An HD-DVD uses the same principles -- it contains a bumpy layer that reflects light from a laser to a sensor, creating a digital signal. HD-DVDs are even exactly the same size as DVDs (120 millimeters in diameter and 1.2 millimeters thick). But three important differences allow them to hold quite a bit more information than DVDs:
- They use 405-nanometer blue-violet lasers rather than 650-nanometer red lasers.
- The pits are smaller and the tracks are closer together.
- They use more efficient compression to cut down the size of the files they store.
The color of the laser may seem like a trivial change to make, but the shorter wavelength of the blue-violet laser is what allows HD-DVD's pits to be smaller and arranged closer together. In other words, it allows the disc to have a much narrower track pitch. Regular DVDs have a track pitch of 0.74 micrometers, and HD-DVDs have a track pitch of 0.40 micrometers. You can imagine this as the difference between writing with a magic marker and a fine-tipped pen.
The other big difference between DVDs and HD-DVDs involves how the information on the disc is compressed. Most DVDs use MPEG-2 compression. HD-DVDs can use MPEG-2, but they typically use the more efficient MPEG-4, which allows higher video quality with a smaller file size. HD-DVDs can also use VC-1 (or Windows Media) compression.
Finally, because of general improvements in the technology, an HD-DVD player can read information from the disc and deliver it to the TV about three times as fast as a DVD player can. It can also send the signal to an HDTV digitally using a High Definition Multimedia Interface (HDMI), preventing the quality loss that conversion to analog causes.
One of the first questions people ask about HD-DVD (besides "Is it better than Blu-ray?") is whether their old DVDs are about to become obsolete. Let's take a look at what is likely to happen with players and discs as people upgrade.