The Blu-ray Disc Association (BDA) have explained the content protection technology in place on a Blu-ray disc, and compared it with that used on the HD DVD format.
Quite apart from the depth data is stored on, the main difference is the copyright protection system employed.
HD DVD uses just one layer of protection - the AACS (Advanced Access Content System), whereas Blu-ray also uses two other layers of technology: BD+ and BD-ROM mark.
There's a detailed description and diagram over at CDRInfo which explains the technology.
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