Inside Blu-ray & HD DVD

When DVD appeared in early 1997, it didn't take much prodding for people to trash their collections of primitive VHS tapes and embrace the shiny new disc. The dramatic boost in picture and sound quality had a lot to do with it - when experienced on a widescreen TV along with 5.1-channel sound, watching a DVD was almost like sitting in a movie theater. And all those cool extras and audio commentaries didn't hurt.

Nine years later, we're about to get our first taste of DVD's even higher-quality successors, HD DVD and Blu-ray Disc. There are plenty of fine points separating the two formats, but both can deliver an unparalleled home theater experience - one that combines high-def pictures with high-rez 7.1-channel audio formats developed especially for these discs. And the advanced interactive features that both Blu-ray and HD DVD bring to the table will make the extras we currently enjoy on DVD look like kid stuff. With so much going for each format, it'll be hard to choose which machine to take home - but most folks will make that decision based on how many movie titles become available for each over the next few years. After all, a player is just a player, but content is king.

Getting Physical: The Discs The new high-def discs are decidedly different creatures from DVD. Most important, they won't work in a DVD player (both HD DVD and Blu-ray players, on the other hand, can handle regular DVDs and CDs). But both types of disc are 12 centimeters in diameter - the same as DVD. And an HD DVD has the same physical construction as a DVD, with two 0.6-mm substrate layers (only the top one contains data) bonded to make a single disc. Blu-ray discs, in contrast, have a single substrate with the data layer residing close (a mere tenth of a millimeter away) to the bottom surface. Since this proximity makes the data layer susceptible to nicks and scratches, the initial Blu-ray design called for a protective caddy - an idea that was eventually scrapped when a scratchproof coating was developed.

A newly developed blue laser, with a shorter wavelength than DVD's red laser, allows both formats to store a lot more data on a disc. (See the diagrams above and below.) An HD DVD can hold three times as much as a DVD, while the even smaller beam spot size in Blu-ray players enables those discs to hold five times as much. As with DVD, multiple data layers allow even more information. For example, a dual-layer HD DVD can contain 30 gigabytes of data, enough to hold up to 8 hours of high-def video - plenty of room for the entire Terminator trilogy. And depending on what type of encoding is used, a 50-GB dual-layer Blu-ray disc could handle all three installments of Peter Jackson's Lord of the Rings cycle!


Of course, this massive storage capacity will matter most when Blu-ray and HD DVD recorders begin to appear over the next year or so. By then, you can expect to see even higher-capacity discs. The HD DVD camp is working on triple-layer discs that can hold 45 GB of data, and a dual-sided disc that doubles that amount. Meanwhile, Blu-ray has already demonstrated a 100-GB multilayer disc.

Both HD DVD and Blu-ray use Advanced Access Content System (AACS) copy protection. This bit of digital number-crunching works behind the scenes to prevent anyone from distributing pristine high-def copies of movies over the Internet. But AACS's Mandatory Managed Copy feature will let you copy discs to a Media Center PC or a home server for distribution over a local area network. You should also be able to copy movies to a portable video player like an iPod or an Archos mobile DVR.