Microsoft Office Tutorials and References
In Depth Information
Computer Hardware
Storing data on an optical disk
Figure 21
When a DVD or
CD-ROM disk is
manufactured, a laser
burns pits into a
reflective surface. These
pits become dark,
non-reflective areas
of the disk.
Areas that
are not pits
have a
Reflective cover
plastic coating
When the laser
strikes a reflective
surface, light
bounces back into
the read head.
The patterns of
dark spots and
light spots
represent data.
When the DVD or CD-ROM drive
reads the data on the disk, it uses
a laser beam. When the beam
strikes a pit, no light is
The most common uses of CD-ROMs or DVDs are for software distribution and storing
large files that typically include graphics, animation, and video. Optical storage media are
very durable. Unlike magnetic media, such as floppy and hard disks, CD and DVD plat-
ters are not susceptible to humidity, dust, fingerprints, or magnets. They are not indestruc-
table, however. Take care not to scratch the disk surface or expose the disk to high
CD-ROMs are for “read-only” access, meaning you can read data stored on them, but
you cannot use them to record or store your own data. In order to record data on a CD,
you need a CD-R (compact disc recordable) drive and a CD-R disk. Instead of storing data
in pits made on the surface of the disk, as with a CD-ROM drive, the drive is designed so
that a laser changes the reflectivity of a dye layer on a blank CD-R disk, creating dark
spots on the disk’s surface that represent the data. Once the data is recorded, you cannot
erase or modify it, but you can append new data to the data currently stored on the CD-R
disk. A CD-RW (compact disk rewritable) drive is designed so that you can write data on a
special CD-RW disk and continually access and modify that data. CD-R disks can be read
by a standard CD-ROM drive or a DVD drive; CD-RW disks can be read only by CD-RW
drives or CD-ROM drives labeled “multi-read.”
Both CD-Rs and CD-RWs are useful for storing large amounts of data, or for transfer-
ring large files from one computer to another. The original CD-ROM drive had a relatively
slow access time: 600 ms. As the technology has improved, that access time has
decreased to less than 200 ms. A lower number means faster access. Also consider the
drive’s data transfer rate, measured in kilobits per second ( Kbps ), to classify it as 1X (the
original), 2X (twice the original), 3X, and so on.
Recordable DVD drives are becoming more common. As with CDs, you can buy a
DVD to which you can record only once, or a rewritable DVD to which you can record
and then re-record data. Recordable and rewriteable DVDs come in several formats; for
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