DVDs - "How they Work"
A DVD is the same diameter and thickness
as a CD, and is 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. Typically DVDs are produced using
replication or DVD duplication for smaller runs.
Each writable layer of a DVD has a spiral track of
data. On single-layer DVDs, the track always circles
from the inside of the disc to the outside. Because
the spiral track starts at the centre, a single-layer
DVD can be smaller than 12 centimeters if desired.
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!