High definition DVD, also known as HD-DVD (which actually stands for High Density DVD), is one of two competing high definition storage format - the other being Blu-ray.

The need for a new, high capacity storage format, has been primarily brought about by the rapid rise in popularity of HDTV in Japan and the US. HDTV has much higher bandwidth than either NTSC or regular DVD discs, so in order to record programs from HD- DVD higher capacity discs, of at least 30GB, are required.

High definition video is also being used increasingly to make Hollywood movies as it offers comparable quality to film at much less cost. Therefore, the studios plan to release future movies on one or both high definition formats.

HD-DVD was developed by Toshiba and NEC and has the support of the DVD Forum, along with a number of Hollywood studios. Currently those studios which have announced support for HD-DVD are; Universal Studios, Paramount Studios, Warner Bros., and New Line Cinema. It has a capacity of 15GB for single-sided discs and 30Gb for double-sided. It doesn't need a caddy or cartridge and the cover layer is the same thickness as current DVD discs, 0.6mm. The numerical aperture of the optical pick-up head is also the same as DVD, 0.65mm.

Because of its similarities to current DVD, high definition DVD is cheaper to manufacture than Blu-ray, because it doesn't need big changes in the production line set-up. Both HD-DVD and Blu-ray have backward compatibility with existing DVDV discs. That is that current DVDs will play in HD-DVD player, although new high definition DVD won't play in older DVD players.

High definition DVD currently supports a number of compression formats, including MPEG-2, VC1 (based on Microsoft's Windows Media 9), and H.264 which is based on MPEG-4 and will be supported by the next version of Apple's QuickTime software, which will be included with Mac OS X Tiger.

Kenny Hemphill is the editor and publisher of The HDTV Tuner - a guide to the kit, the technology and the programming on HDTV.