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Even though both CD and DVD disks have the same
media size and shape, the things they have in
common ends there. There are many different
things between the two, such as what they hold
and how much they hold.

Data pits and lasers
A disc has microscopic grooves that will move
along in a spiral around the disc. CDs and
DVDs both have these grooves, with laser breams
applied to scan these very grooves.

As you may know, digital information is represented
in ones and zeroes. Inside of these discs, very
tiny reflective bumps known as lands and non
reflective holes known as pits, which can be
found beside the grooves, reflect both the ones
and the zeroes of digital information.

By reducing the wave length of the laser to 625mm
or more infrared light, DVD technology has
managed to write in smaller pits when compared
to the standard technology of CD. This will
allow for a greater amount of data per track
on the DVD. The minimum length allowed for a
pit in a single layer DVD-R is .4 micron, which
is obviously more than the .0834 micron that a
CD offers.

The tracks of a DVD are narrower as well, which
allows for more tracks per disc, which also
translates into more capacity than a CD. The
avaerage single layer DVD holds 4.5 GB of data,
while a CD holds a mere 700 MB.

Layers
As stated above, a DVD has smaller pits and the
lasers need to focus on them. This is actually
achieved by using a thinner plastic substrate
than in a CD, which means that the laser needs
to pass through a thinner layer, with less
depth to reach the pits. It's this reduction in
thickness that's responsible for the discs
that were only 0.6mm thickness - which is half
that of a CD.

Data access speed
DVDs will access data at a much faster rate than
a CD can. The average 32X CD-ROM drive reads
data at 4MB a second, while a 1X DVD drive reads
at 1.38MB a second. This is even faster than
an 8X CD drive.

Universal data format
The recording formats of CDs and DVDs are quite
different, as DVDs use UDF, or the Universal
Data Format. This format allows data, video,
audio, or even a combination of all three to
be stored in a single file structure. The
advantage to this is any file can be accessed
by any drive, computer, or even consumer video.
CDs on the other hand aren't compatible with
this format.