Information Technology Reference
In-Depth Information
Optical Drives
An
optical drive
uses a light beam and sensor to read the data. The sur-
face of a blank optical disc is shiny and reflects light strongly. When data
is written to an optical disc, certain areas are burned with a laser so they
are less reflective. The shiny areas are called
land,
and the less-shiny areas
are called
pits.
Recall from the preceding section that on an HDD, transi-
tions between positive and negative magnetic polarity indicate a 1 bit, and
lack of transition indicates a 0 bit. On an optical disc, transitions between
areas of greater and lesser reflectivity indicate a 1, and a consistent level of
reflectivity indicates a 0. See Figure 3.29.
optical drive
A drive that reads discs that are
stored in patterns of greater and lesser reflectivity,
such as a DVD or CD.
Figure.3 29
Magnetic and optical storage both look for transitions in the state of the disc
surface.
There are several types of optical drives and discs. The oldest and most
basic type is a
compact disc (CD)
, which holds up to 900MB of data.
CDs are used for small amounts of data and also for audio recordings.
A
digital versatile disc (DVD)
can store up to 4.7GB per disc (single-
sided, single layer). DVDs can also be double-sided, with recordings
on both sides rather than recording on one and a label on the other.
DVDs can also be dual-layer, where the top layer is semitransparent and
read using a laser and sensor at a different angle than the lower layer. A
double-sided, dual-layer DVD can hold up to 17GB of data. DVDs are
used to distribute large applications, large amounts of data, and standard-
definition movies.
Blu-ray discs (BD)
can store up to 128GB in up to
four layers. They are used to distribute even larger amounts of data, or
high-definition movies.
compact disc (CD)
An optical disc used for
storing music and data, holding up to 900 MB.
digital versatile disc (DVD)
An optical disc
used for storing standard-definition movies and
data, holding up to 4.17 GB per side per layer.
Blu-ray disc (BD)
An optical disc used for stor-
ing high-definition movies and data, holding up to
25 GB per layer.
All three types of discs can be read-only (ROM), recordable once (R), or
rewriteable (RW). To record or rewrite a disc, you must have a drive with
that capability that supports the type of disc you are using.
DVDs come in two competing recordable and rewriteable standards,
abbreviated as plus and minus signs, like this: DVD+R and DVD-R.
Some older home stereo systems do not support reading from +R and
+RW discs, but otherwise the differences are unnoticeable to most
consumers.
