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HD DVD Technology
HD-DVD (for High-Density Digital Versatile Disc) is a digital optical media format which is being developed as one standard for high-definition DVD. HD-DVD is similar to the competing Blu-ray Disc, which also uses the same CD sized (120 mm diameter) optical data storage media and 405 nm wavelength blue laser. HD-DVD is promoted by Toshiba, NEC, and Sanyo, and backed by four major film studios. The Blu-ray Disc is proposed by Sony.
HD-DVD has a single layer capacity of 15 GB and a dual-layer capacity of 30 GB. Toshiba has announced a triple-layer disc is in development, which would offer 45GB of storage. The surface layer of an HD-DVD disc is 0.6 mm thick, the same as DVD but thicker than the Blu-ray Disc's 0.1 mm layer. The numerical aperture of the optical pick-up head is 0.65, compared with 0.6 for DVD. These factors mean that HD-DVD media is less expensive to manufacture than Blu-ray Disc which require re-tooling of DVD disc production lines. Existing DVD media are playable on HD-DVD drives with only minor modification of the optical pickup. Blu-ray Disc players however, are expected to include backward compatibility with DVDs. The two formats employ nearly identical video compression techniques, including MPEG-2, Video Codec 1 (VC1, based on the Windows Media 9 format) and H.264/MPEG-4 AVC.