Originally Posted by shore
I am not an optical engineer, but this is not how I was told these discs are made
. In both formats the spacer
layers between data layers are much
too small to be independently molded. In DVD the spacer layer is 55 +- 15 microns
, HD-DVD is 20 +- 5 microns
and Blu-ray is 25 microns
. By these tolerances, you could have a HD-DVD with the same spacer layer as a Blu-ray disc. These are more like the jam inside a 1200 micron sandwich than the bread.
I am not saying that the cover layer thickness does not have an effect on read/write (particularly tilt), just that it does not really effect number of layers.
For that matter, it is possible to have a red laser DVD like format that has at least 4 layers
Optically, the cover layer is the major determining factor in the NA of the laser to be used, and therefore the power of the laser required to be able to read/write addtional layers. So while manufacturing may allow for many many layers, they are useless if the laser is unable to read them...
Section 1.2 in this document
explains the optical importance of the cover layer. The relevant information to DL (and in expansion, multi-layer as well) is on the top of page 10. Section 1.4 also has some construction information that I believe you would be interested in. (You seem more interested in the construction than the other aspects if you dont mind me saying that)
Basically as you move up through the layers (optically) you must either have a more powerful laser, or you must move the layers closer to the laser by reducing the cover layer thickness. Since HDDVD's cover layer is thicker, HDDVD will require a more powerful laser than BD if they would like to make up for data capacity by having more layers. The reality of the point is that both companies will have very similarly powered lasers, independent of what advances are made. So the reduced cover layer of BD is the determining factor.
When I say the layers are thicker... I am thinking from an optical point of view. No matter what layer you are reading/writing to, you have to get through the cover layer for all, plus however many additional layers are present below the one you are attempting to read.
Both the abilities of the disc construction and the abilities of the laser design must be combined to get a true view of each format's capability.
I am always eager to discuss and learn, so if you think I'm in error in any way, point it out and I will re-asses what I think I know :)
I also agree with Darin in that the closer you get to a real time application, the more important transfer rate becomes vs storage, once minimum storage condition is met. But that is dependent on the application.
The multilayer debate may also be more "debateable" in the non pressed medias. Dye media, etc. The power levels are much more interesting in that arena as layers increase.