Special Coverage: HD DVD

What Is HD DVD?

Table of Contents

HD DVD was developed by Toshiba, NEC, Sanyo, and others as the next-generation optical media-disc format. Originally called Advanced Optical Disc (AOD), HD DVD was adopted by the DVD Forum in 2003 as its official high-definition optical-disc format. The DVD Forum is a consortium of more than 200 companies that oversees the development of all things DVD, making their endorsement a significant one.

There are three basic types of HD DVD discs. HD DVD-ROM is a read-only version intended for prepackaged material, much like DVD-Video and DVD-ROM today. HD DVD-R is the write-once recordable version, and HD DVD-RE (rewritable) can record and re-record data on the same disc.

The technical details of HD DVD are best summarized in a table (see DVD vs. HD DVD Specs). The disc itself is 12 centimeters in diameter and 1.2 millimeters thick -- exactly the same dimensions as CDs and DVDs. In fact, HD DVD was designed to be as compatible as possible with the existing disc manufacturing infrastructure. As a result, its disc structure is very similar to DVD's, with two 0.6-millimeter sides bonded together; each side can have one, two or three layers of data. In addition, DVD and HD DVD layers can be combined on the same side of a disc (one layer each) or on opposite sides (each side can be dual-layer).



Each layer of an HD DVD-ROM disc can hold up to 15GB of data, allowing a total of up to 45GB per side. How can all that data fit on a disc the same size as a DVD? By using smaller pits in the surface of each layer. How can these smaller pits be read? By using a blue laser, the light from which is nearly 40 percent shorter in wavelength than the red laser used for DVD, and focusing that light to a smaller spot size.


The pits on HD DVD (a) are much smaller than those on DVD (b), which are, in turn, smaller than those on CD (c). Along with the shorter wavelength of blue laser light, this allows HD DVD discs to store much more data than either of the other formats.

One of the format's primary selling points is that the lens structure is highly compatible with DVD and CD, making it easy to design players that can handle all three formats. The HD DVD player's pickup head consists of one lens riding about 1mm or so above the disc surface.

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