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Unformatted text preview: o write to the recording
surface by etching microscopic pits on the disk surface, and another laser beam (of lesser
intensity) is used to read the data from the light-sensitive recording surface. That is, data
recording is done by focusing the stronger laser beam on the surface of the spinning disk.
The laser beam is turned on and off at a varying rate because of which tiny pits (visible
only through a powerful microscope) are burnt into the metal coating of the disk along its
tracks. To read the stored data, the less-powerful laser beam is focused on the disk
surface. As shown in Figure 8.24, this beam is strongly reflected by the coated surface
(known as a land) and weakly reflected by the burnt surface (known as a pit), producing
patterns of on and off reflections that are converted into electronic signals of binary Is
and 0s by a sensor. Note that a mechanical read/write access arm is not needed in this
case because a light beam can be easily deflected to the desired place on the optical disk. Access Time
Since a mechanical read/write access arm is not needed, the delay that occurs in case of
magnetic disks due to mechanical movements of the access arm will not be there in case
of optical disks. Hence you may think that data accessing from optical disks will be much
faster (access time will be less) as compared to that from magnetic disks. However this is
not true. Optical disk drives are slower (have larger access time) than magnetic disk
drives due to the following reasons:
1. We saw that the sectors of an optical disk are arranged on a continuous spiral track.
This data organization results in a slower random access time than the concentric tracks
used by magnetic disks, whose sectors can be located faster because they are always
found on a given track at a fixed distance from the center.
2. We also saw that with optical disks each sector has the same length regardless of
whether it is located near the disk's center or away from the center. This data
organization requires a more complicated drive m...
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This document was uploaded on 04/07/2014.
- Spring '14