Compact Discs - CD-ROM, CD-R, CD-RW

Q. How do CD-ROM drives read data?

A. A CD-ROM drive contains a tiny laser that’s part of a read head assembly. The read head assembly moves over the face of a CD-ROM, shining the laser onto the disc’s surface. At the surface, a CD-ROM includes a series of tiny pits stamped into its plastic layer as it is manufactured. The empty spaces between the pits are called lands.

CD-ROMs also have a reflective aluminum layer. Using the characteristics of these layers and its laser, a CD-ROM drive’s sensor is able to distinguish between pits and lands according to the amount of light reflected back. Lands reflect more light than pits do because when the laser’s light strikes a pit, some of the light diffuses.

Next, the CD-ROM drive’s sensor interprets the pits, lands, and the leading edges of the pits as strings of 1s and 0s, which the computer can then process as digital information.

Q. What’s the difference between a CD-R drive and a CD-RW drive?

A. CD-R drives write data only to CD-Rs, whereas a CD-RW drive can write data to both CD-Rs and CD-RWs. CD-R drives were somewhat popular a few years ago, but CD-RW drives became much more popular among consumers as their prices dropped to the level of CD-R drives. Today, CD-R drives for consumers are virtually nonexistent because CD-RW drives are just more practical.

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