High Definition Digital Versatile Disc, or HD DVD, though initially called AOD for Advanced Optical Disc, is an optical disc that uses state-of-the-art blue-violet laser technology to enable extremely large-capacity recordings and high speed data transfer rates. Backed jointly by Toshiba/NEC, amongst 60 other manufacturers, these companies proposed and developed the basic format specifications. Although HD DVD was primarily designed to be a "consumer high-definition video recording format", its very high storage capacities and high-speed data transfer rates also make HD DVD suitable for storage libraries and ultimately other applications.

An HD-DVD disc has the same physical size as a DVD (12cm) but has higher data and track densities that give it between roughly three to six times the storage capacity of a standard 4.7GB DVD-R. This feat is made possible using a 405 nm (405 billionth of a meter) blue-violet laser, actually violet-purple, (see figure 1) and an optical pickup head with a 0.65 numerical aperture (NA) lens. Because a blue-violet light laser has a shorter wavelength (405 nm) than the red light (650 nm) used in CD and DVD systems, it allows the laser beam to make a smaller spot on the disc surface. With each bit of data taking up less space on the disc, more data can be stored on a 4.7-inch disc.

High-definition video (720p or 1080i) requires five times the recording capacity of standard definition video (480i). The actual HDTV transmission is based on a 19.4 Mbps (Mega bits per second) digital data stream but the maximum data transfer for DVD is about 10 Mbps. Thus, there’s simply not enough bandwidth to put an HDTV program on a recordable DVD format. To achieve the density necessary to put this amount of data on a single-sided 12 cm optical disc, the size of the spots burned into the disc need to be smaller. In addition, the high-definition video will need to undergo compression to be able to store this high-definition picture.

The HD-DVD format employs MPEG-2, Video Codec 9 (VC-1 based on the Windows Media 9 format), or H.264/MPEG-4 AVC video compression techniques in order to lower the data rate (i.e., use less digital data) of the high-definition video. The excellent efficiency of the latest MPEG-4 AVC and VC-1 codec allow image data to be compressed to about one-third the size achieved by MPEG-2 and replay of high quality digital images.

But regardless of the codec utilized, the idea is to compress enough to decrease the bit rate to data capacity levels while trying to preserve the high-definition picture quality to a reasonable standard. This allows the high-definition signal to be recorded without excessive compression, preserving the detail of the original high-definition picture.

The audio formats for HD DVD extend beyond the current DVD specifications to include every type of audio codec available. Though, at this writing, the more robust DTS Digital Surround had been selected as the audio technology of choice for the High Definition Digital Versatile Disc (HD DVD).

What are the HD DVD Disc Capacities? HD-DVD has a single-sided single-layer 15GB, a single-sided dual-layer 30GB, a single-sided triple-layer 45GB, and a 20GB (32GB under development) single-layer rewritable capacity compared to current DVDs which can hold between 4.7GB and 9.4GB of data (see Figure 2).	As of this writing, the HD DVD backers had proposed a "Hybrid" (read only) double-sided dual-layer structure disc (see figure below) containing an 8.5GB capacity standard-definition and a 30GB capacity high-definition version.

HD DVD maintains backward compatibility with current DVD discs since it adopts the same bonded disc structure as current red-laser DVD systems now in use, including the same thickness of the surface layer and the same process for replication. In fact, the surface layer of an HD-DVD disc is 0.6 mm thick, the same as that of a standard DVD.