How CD ROM drive works

by Terry McLean.
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A CD-ROM drive operates by using a laser to reflect light off the bottom of the disc. The reflected light is then read by a photo detector. The overall operation of a CD-ROM drive is as follows:


  1. The laser diode emits a low-energy infrared beam toward a reflecting mirror.

  2. The servo motor, on command from the microprocessor, positions the beam onto the correct track on the CD-ROM by moving the reflecting mirror.

  3. When the beam hits the disc, its refracted light is gathered and focused through the first lens beneath the platter, bounced off the mirror, and sent toward the beam splitter.

  4. The beam splitter directs the returning laser light toward another focusing lens.

  5. The last lens directs the light beam to a photo detector that converts the light into electric impulses.

  6. These incoming impulses are decoded by the microprocessor and sent along to the host computer as data

When first introduced, CD-ROM drives were too expensive for widespread adoption. In addition, drive manufacturers were slow in adopting standards, causing a lag time for the production of CD-ROM titles. Without a wide base of software to drive the industry, acceptance was slow.

After the production costs of both drives and discs began to drop, however, CD-ROMs were rapidly assimilated into the PC world. This was particularly due to the ever-expanding size of PC applications. Virtually all software is now supplied on CD-ROM, even if the disc doesn't contain data representing a tenth of its potential capacity. In some cases, particularly large programs are now distributed on DVD as well as CD.

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