Hardware Reference
In-Depth Information
are under 133 minutes. Nevertheless, some applications such as multimedia games
or reference works may need more, and Hollywood would like to put multiple
movies on the same disk, so four formats have been defined:
1. Single-sided, single-layer (4.7 GB).
2. Single-sided, dual-layer (8.5 GB).
3. Double-sided, single-layer (9.4 GB).
4. Double-sided, dual-layer (17 GB).
Why so many formats? In a word: politics. Philips and Sony wanted single-sided,
dual-layer disks for the high-capacity version, but Toshiba and Time Warner
wanted double-sided, single-layer disks. Philips and Sony did not think people
would be willing to turn the disks over, and Time Warner did not believe putting
two layers on one side could be made to work. The compromise: all combinations,
with the market deciding which ones will survive. Well, the market has spoken.
Philips and Sony were right. Never bet against technology.
The dual-layering technology has a reflective layer at the bottom, topped with
a semireflective layer. Depending on where the laser is focused, it bounces off one
layer or the other. The lower layer needs slightly larger pits and lands to be read
reliably, so its capacity is slightly smaller than the upper layer's.
Double-sided disks are made by taking two 0.6-mm single-sided disks and glu-
ing them together back to back. To make the thicknesses of all versions the same,
a single-sided disk consists of a 0.6-mm disk bonded to a blank substrate (or per-
haps in the future, one consisting of 133 minutes of advertising, in the hope that
people will be curious as to what is down there). The structure of the double-
sided, dual-layer disk is illustrated in Fig. 2-28.
Polycarbonate substrate 1
Semireflective
layer
0.6 mm
Single-sided
disk
Aluminum
reflector
Adhesive layer
Aluminum
reflector
0.6 mm
Single-sided
disk
Semireflective
layer
Polycarbonate substrate 2
Figure 2-28.
A double-sided, dual layer DVD disk.
DVD was devised by a consortium of 10 consumer electronics companies,
seven of them Japanese, in close cooperation with the major Hollywood studios
(some of which are owned by the Japanese electronics companies in the consor-
tium). The computer and telecommunications industries were not invited to the
