The wavelength of the laser is reduced(from the 780mm
infrared light used in the CD) to 625mm cos of which DVD
technology has managed to write in smaller ‘pits’ as
compared to the standard CD. This allows for a greater
amount of data per track.The tracks of DVDs are narrower
which allows more tracks per disc this again translates into
more capacity than a CD.
Although both CD and DVD have the same media size and
shape, the similarity ends there. There are numerous points
of difference between the two, as listed below:
Data pits and lasers
A disc has microscopic grooves that move along in a spiral
around the disc. Both CDs and DVDs have these grooves.
Laser beams are applied to scan these grooves. As you may
be aware, digital information is represented in ones and
zeroes. In these discs, very tiny reflective bumps
(called ‘lands’) and non-reflective holes (called ‘pits’),
which are found alongside the grooves, reflect the ones and
zeros of digital information.
Here lies the difference – by reducing the wavelength of
the laser (from the 780mm infrared light used in the CD) to
625mm or more infrared light, DVD technology has managed to
write in smaller ‘pits’ as compared to the standard CD.
This allows for a greater amount of data per track. The
minimum length of a pit in a single layer DVD-RAM
[http://computer-information.info] is 0.4 micron, as
compared to 0.834 micron for a CD.
Also, the tracks of DVDs are narrower, allowing for more
tracks per disc, which again translates into more capacity
than a CD.
As explained above, DVDs have smaller ‘pits’ and the lasers
have to focus on them. This is done by using a thinner
plastic substrate than in a CD, which means that the laser
has to pass through a thinner layer, with less depth to
reach the pits. It was this reduction in thickness which
was responsible for discs that were only 0.6mm thick – half
that of a CD.
Data access speeds
DVDs access data at a much faster rate that do CDs. Here is
a comparison – a 32X CD-ROM drive reads data at 4M bytes
per second while a 1X DVD drive reads at 1.38M bytes per
second. That’s even faster than an 8x CD drive!
UDF (Universal Data Format)
Recording formats of CDs and DVDs are quite different. DVDs
use UDF (Universal Data Format [http://pda-devices.info]).
This allows data, video, audio or a combination of all
three, to be stored in a single file structure. The
advantage of this is that any file can be accessed by any
drive, computer or consumer video. CDs, however, are not
compatible with this format.