Chapter 1

36 Everything You Ever Wanted to Know About DVD
audio track (in a few rare cases they have a PCM track). See “What’s the
Deal with DTS and DVDs?” for more on DTS.
Don’t assume that the Dolby Digital label is a guarantee of 5.1 channels.
A Dolby Digital soundtrack can be mono, dual mono, stereo, Dolby Surround
stereo, and so on. For example, Blazing Saddles and Caddyshack
have monophonic soundtracks, so the Dolby Digital soundtrack on these
DVDs has only one channel. Some DVD packaging has small lettering or
icons under the Dolby Digital logo that indicates the channel configuration.
In some cases, a soundtrack has more than one Dolby Digital version: a
5.1-channel track and a track specially remixed for stereo Dolby Surround.
It’s perfectly normal for your DVD player to indicate the playback of a Dolby
Digital audio track while your receiver indicates Dolby Surround. This
means the disc contains a two-channel Dolby Surround signal encoded in
Dolby Digital format. See Chapter 3 for more audio details.
Can DVDs Have Laser Rot?
Laserdiscs are subject to what is commonly called laser rot, the deterioration
of the aluminum layer due to oxidation or other chemical changes. This
often results from the use of insufficiently pure metal for the reflective coating
created during replication, but it can be exacerbated by mechanical
shear stress due to bending, warping, or thermal cycles. The large size of
laserdiscs makes them flexible, so that movement along the bond between
layers can break the seal, which is called delamination. Deterioration of the
data layer can be caused by chemical contaminants or gasses in the glue,
or by moisture that penetrates the plastic substrate.
Like laserdiscs, DVDs are made of two platters glued together, but DVDs
are more rigid and use newer adhesives. DVDs are molded from polycarbonate,
which absorbs about 10 times less moisture than the slightly hygroscopic
acrylic (PMMA) used for laserdiscs.
DVDs can have delamination problems, partly because some cases or
players hold too tightly to the inner hub of the disc. Delamination by itself
can cause problems (because the data layer is no longer at the correct distance
from the surface) and can also lead to oxidation.
So far DVDs have had few “DVD rot” problems. Reports have been made
of a few discs going bad, possibly due to delamination, contaminated adhesive,
chemical reactions, or oxidation of the reflective layer (see www.
mindspring.com/~yerington/ and www.andraste.org/discfault/discfault.
htm). Occurrences of “cloudiness” or “milkiness” in DVDs have been reported,
a possible cause being improper replication. An example of this
would be when the molten plastic cools off too fast or isn’t under enough