DVD-9 discs are
one-sided, double layer recordable DVDs with a capacity
of 8.5GB. The numbering after DVDs can be approximated
to the capacity of the discs. For example DVD-5 discs
can hold 4.7GB on a one-sided, single layer disc
and DVD-18 can hold 17.4GB on a dual-side, dual-layer
disc.
In double layer
(also referred to as 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) |
|
 |
DVD-9 discs are
12cm discs and can have the content data added through
the production processes of either DVD
duplication or DVD
replication. The DVD-9 format is not
as common as the DVD-5 format as the 4.7GB capacity
of the DVD-5 discs is usually sufficient for most
business uses, although many DVD films will use DVD-9
format if the film length is over 120 minutes.
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 DVD-9 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 DVD-9
discs are not available in re-writable
formats of DVD-RW or DVD+RW.
View other CD
and DVD production FAQs.
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