HD DVD, High Density DVD, or High-Definition DVD is a high-density optical disc format designed for the storage of data and high-definition video.
The HD DVD disc is designed to be the successor to the DVD format and can store roughly 3-4 times the amount of data as its predecessor. Although it uses the same blue-violet 405 nm laser as Blu-ray Disc uses, technical differences make the two formats incompatible. The additional capacity of HD DVD means the discs are more suited to high-definition video than standard DVDs. In comparison to its main competitor, Blu-ray Disc, HD DVD has less information capacity per layer (15 gigabytes instead of Blu-ray Disc’s 25). HD DVD shares the same basic disc structure as a standard DVD: back-to-back bonding of two 0.6 mm thick, 120 mm diameter substrates.
The primary rival to Blu-ray Disc is HD DVD, championed by Toshiba, NEC Corporation, Microsoft, and Intel. HD DVD has a lower theoretical disc capacity per layer (15 GB vs 25 GB). The Blu-ray Disc version of the Adam Sandler movie Click was released on October 10, 2006 as the first ever dual-layer release. Sony’s goal is to use 50 GB dual-layer discs to store up to nine hours of HD video content. Alternatively, studios releasing movies on Blu-ray Disc can choose to use VC-1 or H.264/AVC instead of MPEG-2 as an alternative way to put four hours of high-definition content on a (single layer) BD.
In terms of audio/video compression, Blu-ray Disc and HD DVD are similar on the surface: both support MPEG-2, VC-1, and H.264 for video compression, and Dolby Digital (AC-3), PCM, and DTS for audio compression. The first generation of Blu-ray Disc movies released used MPEG-2 (the standard currently used in DVDs, although encoded at a much higher video resolution and a much higher bit rate than those used on conventional DVDs), while initial HD DVDs releases used the VC-1 codec. Due to greater total disc capacity, the Blu-ray Disc may choose in the future to utilize a higher maximum video bit rate, as well as potentially higher average bit rates . In terms of audio, there are some differences. Blu-ray Disc allows conventional AC-3 audiotracks at 640 kbit/s, which is higher than DVD/HD DVD’s maximum, 448 kbit/s. Nevertheless, Dolby Digital Plus support is mandatory for standalone HD DVD players at a maximum of 3 Mbit/s, while optional for BD players and support up to a higher bitrate of 4.736 Mbit/s.
Both HD DVD and Blu-ray Disc support the 24p (traditional movie) frame rate, but technical implementations of this mode are different among the two formats. Blu-ray Disc supports 24p with its native timing, while HD DVD uses 30i timing for 24p (replacing missing frames with “repeat field flags”). Decoders can ignore the “flags” to output 24p. There is no impact on picture resolution or storage space as a result of this, as the HD DVD format uses the exact same video information — it simply adds notational overhead.
On November 29, 2004 four Hollywood studios (New Line Cinema, Paramount Pictures, Universal Studios and Warner Bros.) announced non-exclusive agreements to support HD DVD. Since that time, Paramount and Warner have chosen to release titles in both Blu-ray Disc and HD DVD, while only Universal has since announced exclusive support for HD DVD. New Line has not released movies in either format yet but since it is owned by Warner it is expected to follow the parent company’s lead.
