SUMMARY—CD-ROM STORAGE
This chapter has introduced you to the CD-ROM
and CD-ROM drive. The following information
summarize important points you should have learned.
COMPACT DISC— A compact disc is an optical
storage medium that can store over 500 megabytes of
information.
PHYSICAL CHARACTERISTICS— The disc is
120 mm in diameter with a 15 mm hole in its center.
The disc is made of a polycarbonite plastic and coated
with a reflective material. Data is stored by etching
small holes in the reflective material called pits. The
nonetched areas that reflect light are called lands.
ADVANTAGES OF CD-ROM— The advantages
of using CD-ROM include:
Capability to store large amounts of information
Ability to store data, graphics, audio, and video
on the same disc
Durability—since the optical head of the
CD-ROM drive never contacts the disc, there is
no danger of a head crash, wear and tear, or
accidental data corruption that magnetic media
suffer.
DISADVANTAGES OF CD-ROM— The
disadvantages of CD-ROMs include:
* High initial cost to produce a single disc
l Slow access and data transfer times compared
with high performance fixed disk systems
DATA STORAGE STRUCTURE— Data is
stored on a CD-ROM disc in a continuous spiral that
starts at the inside of the disc. The spiral is divided into
sectors that each hold 512 bytes. Sectors are addressed
by minute: second: sector. The number of sectors per
revolution of the disc varies as the spiral moves toward
the outer edge. The disc drive varies the speed of the
disc so that the data passes over the optical head at a
constant 1.3 meters per second. This is known as
constant linear velocity. Data is encoded on the disc
using a method known as eight-to-fourteen modulation.
Eight-to-fourteen modulation uses 14 bits to represent
1 byte and aids in error detection and correction.
CD-ROM DRIVES— The CD-ROM drive reads
the information stored on a compact disc. The methods
used to read data from the disk and the laser systems
used in CD-ROM drives vary by manufacturer, but have
several similarities.
The basic components of the
CD-ROM drive are the optical head, a turntable, a
computer interface, and a microprocessor-based control
system.
OPTICAL HEAD— The optical head is the heart
of the CD-ROM drive. It contains a small laser diode
to read the data on the disc. The optical head also
contains circuitry and optics to control the tracking and
focus of the laser beam.
CD CONTROLLER— The CD controller
receives the raw data signals from the optical head and
converts the eight-to-fourteen encoded data to eight-bit
bytes. The controller also prepares the data for transfer
to the computer via the interface and controls the speed
of the turntable.
INTERFACE SECTION— The interface section
controls the data exchange between the computer and
the CD-ROM drive. CD-ROM drive interfaces can be
SCSI or proprietary systems.
CD-ROM APPLICATIONS— CD-ROMs are
used to distribute large amounts of information, such as
databases and publications. CD-ROMs can also
combine types of information, such as audio, video,
data, and graphics. These systems are compact disc
interactive or CD-I.
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