Even though both CD and DVD disks have the same media size and shape, the things they have in common ends there. The two are very different, 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; a laser beam scans these very grooves to read the data on the disc.
As you may know, digital information consists of ones and zeroes.
Inside of these discs, very tiny reflective bumps known as lands and non reflective holes known as pits 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 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 average single layer DVD holds 4.5 GB of data, while a CD holds a mere 700 MB.
As stated above, a DVD has smaller pits and the lasers need to focus on them. DVD readers achieve this 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 readers will access data at a much faster rate than a CD reader can attain. 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 is accessible by any drive, computer, or even consumer video.
CDs, on the other hand, aren’t compatible with this format.