disc, the conversion from coded information back to a
byte is accomplished from a look-up table. This table
is in a ROM on the disc drive. Three additional bits are
added to each 14-bit code to provide separation and
low-frequency suppression.
TOPIC 2—CD-ROM DRIVES
Although still relatively new, CD-ROM drives are
becoming popular as a tertiary storage media device.
CD-ROM drives vary by manufacturers in the method
the data is read from the disk and the laser system used,
but the basic operation is similar. In this section, we
cover the common components and operation of
CD-ROM drives.
The basic components of the
CD-ROM drive are the following:
Optical head
Turntable
Computer interface section
Microprocessor based control system
Figure 11-3 shows a basic block diagram of a CD-ROM
drive.
OPTICAL HEAD
The optical head maintains the circuitry to read the
data from the disc. This unit usually consists of four
main subassemblies; (1) the laser, used to generate a
light beam; (2) a lens system, to focus the laser beam
on the disc and to direct the reflected light to the
photodetector; (3) a series of servomotors that controls
the position of the laser and lenses to ensure proper
tracking and focus; and (4) a photodetector, that
evaluates the reflected light and converts the light to
electrical impulses.
Laser
The laser in a CD-ROM drive is generally a small
injection laser diode that emits light in the infrared
band. An injection laser is energized by injecting it with
an electric current across a semiconductor junction.
Injection laser diodes are the smallest laser light source.
They are highly efficient and mass produced.
Figure 11-3.—A CD-ROM drive block diagram.
11-4
