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Unformatted text preview: echanism because the rotation speed of
the disk must vary inversely with the radius; the drive must slow down the disk's rotation
speed to read sectors towards the outside of the disk and speed it up to read sectors
towards the center of the disk. This access mechanism leads to slower data access (larger
access time) as compared to magnetic disks, in which case the disks rotate at a constant
speed irrespective of the location of the data to be accessed.
Access times for optical disks are typically in the range of 100 to 300 milliseconds.
Compare this with the access times of floppy disks which are typically in the range of
100 to 200186
Optical Disk Drive
An optical disk has to be mounted on an optical disk drive before it can be used for
reading or writing of information. An optical disk drive contains all the mechanical,
electrical and electronic components for holding an optical disk and for reading or
writing of information on to it. That is, it contains the tray on which the disk is kept, the
read/write laser beams assembly, and the motor to rotate the disk. A typical optical disk
drive is shown in Figure 8.25.
An optical disk drive differs from a magnetic disk drive in the following manner:
1. As shown in Figure 8.24, it uses laser beam technology for data reading/writing and
has no mechanical read/write access arm.
Magnetic disk drives use a constant angular velocity (CAV) encoding scheme
whereas optical disk drives use a constant linear velocity (CLV) encoding scheme. That
is, in case of hard disk drives the disk spins at a constant rate of speed and each sector
occupies the area subtended by a fixed angle. On the other hand, in case of optical disk
drives, the rotational speed of the disk varies inversely with the radius; the motor slows
down the disk to read sectors towards the outside of the disk and speeds it up to read
sectors towards the inside of the disk. Doing this requires a more complicated drive
mechanism that can overcome the inertia of the disk when accelerating and the
momentum when decele...
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This document was uploaded on 04/07/2014.
- Spring '14