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DVD recordable Media
With the advent of the DVD recorder the media producers are cranking out all sorts of DVD writable or re-writable disks. But what makes them different and why ?
Formats
DVD recordable disks come in a number of formats. First of all there is the DVD-R. This is the original recordable DVD disk. Later came the DVD-RW which is the DVD-R's re-writable counterpart. The emergence of the DVD+ standard brought 2 new disks : DVD+R and DVD+RW. And then there is still DVD-Ram. Below is a table of these formats and what distinguishes them from each other
* = double sided disc.
** - double sided and only 8 cm diameter instead of the usual 12cm.
The reflectivity is a measure for the level of difference between a one and a zero. Because of the way the re-writable media are working the reflectivity is lower making them harder to be read. That explains why cheap video players have trouble with the re-writable media , and less trouble with the write once media. Older drives are less sensitive then the newer ones and thus have problems reading the rewritables. Often they don't detect a dic at all , or suffer from a lot of signal corruption and thus reject the disc.
Disk Structure
The above image shows the different layer stacks of the DVD media. The Active layer for a DVD-R or DVD+R is basically a colored dye. Whenever the dye is punctured the Reflective layer is exposed. On a re-writable the Active layer is a crystal ( typically AglnSbTe ). When in its crystalline form it allows light to pass trough to the reflective layer and back. When damaged by the writing laser it becomes opaque and practically no light passes trough.
The storage process
On dye-type disks ( DVD-R and DVD+R) storing information done be shooting a laser to the disk and damaging the storage layer. In case of a one-time writable disk this is just a matter of shooting holes in a dye layer. The open hole exposes the reflective layer above to the read laser and presto. Once a hole has been shit there is no way to 'repair' it.
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