CHAPTER 13

720 Chapter 13 Optical Storage
DVD Technology
DVD technology is similar to CD technology. Both use the same size (120mm diameter, 1.2mm thick,
with a 15mm hole in the center) discs 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 as well. Each layer
is separately stamped, and they are all 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 and 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, 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; otherwise, if it’s double-sided, only a small ring near the
center provides room for labeling.
Reading the information back 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), no light is reflected back.
As the disc rotates over the laser and receptor, the laser shines continuously while the receptor sees
what is essentially a pattern of flashing light as the laser passes over pits and lands. Each time the
laser passes over the edge of a pit the light seen by the receptor changes in state from being reflected
to not reflected or vice versa. Each change in state of reflection caused by crossing the edge of a pit is
translated into a 1 bit digitally. Microprocessors in the drive translate the light/dark and dark/light
(pit edge) transitions into 1 bits, translate areas where there are no transitions into 0 bits, and then
translate the bit patterns into actual data or sound.
The individual pits on a DVD are 0.105 microns deep and 0.4 microns wide. Both 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).
See the section “CD-ROM 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)