In 2006, two potential successors to DVD were introduced: BluRay and HD-DVD. With sizes of 15–50GB, they provide the potential for storing a lot more data than DVDs. In theory, DVDs can store almost 16GB, but it requires a dual-sided disc.
They are uncommon because they require either the disc to be turned over or a drive with two lasers. In practice, DVDs can store only a little under 8GB. HD-DVD supports up to 15GB per layer, with triple-layer discs promised soon, while BluRay supports 25GB layers.
As if the format war weren’t crowded enough, a new player recently appeared: HD-VMD. These discs are effectively DVDs with more layers, allowing 20GB per disc, which is less than the amount that is actually used on current HD discs. HD-VMD players are much cheaper to make than those for the other formats.
Since the layers are the same as DVDs, they can be read with the same kind of laser found in DVD players—just with a slightly more complicated arrangement of mirrors to allow focusing on more layers.
Both HD-DVD and BluRay use a shorter wavelength laser in the blue part of the spectrum. These lasers are still a comparatively new invention and are much more expensive. In order to read older DVDs and CDs, HD-DVD and BluRay players need to include a second (red) laser.
While the HD-VMD format has a lot less headroom than its competitors—a single BluRay layer is close to the maximum total storage capacity of HD-VMD—the cheap price may gain it some traction in the short term.
How Many Pixels Are "Good Enough?"
The main selling point for these new discs is the fact that they make it possible to store high-definition content. This is, perhaps, not the best way of marketing the new discs, considering the relatively low market penetration of high resolution TVs.
In addition to more data, the newer formats also support better video CODECs, allowing better quality video to be supported with the same data rate. In theory, they could be used to improve the quality of DVD video. In practice, the large installed base of DVD players makes this impossible because it would not be possible to upgrade the players to support a newer encoding in software.
Predictions along the line of "no one will ever need..." are invariably wrong, so I won’t make the mistake of claiming that no one will ever need (or want) HD video. There is a limit to the number of pixels a human eye can perceive, and so eventually there will be no point in increasing quality, but it has not been reached yet.
There is a much closer limit, however. In consumer technologies, the barrier is not "perfect," but it is "good enough." Once a particular aspect of technology is "good enough," differences in the other aspects become more important.
Audiophiles have been complaining about the quality of CD audio since it was introduced. For most people, however, CD quality is good enough. If you compare a CD recording of a symphony to a higher definition recording, you might be able to tell the difference, especially if you paused various sections and listened to particularly complex parts, but only on particularly high-end equipment.
In fact, for a lot of people, CDs are better than "good enough." Something around the quality of 128Kb/s MP3 recordings are good enough. At this quality, the ability to store an entire music collection in a pocket becomes more important than audio fidelity.
For some people, VHS was good enough. I know people who claim not to be able to distinguish between VHS and DVD quality. I suspect this isn’t quite true; if you showed them the VHS and DVD recordings next to each other (spacially or temporally), they would be able to tell which was which.
The important thing is that once the video is VHS quality, it’s acceptable. People still buy DVDs because they are smaller, have better quality sound, and don’t need rewinding.
The larger capacity of the newer discs makes it possible to fit an entire season of a standard definition TV show on a single disc. This may well be more attractive than a smaller amount of HD content, since recent DVD sales have shown TV boxed sets increasing at the expense of films.
This trend started with DVDs; where previously TV shows were sold on VHS tapes with two or three episodes, making collecting entire seasons expensive (and very space-consuming), now they are sold in small boxed sets of a few DVDs.
Once the quality is good enough, the quantity becomes a more important factor to buyers.