to be multiplied. It is associated to a computer by IDE and SCSI interfaces. The main application of CD-ROM is in distributing large text, audio and video. For illustration the whole Encyclopaedia could be stored in one CD-ROM. A 640MB CD-ROM can store 74 minutes of music.