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.

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.