Chapter 10: CD-ROMs and DVDs^211
travel over a much longer linear path than when they are over the inside tracks. Another
speed measurement on a disk is itslinearvelocity, which on most disk drives is not constant.
However, many of the latest hard disk designs now store more information on the outer
tracks of the disk than they do on the inner tracks (a process calledzoned bit recording)to
take advantage of this condition.
A CD-ROM drive adjusts the speed of the spindle motor to keep the linear velocity of
the disc constant. When the read head is near the outside of the CD, the motor runs
slower; when the head moves near the inside edge of the disc, the motor runs faster. This
ensures that the same amount of data goes past the read head in any amount of time. The
process used by the CD-ROM is calledconstant linear velocity (CLV).
Early CD-ROM drives operated at the same speeds as standard CD-DA players,
which was about 210 to 539rpm (revolutions per minute), with a standard transfer
rate of 150KB or 1X (times) the CD-DA transfer rate. Increasing the spindle motor’s
speed and beefing up the CD-ROM’s electronics increased the transfer rate of the
CD-ROM drive. This allowed the CD-ROM drive to transfer data fast enough to support
multimedia software.
Uptotransferratingsof12X(12timesthetransferrateofaCD-DA),thespindlemotor
speed is varied to maintain CLV. Newer CD-ROM drives now incorporate the CAV
method and vary the transfer rate depending on where the read head is on the disc. On a
CAVCD-RM,the“X”transferspeedratingisanindicationofthebestpossiblespeeddata
(near the outside edge) can be transferred. For example, a CAV drive with a claim of 50X
data transfers can’t really transfer data at that rate over the entire CD.
As the spindle motor speeds of today’s CD-ROM drives approach 13,000rpm, the
change back to CAV is being made to avoid the difficulty of changing the motor speed
from 5,000rpm to 13,000rpm and then back to 5,000rpm. In spite of these higher, faster
speeds, the spin-up and spin-down of the CD-ROM drive is a factor of how slow a
CD-ROM drive performs, especially when doing random accesses on data located at
different edges of the CD.
The Disc Loading Mechanism
Thedisc loading mechanismis the mechanical or physical way that the CD is loaded into
the CD-ROM drive so it can be accessed or played. There are three distinct ways used to
load CD-ROM media into a CD-ROM drive:
Tray-loading This is the most common loading mechanism in use on PCs. The
tray-loading method uses a plastic horizontal tray (jokingly called a “cup holder”)
that is opened and closed by motorized gears in the drive (see Figure 10-6).Pressing
the eject button on the front of the CD-ROM drive activates the gears and servos
to extend the tray out of the drive. The CD is then placed in the fitted portion of the
tray and either a gentle nudge or pressing the eject button pulls the tray and the
CD back into the drive. To remove the CD, the tray is extended, the CD removed,
and the tray returned inside the drive. On some PC cases, the CD-ROM drive is
installed vertically; these CD-ROM drives use tabs that extend and retract to hold
the disc in place until the drive is closed.