|
What's the Difference Between a DVD and a CD?
At first glance a CD and DVD are very similar. Both are the same physical size (120mm in diameter, 1.2mm in depth), but that's where the similarities end. CDs consist of a single layer, whereas DVDs comprise two 0.6mm bonded polycarbonate layers. The spirals of data on a DVD are also more tightly packed and the data pit length half that of a CD. As a result of these differences, the laser used to read a DVD is much narrower (650/635nm as opposed to 780nm on CD). DVDs also spin at more than twice the speed of a CD. This speed increase and pit/spiral difference results in a maximum end-stage composite data transfer-rate of up to 10.08Mbps from DVD, over seven times the capability of CD at 1.4112Mbps.
DVDs come in several densities. The basic single layer single side variant is known as a DVD-5 and stores up to 4.7GB of data; dual-sided DVD-5s are known as (surprise) DVD-10s. A dual-layered single-sided DVD is known as a DVD-9 and contains up to 8.54GB. The second layer of a DVD-9 (layer 1) contains slightly less data than the first layer (layer 0), as the data pits are spaced slightly further apart. This allows more accurate reading of the second layer through the first data layer and the transparent glue used to bond the two data layers together.
A dual-layered, dual-sided disc is known as a DVD-18, and can contain up to 17.08GB. DVD-18 and 14 discs use a slightly different and more difficult construction technique than that used by DVD-5, 9 or 10 discs so are still relatively uncommon.
Adam
|