Page 55 - DCAP101_BASIC_COMPUTER_SKILLS

Basic Computer Skills

Notes 2. In case of optical disks, each sector has the same length regardless of its position from
the center. This data organization requires a more complicated drive mechanism
because disk’s rotation speed must vary inversely with the radius. That is, the drive
must slow down disk’s rotation speed to read/write sectors towards the outside of the
disk and speed it up to read/write 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.

3. Since optical disk is a removable media, it is prone to scratches, dust, sticky prints
(including fingerprints), etc. while handling. This causes the read mechanism to employ
error correction mechanisms, like re-read, varying angular velocity, data reconstruction
using parity, etc.

4. The entire read/write and disk assembly is not a sealed unit. Hence, the disk drive
cannot spin the disk at fast speed as it may damage the disk and other parts.

Access times for optical disks are in the range of 100 to 300 milliseconds. Compare this
with the access times of floppy disks, which are in the range of 100 to 200 milliseconds,
and that of hard disks, which are in the range of 10 to 30 milliseconds.
4.1.2.1 Types of Optical Disks

All optical disks are round platters. They come in different sizes and capacities. Commonly
used types of optical disk are CD-ROM, WORM (CD-R), CD-RW, and DVD disks. They are
described below.
CD-ROM

CD-ROM stands for Compact Disk-Read-Only-Memory. It is a spin-off of audio CD technology
and works much like audio CDs used in music systems. In fact, if your computer has sound
card and speakers, you can play audio CDs with your computer.
CD-ROM disk is a shiny, silver color metal disk usually of 5/4-inch (12 cm) diameter. It is
made of polycarbonate plastic and thin layer of pure aluminum is applied to make the
surface reflective. For some very good quality disks, gold layer is used. A thin layer of
lacquer protects it. It has storage capacity of about 650 Megabytes or 700 Megabytes in newer
ones. It is so called because of its large storage capacity on a compact-size disk and because
it is a read-only storage medium. “That is, these disks come pre-recorded and information

stored on them cannot be altered.
Since an optical disk is read from the center to outer edge, several sizes are possible. Apart
from the most common 120mm 74/8 minute disk having storage capacity of 650 MB to 700
MB, 80mm 21 minute disk (called Mini-CD) having 184 MB storage is also available. They
can be used with the disk drive of a 120 mm disk. Mini CDs are designed in
various fancy shapes like round, oval, triangle, etc., while ensuring that the storage area is
affected.

WORM Disk/CD-Recordable (CD-R) Disks
WORM stands for write-once, read-many. WORM disks allow users to create their own CD-
ROM disks by using a CD-recordable (CD-R) drive attached to a computer as a regular
peripheral device. WORM disks look like standard CD-ROM disks, are purchased blank, and
later encoded using a CD-R drive. The information recorded on a WORM disk by a CD-R
drive can be read by any ordinary CD-ROM drive. As the name implies, data can be written
only once on a WORM disk but can be read many times. That is, like a CD-ROM disk, once

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