High Definition Facts
If you follow tech news, you’ve probably heard about High Definition movies and the two competing "shiny disc" formats -- HD DVD and Blu-ray -- and the new copy-protection system, AACS. Unfortunately there is a lot of misinformation out there about these technologies, probably because the facts change over time and the press essentially plays a game of telephone with each story repeating "facts" found in an earlier story.
Whilst this post won’t go into great details about either of the disc formats, it will talk about a few key things they both have in common. Thanks to other members of the HD DVD team here at Microsoft for providing some of the more esoteric information 🙂
Both formats support (up to) 1920 x 1080p content. Some news stories claim that HD DVD only supports 1080i, but that claim is false. Additionally, both formats support a wide variety of framerates, including 1080p24 (24 frames per second) which is the native frame rate of film and the most accurate way to store motion picture content. Players (in both formats) may choose to output at 1080i for various reasons -- component cost vs. small number of 1080p-capable displays; etc. -- but the content on the disc will almost certainly be at 1080p24. Given that many present-day TVs can only accept 720p or 1080i input anyway, this is not such a big deal for consumers who already purchased "HD-ready" TVs. In time, as they upgrade their TVs to 1080p they can also update their players to 1080p and take advantage of lower price-points at that time. Some displays can also convert the 1080i signal back into a 1080p signal for final display.
Of course, it is up to the Studio to decide which resolution to use on the disc, but it would be hard to imagine them choosing anything other than the native frame rate of film, which will give the very best picture.
Both HD DVD and Blu-ray support the AACS copy protection system, which is based on standard cryptographic algorithms and is published on the AACS LA web site. Blu-ray supports an additional copy-protection system named "BD+" that is based on reading a small program off each Blu-ray disc and running it to ensure the player is compliant. Not much is known about how this works, although you can find some basic information about SPDC. Blu-ray also includes a third system named "ROM Mark" that makes it hard to duplicate entire discs.
In theory it is up to the Studio to decide if they want to protect their content or not, but in practice... well, you can figure that out for yourself 🙂
(Side note: Since all HD DVD players will play standard DVDs as well, they will support the older CSS copy-protection system).
"Down-rezing" is the act of taking a full 1920 x 1080 image and converting it into a 960 x 540 image when the image is output over unprotected analog connections (eg, component video). This is one quarter of the total resolution of full high-definition, but still 50% more than a 720 x 480 standard-definition picture. Down-rezing is controlled by the Image Constraint Token -- or ICT -- as defined by AACS and is enforced by both formats. ICT does not apply to HDCP-protected digital outputs.
Again, it is up to the Studio to decide whether they want to assert ICT or not on any particular disc, and many studios have announced that they will not do so. Any discs (in either format) that asserts the ICT will contain product labelling to inform customers that they will not get a full HD picture over unprotected outputs.
Digital Only Token
The Digital Only Token -- or DOT -- is also defined by AACS (and will be enforced by both formats) and is even stronger than the ICT. If a player detects the DOT, it must not allow any output over unprotected connections; only HDCP-protected digital outputs will be supported. Users will see a blank screen if they try to watch a DOT movie over unprotected connections.
In the future, it will be up to the studio to decide if they want to assert DOT on any particular disc or not, but for the time being studios are not permitted to use the DOT. This means you won't see any DOT-enabled titles for some time, and right now we don't really know what the DOT will be used for (other than "New Business Models" which may be entirely dissimilar to traditional consumer products purchased at retail).
Extra Geeky Info
HD DVD encodes audio and video in Program Streams and uses field syntax with 30 frame per second timing to represent both 30 fps interlace and 24 fps progressive content. “Missing fields” for 24P content are replaced by “repeat field flags”, the same as DVD-Video. Decoders can ignore the “flags” to output P24 or use them to output 30i. Blu-ray encodes audio and video in Transport Streams and has the option to use frame syntax to encode 24fps content with 24fps timing.