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.

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