Dual layer and
double layer DVDs are the same thing and are also
referred to as DVD-9
discs and have a capacity of 8.5GB.
In dual layer
DVD-9 discs, two layers of standard DVD-5 are joined
together with a transparent spacer and a thin reflector
between the two. The bottom layer is read and
written to in exactly the same manner as DVD-5. Reading
and writing to the second layer is achieved by the
laser focusing a fraction of a millimeter beyond
the first recording layer.
The diagram below
shows the difference between single (DVD-5) and dual
layer discs (DVD-9):
Single layer DVD
(4.7GB)
Dual layer DVD (8.5GB)
Dual layer DVD discs
are 12cm in diameter and can have the content data
added through the production processes of either DVD
duplication or DVD
replication. Dual layer discs are not
as common as the single layer format as the 4.7GB
capacity of the single layer discs is usually sufficient
for most business uses and dual layer discs are also
a lot more expensive.
Will
my DVD drive be able to record dual layer
DVDs?
Typically only the newer DVD recorder
drives will be able to write to dual
layer or double layer discs. You
should look on the face of the drive
tray for the logo shown to the right.
The extra rectangular border in the
top right and the 'DL' are the key
differences to mark these drives
out from standard DVD writer drives
that only record to single layer
discs.
DVDs have
much higher storage capacities than CDs by
using smaller pit sizes and narrower track pitches
in the spiral groove that runs around the
discs. These smaller pits and grooves require a different
wavelength of laser light (650nm) to that used in CD drives (780nm) and therefore DVD discs cannot be read
in standard CD drives.
Below is a diagram
illustrating the difference in track pitch between
CDs and DVDs:
The writable versions
of dual layer discs that can be bought in the shops
are often referred to as DVD-R DL, with the DL referring
to 'Dual Layer'.
Note that dual
layer DVDs are not available
in re-writable formats of DVD-RW or DVD+RW.
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