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some issues

Posted by Joe Murphy Jr on July 29, 2007 at 20:14:36:
Pit density and laser focus are two areas of difference. An HD DVD laser won't read a Blu-ray disc because the laser that reads Blu-ray discs has a narrower focus to be able to read a denser pit structure on a BD. While it's not the % of difference between CD and DVD, it is the reason 25GB of data fits on a single-layer BD and only 15GB of data fits on a single-layer HD DVD. The lasers in these players can be made to focus at different depths/layers, but not stricter tolerances (just like DVD lasers can read CDs, but not the other way around).

The processing of the data is pretty much the same -- it's how the data is encoded on the disc that is different. The Blu-ray format is allowed the use of direct 24p encoding, while HD DVD uses an interlaced encoding (the interlaced encoding can be translated to progressive, so the exact same output is possible).

For future use, Blu-ray has a better chance of offering even more capacity. Because a Blu-ray disc puts the data closer to the surface than HD DVD, .1mm vs .6mm, there's more room left over for additional layers. About the best HD DVD will ever achieve, if it even gets out of the lab, is a triple layer disc (51GB) -- which Blu-ray more or less equals in its dual-layer state. As I'm sure you've read/heard, TDK has 200GB Blu-ray discs in the lab. I seriously doubt we will ever see anything over 50GB for the movie formats on consumer Blu-ray players, but the computer world is an entirely different matter. Will such discs take off? If the price is right, who knows? At least the possibility exists with the BD format.