DVD
Single-layer	4.7GB
Dual-layer	8.4GB
HD-DVD
Single-layer	15GB
Dual-layer	30GB
Blu-ray
Single-layer	25GB
Dual-layer	50GB

You've probably heard about HD-DVD and Blu-ray, a pair of competing next-generation DVD technologies that hit store shelves earlier this summer. You may also know that, unlike standard DVDs, they can deliver movies in high-definition format (720p, 1080i, and 1080p). But do you know how these discs, which to the naked eye look virtually identical to a standard DVD, go about accomplishing such an impressive feat?

The key lies in their data capacity. An average movie with a non-high definition resolution of 480p and encoded using MPEG-2 (the standard DVD compression format) is about 4-gigabytes in size, and just barely fits on a single-layer DVD. The same movie stored at the much higher resolution of 1080p and encoded using the same compression format would require far more space — about 25GB.

Put another way, you'd need at least three dual-layer DVD discs to hold it. Or you could just use one HD-DVD or Blu-ray Disc.

So what's the secret to packing more video onto a single disc?

The first way Blu-ray and HD-DVD trump standard DVDs is through the use of a more efficient method of data compression, called MPEG-4. This standard uses a variety of technologies to shrink the size of the content typically found on a DVD, including video, audio, interactive features and subtitles, which significantly reduces the amount of space required on the disc.

More physical storage

The second — and perhaps more impressive — means by which next-generation DVD technologies approach the data capacity problem is by providing more physical storage without increasing the surface area of the disc. This is where some high-tech light and magic come into play.

Optical discs, including CDs, DVDs, HD-DVDs, and Blu-ray Discs, store data in long spirals composed of millions of microscopic bumps. To retrieve data, a media player focuses a narrow laser on these bumps as the disc spins. A sensor collects the light reflected off each bump, reading it as a digital signal.

The amount of data, or bumps, stored on a disc depends on several factors, most notably the wavelength of the laser that will be used to access it. The smaller the wavelength, the smaller you can make the minimum distance between spiraling lines of data and the bumps within them without compromising readability.

Think of it as a page of typed text. The more you reduce the size of the typeface and the space between lines of text, the more words you can put on a single page. But the problem you'll eventually encounter is that the smaller the words get, the more difficult they are to read.

Optical disc technologies experience the same obstacle. DVD players employ red lasers with a 650 nanometer wavelength that is capable of reading bumps of data 0.4 microns in size on spiral tracks 0.74 microns in width. (To put this in perspective, a grain of sand measures about 55 microns.) If you want to store more data in the same area, you have to make the bumps smaller — but if the bumps were made any smaller or placed more closely together, a red laser wouldn't be able to read them.

Blue lasers

That's why HD-DVD and Blu-ray players don't use red lasers, but rather blue ones that have an even smaller wavelength of 405 nanometers. This allows bumps of data to be decreased in size to about 0.15 microns and spiral track widths reduced to 0.32 microns.

The end result? You can fit a heck of a lot more stuff onto a single disc.

The table attached to this article shows just how much more information HD-DVD and Blu-ray formats can store compared with a DVD disc. (Note: dual-layer discs have two spirals of data, one overlapping the other, that can be distinguished by a player's laser/sensor mechanism. The laser can see through one layer to read the layer below, which increases the surface area available for storing data.)

The table shows Blu-ray as the clear leader in terms of sheer storage capacity, but there are a couple of tradeoffs that come along with the bigger suitcase.

The first is cost. Blu-ray strays from the DVD standard of sandwiching data spirals between two layers of plastic. Instead, the data layers of a Blu-ray disc are positioned atop a single layer of plastic and covered with a thin, hard laminate to prevent scratches. Not only does this method allow the laser to read smaller bits of data packed more closely together (hence the greater storage capacity), it also makes for a cleaner reading with less chance of laser distortion.

Blu-ray discs more expensive to produce

But it means as well that Blu-ray discs require entirely new manufacturing facilities - an added cost that consumers will notice when they compare price tags at retail stores. HD-DVD, on the other hand, is close enough in format that it can make use of existing DVD manufacturing plants.

The second disadvantage of Blu-ray is compatibility. Since HD-DVD uses the same basic format as DVD, all of your old DVDs can be played in an HD-DVD player. In fact, there's also the potential for HD-DVDs to be playable in standard DVD players; all it would require is for an HD-DVD manufacturer to decide to produce dual layer HD-DVDs with one layer readable by an HD-DVD player's blue laser and a second readable by a DVD player's red laser.

Blu-ray discs, unfortunately, aren't playable in a standard DVD player, and it's up to individual manufacturers to decide whether they want to equip their Blu-ray players with additional technology that would make them capable of playing standard DVDs.

Now that you know how next generation DVD technology works, all that's left is to figure out which format — if either — you should buy. But that's a decision few people are willing to make. Based solely on an analysis of the technology involved, there is no clear winner.

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