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A DVD is very similar to a CD,
but it has a much larger data capacity. A standard
DVD holds about seven times more data than a CD does.
This huge capacity means that a DVD has enough room
to store a full-length, MPEG-2-encoded movie, as well
as a lot of other information.
Here are the typical contents of a DVD movie:
Up to 133 minutes of high-resolution
video, in letterbox or pan-and-scan format, with 720
dots of horizontal resolution (The video compression
ratio is typically 40:1 using MPEG-2 compression.)
Soundtrack presented in up to eight languages using
5.1 channel Dolby digital surround sound
Subtitles in up to 32 languages
DVD can also be used to store almost eight hours of
CD-quality music per side.
Storing Data on a DVD
DVDs are of the same diameter and thickness as CDs,
and they are made using some of the same materials
and manufacturing methods. Like a CD, the data on
a DVD is encoded in the form of small pits and bumps
in the track of the disc.
A DVD is composed of several layers of plastic, totaling
about 1.2 millimeters thick. Each layer is created
by injection molding polycarbonate plastic. This process
forms a disc that has microscopic bumps arranged as
a single, continuous and extremely long spiral track
of data.
Once the clear pieces of polycarbonate
are formed, a thin reflective layer is sputtered onto
the disc, covering the bumps. Aluminum is used behind
the inner layers, but a semi-reflective gold layer
is used for the outer layers, allowing the laser to
focus through the outer and onto the inner layers.
After all of the layers are made, each one is coated
with lacquer, squeezed together and cured under infrared
light. For single-sided discs, the label is silk-screened
onto the nonreadable side. Double-sided discs are
printed only on the nonreadable area near the hole
in the middle.
What cannot be impress upon you
is how incredibly tiny the data track is -- just 740
nanometers separate one track from the next (a nanometer
is a billionth of a meter). And the elongated bumps
that make up the track are each 320 nanometers wide,
a minimum of 400 nanometers long and 120 nanometers
high.
You will often read about "pits"
on a DVD instead of bumps. They appear as pits on
the aluminum side, but on the side that the laser
reads from, they are bumps.
The microscopic dimensions of
the bumps make the spiral track on a DVD extremely
long. If you could lift the data track off a single
layer of a DVD, and stretch it out into a straight
line, it would be almost 7.5 miles long! That means
that a double-sided, double-layer DVD would have 30
miles (48 km) of data!
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