PC Hardware A Beginner’s Guide

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screw holes are tapped into the sides of the enclosure that allow for mounting it directly
into a standard drive bay, as shown in Figure 10-4. On some older PCs, a CD-ROM, as
well as a hard or floppy disk drive, is mounted in the PC bay with mounting rails that
attach to the sides of the drive and then slide into the drive bay.


The Laser and Head Assembly


The laser in a CD-ROM drive is a beam of light that is emitted from aninfrared laser
diode. The laser is aimed not directly on the CD but toward a reflecting mirror in the read
head assembly. The read head moves along the spiraling track of the CD just above the
surface of the disc. The light from the laser reflects off the mirror and then passes through
a focusing lens that directs the light directly on a specific point on the disc. The light is
reflected back from the shining metallic coating on the disc. The amount of light reflected
depends on whether the laser is hitting a flat or a pit. The reflected light is passed through
a series of collectors, mirrors, and lenses that are used to focus the reflected light and send
it to aphoto detector. The photo detector converts the light into an electrical signal, the
strength of which is determined by the intensity of the reflected light. Figure 10-5 illus-
trates the components of the CD-ROM drive’s head assembly.
Since the disc spins, most of the components used to “read” the CD are fixed in place,
with only the read head assembly, which contains the mirror and read lens, actually moving.
The CD-ROM is a single-sided media and data is recorded on one side. This means that
the CD-ROM drive requires only one read head and head assembly, resulting in an over-
all design that is relatively simple.
Probably the biggest problem for the CD-ROM drive is that because it uses light, the
laser must not be obstructed. Dust or other foreign material on the disc or on the focus
lens in the read head can cause problems for a CD-ROM drive to the point of causing
errors or even a drive failure.


Chapter 10: CD-ROMs and DVDs^209


Figure 10-4. A CD-ROM drive. Photo courtesy of Kenwood Corporation
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