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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.
Then, TVs got a whole lot bigger.
DVDs look best on screens that are smaller than 36 inches, so they're not always up to the challenge of today's high-definition (HD) sets. 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.
DVDs & HD-DVDs
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. Check out How DVDs Work to learn more about how a DVD player does this.
A simplified view of what happens in a DVD player. An HD-DVD player is a lot like this, but it can send the signal digitally rather than converting it to analog.
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-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.
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.
Standard vs. High-definition
Wondering what the difference is between standard and high definition? Standard definition (SD) uses 525 lines of pixels from top to bottom. High definition (HD) uses up to 1125 lines. Not all of these lines are visible on the screen, but they're included as part of the signal.
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The HD-DVD was voted the successor to the DVD by DVD Forum, an international organization comprising hardware, software, media and content companies that use and develop the DVD format. But many companies support Blu-ray. In the battle of HD-DVD vs. Blu-ray, who do you think will win? Why?
