Hardware Reference
In-Depth Information
DVD Construction and Technology
DVD technology is similar to CD technology. Both use the same size discs (120mm dia-
meter, 1.2mm thick, with a 15mm hole in the center) with pits and lands stamped in a
polycarbonate base. Unlike a CD, though, DVDs can have two layers of recordings on
a side and be double-sided. Each layer is separately stamped, and the layers are bonded
together to make the final 1.2mm-thick disc. The manufacturing process is largely the
same, with the exception that each layer on each side is stamped from a separate piece
of polycarbonate plastic. These are then bonded together to form the completed disc. The
main difference between CD and DVD is that DVD is a higher-density recording read by
a laser with a shorter wavelength, focused more closely to the disc, which enables more
information to be stored. Also, whereas CDs are single-sided and have only one layer of
stamped pits and lands, DVDs can have up to two layers per side and can have informa-
tion on both sides.
As with CDs, each layer is stamped or molded with a single physical track in a spiral
configuration starting from the inside of the disc and spiraling outward. The disc rotates
counterclockwise(asviewedfromthebottom),andeachspiraltrackcontainspits(bumps)
and lands (flat portions), just as on a CD. Each recorded layer is coated with a thin film
of metal to reflect the laser light. The outer layer has a thinner coating to allow the light
to pass through to read the inner layer. If the disc is single-sided, a label can be placed on
top; if it's double-sided, only a small ring near the center provides room for labeling.
Just as with a CD, reading the information back on a DVD is a matter of bouncing a low-
powered laser beam off one of the reflective layers in the disc. The laser shines a focused
beam on the underside of the disc, and a photosensitive receptor detects when the light
is reflected back. When the light hits a land (flat spot) on the track, the light is reflected
back; when the light hits a pit (raised bump), the phase differential between the projected
and reflected light causes the waves to cancel and no light is reflected back.
The individual pits on a DVD are 0.105 microns deep and 0.4 microns wide. The pits and
lands vary in length from about 0.4 microns at their shortest to about 1.9 microns at their
longest (on single-layer discs).
Refer to the section “
CD Construction and Technology
,
” earlier in this chapter, for more
information on how the pits and lands are read and converted into actual data, as well as
how the drives physically and mechanically work.
DVD uses the same optical laser read pit and land storage that CDs do. The greater capa-
city is made possible by several factors, including the following:
• A 2.25 times smaller pit length (0.9-0.4 microns)
• A 2.16 times reduced track pitch (1.6-0.74 microns)
