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 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.
Layers 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). 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.
About the author:
Logan writes about various topics. This article is free to
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