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

at