is off to the left, off to the right, or on track. The output
of the photodiode is fed to a differential amplifier. If
the laser is on track, the output voltage of the
photodiodes is equal. If the laser beam is off to the left
or right, a control voltage will be generated by the
differential amplifier that is fed to the track following
servo. The amplitude and polarity of this control
voltage determines the direction and distance of
correction needed.
The same four photodiodes are also used to
determine the focus of the laser beam as it strikes the
disc. Figure 11-4 shows how the photodiodes will react
to detect if the disc is too close to the laser or too far
away from the laser, or if the laser is in focus. Again,
the output of the photodiodes is fed to an amplifier and
correction of focus is made by moving the objective
lens.
The output of these four photodiodes is also
summed and contains the encoded data on the disc. It
is then sent to the control section for decoding.
CD CONTROLLER
The CD controller processes the signals received
from the optical head, attempts to correct any errors in
the data, and controls the speed of the turntable. The
information from the photodiodes that is received by
the controller is still encoded in eight-to-fourteen
modulation (EFM) data.
The decoding of EFM data is done by the
microprocessor.
The code addresses a ROM that
contains the proper byte for the encoded data. The
output of the ROM is stored in a RAM where it is
checked for errors.
TURNTABLE
The turntable rotates the disc and is driven by a
servomotor. Since the data is written in a continuous
spiral, the speed of the turntable must be adjustable so
that the information passes over the optical head at a
constant speed. The audio CD requires a speed of 1.3
meters per second. This speed was adapted for use in
computer applications, but proved to be extremely slow
when compared to the processing and data transfer
speeds of modem computers. The 2X CD-ROM drive
doubled the speed the data track passed over the optical
head. The 4X, 6X, and 8X CD-ROM drives spin the
disc even faster.
The speed multiplication factor is
based on the original speed of 1.3 meters per second.
Initial speed adjustments are made when the optical
head is positioned in the approximate area of the data.
The header of each sector contains a synchronization
pulse that is fed into a sawtooth wave generator. The
sawtooth wave is fed to the turntable servomotor. The
frequency of the wave is used to make fine adjustments
to the turntable speed.
INTERFACE SECTION
The interface section provides for the transfer of
data between the computer and the CD-ROM drive.
Many CD-ROM drives are manufactured with the small
computer systems interface (SCSI), although some
proprietary interface units are available.
TOPIC 3—CD-ROM APPLICATIONS
Applications that use CD-ROM are rapidly
expanding throughout the Navy as systems are updated
and the need for reliable storage of large amounts of
information increases.
DATABASES AND PUBLICATIONS
CD-ROMs are used in command and control
systems, intelligence systems, and the supply system.
These applications use large databases. Databases,
such as a part number cross-reference list, can
significantly reduce the amount of paper storage space
required.
The CD-ROMs allow information to be
quickly retrieved, cross-referenced, and displayed to
the user.
Many publications and instructions are also being
stored on CD-ROM in an effort to reduce printing and
mailing costs. As publications are updated, a new disc
is made and sent to all users, who then replace the old
disc.
MULTIMEDIA (CD-I) APPLICATIONS
Multimedia or compact disc-interactive (CD-I)
applications combine machine executable code
(programs), text, audio, video and graphics all on the
same CD-ROM. The microprocessor in the CD-ROM
drive reads the code at the beginning of each sector to
determine if the information that follows is audio,
video, graphics, etc. The data is then output on the
appropriate channel of the CD-I drive.
11-6
