| Although both CD and DVD have the same media | | | | amount of data per track. The minimum length of |
| size and shape, the similarity ends there. There | | | | a pit in a single layer DVD-RAM [ is 0.4 micron, as |
| are numerous points of difference between the | | | | compared to 0.834 micron for a CD. |
| two, as listed below: | | | | Also, the tracks of DVDs are narrower, allowing |
| Data pits and lasers | | | | for more tracks per disc, which again translates |
| A disc has microscopic grooves that move along | | | | into more capacity than a CD. |
| in a spiral around the disc. Both CDs and DVDs | | | | Layers |
| have these grooves. Laser beams are applied to | | | | As explained above, DVDs have smaller |
| scan these grooves. As you may be aware, digital | | | | ‘pits’ and the lasers have to focus |
| information is represented in ones and zeroes. In | | | | on them. This is done by using a thinner plastic |
| these discs, very tiny reflective bumps (called | | | | substrate than in a CD, which means that the |
| ‘lands’) and non-reflective holes | | | | laser has to pass through a thinner layer, with less |
| (called ‘pits’), which are found | | | | depth to reach the pits. It was this reduction in |
| alongside the grooves, reflect the ones and zeros | | | | thickness which was responsible for discs that |
| of digital information. | | | | were only 0.6mm thick – half that of a CD. |
| Here lies the difference – by reducing the | | | | Data access speeds |
| wavelength of the laser (from the 780mm | | | | DVDs access data at a much faster rate that do |
| infrared light used in the CD) to 625mm or more | | | | CDs. Here is a comparison – a 32X CD-ROM |
| infrared light, DVD technology has managed to | | | | drive reads data at 4M bytes per second while a |
| write in smaller ‘pits’ as compared | | | | 1X DVD drive reads at 1.38M bytes per second. |
| to the standard CD. This allows for a greater | | | | That’s even faster than an 8x CD drive! |