Hardware Reference
In-Depth Information
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 information 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 (as
viewed from the bottom), and each spiral track contains pits (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 capacity 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)
• A slightly larger data area on the disc (8,605-8,759 square millimeters)
• About 1.06 times more efficient channel bit modulation
• About 1.32 times more efficient error-correction code
• About 1.06 times less sector overhead (2,048/2,352-2,048/2,064 bytes)
The DVD pits and lands are much smaller and closer together than those on a CD, allowing the same
physical-sized platter to hold much more information.
Figure 11.9
shows how the grooved tracks with
pits and lands are just over four times as dense on a DVD as compared to a CD.
