HD DVD Advanced Buying Guide

The principles
High-Definition DVD works on the same principles as regular DVD. It is a natural evolution of the format, with the storage capacity increased by attacking the problem from two angles – HD DVDs hold more data, and the compression techniques used to store that data have also been improved.

HD DVD has a rival format, Blu-ray discs, so a format war similar to the VHS-Betamax war of the 1980s has been waged. It now looks like Blu-ray has won this war.

How it works
HD DVD uses the same structure of ‘bumps’ or ‘pits’ as DVD does now. The HD DVD disc is made of two 0.6mm discs sandwiched together, again just like with regular DVD.

These factors mean that HD DVD discs can be produced on the same production lines as standard definition DVDs. In fact, it is estimated that modifying the manufacturing equipment would take minutes and increase production costs by just 10%.

Where the new discs differ is in the structure of the data layer. By using bumps that are half the size of those on a DVD, and by reducing the space between the data grooves on the disc (known as the ‘track pitch’) from 0.74µm to 0.4µm, far more data can be stored on an HD DVD – 15GB on a single-layered disc, 30GB on a dual-layered disc.

In order to read this tightly packed data, a blue laser is employed. The red laser in a DVD player has a wavelength of 650nm, while that of a blue laser is just 405nm, meaning it can be more tightly focussed.

HD DVD then goes a step further by including compatibility with MPEG-4 video compression (the same used to create DiVX files for distribution over the internet). The first MPEG compression format, MPEG-1, was used on Video CDs in the 1990s and produced results that were comparable with VHS.

MPEG-2, used on DVDs, can compress a two-hour movie into around 4.7GB, enough to fit on to a single-layered DVD.

MPEG-4 encoding compresses data to about a third the size of an MPEG-2 stream. This, coupled with the increased storage capacity of an HD DVD disc, means that around four hours of high-definition programming can fit on to a single-layered HD DVD, or eight hours on a dual-layered disc.

HD DVD is still able to use the old MPEG-2 encoding, and it is possible to include standard-definition versions of a film or extra features in MPEG-2 on the same disc as an MPEG-4 high-definition feature film.

The final element to consider in the way HD DVD delivers an image is in the data transfer speed. A standard DVD can deliver up to 11.08Mbps (Megabits per second), while HD DVD can manage 36.55Mbps, more than enough to deliver a high-definition signal to a compatible monitor.

Glossary
Blu-ray – A rival to HD DVD, offering more storage capacity but still delivering high-definition movies.

DiVX – A video-compression format that can squeeze video files to a fraction of their normal space, allowing near-DVD quality movies to be downloaded from the internet and stored on a CD

HD DVD – A high-definition version of DVD technology

High-definition – A superior delivery format for TV that presents more lines of detail, resulting in a much sharper, more detailed image.