Why dual-layer DVD is valued ?
Can hold more data
Contains a backup of the data stored
Creates alternative sound tracks
Uses a second layer to offer a speed increase
Can hold more data
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.
Dual layer refers to a DVD recording technology that provides users with 8.5-GB of recording space (as opposed to 4.7GB of space) on a standard DVD+R discs). The additional recording space is the direct result of dual layer technology. As the name suggests, dual layer technology provides two individual recordable layers on a single sided DVD disc. To take advantage of dual layer technology when recording DVDs, you must use a dual layer enabled DVD burner and dual layer DVD media. Dual layer technology media can be read (once recorded) by most DVD drives and players currently on the market. Dual layer technology has been used for many years in the production of commercial DVD movies but is now available for home and business recording as well.
Dual Layer is more commonly called Double Layer in the consumer market, and can be seen written as DVD+R DL or DVD-R DL on recordable DVD media and hardware packaging.
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.
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.