The disks are nearly identical in physical dimensions, so I concentrate on the
functional differences:CDs record on one side of the disc, on a single layer of information,
while DVDs can have a pair of layers of information on each side of the
disc.
CDs have a read/write head on one side of the disc, while DVD devices
come at the disc from both sides.The serpentine, or spiral, layout of the single track on CDs and DVDs are
similar, although DVD tracks are less than half the already miniscule
width of a CD track, .74 micron instead of 1.6 microns.
Just as with CDs, any prerecorded DVD should play without problem on
a DVD player in a computer or in a home entertainment system.
However, a DVD created in a DVD recorder may not play in another com-
puter with a DVD player of a different manufacturer.How a DVD drive works .....................................................................
DVDs use the same principle as CDs to read data; a laser shines on a layer of
information recorded in the form of tiny reflective or nonreflective locations.
If you’ve got a DVD player,it can only read DVDs and not record them; you
may, though have a combination CD-RW and DVD player that allows you to
record and erase a CD and play a DVD.Recordable DVDs use a high-powered laser to heat a spot to change it from
reflective crystal form to a nonreflective compound. To erase or rewrite a
block of data, a lower-powered laser reheats spots on the disc to change
them back to reflective crystals. Both CDs and DVDs are 1.2mm thick, but a
DVD’s laser can focus on one or another of two 0.6-mm deep substrates. DVD
drives use a shorter wavelength of light to help them focus on smaller and
thinner locations. A DVD’s pitsand lands(holes and flat surfaces) can be as
small as 0.4 micrometers, less than half that of a CD-ROM, which makes holes
as deep as 0.83 micrometers — I’m talking about tiny dimensions either way,
but you get the idea: DVDs work in even tinier increments than CDs.DVD speed ratings follow a similar theory of relativity to that of CD drives.
The first DVD players were capable of reading data at about 1.3MB per
second; subsequent drives could download at double and quadruple that
speed and are called 2Xor 4Xdrives. Since DVD drives can also read CD
discs, you also see a CD rating, typically in the 32X or 40X range. And then
you’ll also find writing and rewriting speeds for CD and DVD discs.154 Part III: Laying Hands on the Major Parts