If you used to watch movies on videotape, you probably remember the first time you saw one on DVD. Suddenly, the video and sound were of much better quality. You could also pause without distorting the picture, skip from chapter to chapter and zoom in on the screen. When studios started adding commentary tracks, "extras" and multiple sound options on each disc, it seemed like the technology had reached its peak. People couldn't really imagine a better way to watch a recorded movie than on a DVD.

To store and play HD movies, you need a disc that holds more information, like an HD-DVD. In this article, we'll explore how HD-DVDs differ from DVDs and what's happening in the struggle between HD-DVD and Blu-Ray.

The basic idea behind the HD-DVD is really simple -- it looks like a DVD and acts like a DVD, but it holds more information. A DVD holds about two hours of standard definition video, but an HD-DVD can hold about 48 hours.

If you already know how DVDs work, you already know a lot about HD-DVDs. A DVD stores information as a series of microscopic pits arranged in a very long spiral. A red laser reads these pits from the other side, so it sees them as bumps. The bumps reflect the laser's light to a sensor. Electronics within the DVD player read the information from the sensor as a digital signal.

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-DVDs 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 fine-tipped pen and a magic marker.

The difference between a red laser and a blue laser is like the difference between a fine-tipped pen and a magic marker.

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.

If HD-DVDs gain widespread use, you should still be able to buy DVDs -- the majority of homes in the United States don't have HDTVs, and there's no point in upgrading to HD-DVD without one. In addition, HD-DVDs can store regular and high-definition content on the same disc. Twin format discs have two layers -- a DVD layer on top layer uses a red laser, and an HD-DVD layer on the bottom uses a blue laser. The outer layer is transparent to the blue laser.

The other option for including DVD and HD-DVD content on the same disc is a combination format, which uses a two-sided disc. A red laser can read the DVD side, and a blue laser can read the HD-DVD side.

HD-DVDs aren't the only option for high-definition video playback, though. The other likely candidate for DVD's successor is Blu-ray. Some people plan to put off upgrading their movies and players until there's a clear winner in the "format war" between the two.

Several companies have developed alternatives to the existing DVD standard. The two forerunners are HD-DVD and Blu-ray. Competition between the two has escalated, drawing inevitable comparisons to the struggle between VHS and Betamax. Here are the highlights :

• Both formats use blue lasers rather than red.



• Both have the same options for video and audio compression.



• Blu-ray offers significantly more storage space -- 50 GB on a dual-layer disc versus HD-DVD's 30 GB.



• The DVD Forum, which creates DVD standards, has approved HD-DVD and has not approved Blu-ray.



• HD-DVD is less expensive than Blu-ray :



• HD-DVD players hit the market on April 18, 2006, two months before the first Blu-ray player hit the U.S. market in June, 2006.