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HD-DVD versus Blu-ray
After nearly a decade of DVD dominance, optical disc formats are undergoing another revolutionary change. With DVDs offering insufficient storage for high definition content, a new optical format was required. Unfortunately, two competing formats have been developed instead – HD-DVD and Blu-ray. In this article, we explain why we need HD opticals, and the differences between the two competitors.
A single-layer DVD can store 4.7GB of data per side, and a dual-layer one 8.5GB. The latter is enough for a little over two hours of video encoded with MPEG-2 compression at the usual 8Mbits/sec and 720 x 576 resolution. But High Definition is around five times the resolution, so would require five times the storage space using the same MPEG-2 compression for similar image quality.
The venerable DVD clearly wasn’t up to the job. Instead,
a disc capable of many times the capacity was required for HD content.
Both HD optical formats also offer H.264 MPEG-4 AVC and Microsoft’s
VC-1 (a version of Windows Media Video 9) as alternatives, which
are capable of much higher compression with comparable visual quality
than MPEG-2. But even then, DVD was never going to be enough for
the HD revolution.
The HD-DVD format was spearheaded by Toshiba, although a number of other companies are involved. It is being promoted by the DVD Forum. HD-DVD was designed with backwards compatibility in mind. The disc itself is very similar to a DVD, with a 0.6mm surface layer on a 0.6mm substrate. So the manufacturing process is almost the same, which is one of the strengths Toshiba is promoting for the format. The players are also significantly cheaper at launch than Blu-ray players.
In order to fit much more data into the same-sized disc, HD-DVD uses a blue laser operating at 405nm rather than the red one at 650nm of DVDs. It also uses a slightly larger numerical aperture of 0.65, rather than 0.6 for a DVD. These two factors allow a more tightly focused laser so the bits can be packed more densely. The same 12cm discs are used, but the blue laser allows 15GB to be stored per layer, rather than 4.7GB. Initially, a dual-layer version capable of storing 30GB per side has been the basis for the majority of movie releases. But Toshiba is already talking about three or more layers, with storage capabilities of 45GB and greater.
Blu-ray is a format pioneered by a group of companies revolving around Sony, and is promoted by the Blu-ray Disc Association. Although Blu-ray also uses a 405nm blue laser, like HD-DVD, the disc format itself is very different. Instead of a 0.6mm surface layer, a much thinner 0.1mm layer is bonded to a thicker 1.1mm substrate. Blu-ray also uses a 0.85 numerical aperture. This allows an even narrower focus than HD-DVD’s laser, so Blu-ray can store 25GB per layer. But TDK has already announced the creation of a six-layer disc with 33GB per layer, squeezing 200GB onto a single side.
Although the first Blu-ray discs (BD) were supposed to be dual-layer with a capacity of 50GB, manufacturing problems allegedly limited some of the first movie releases to a single layer. Initially, Blu-ray was also criticised because the thin surface layer was too fragile, and a disc caddy was necessary. But a hardened coating has been developed since then, so shipping discs and drives have done away with this requirement. BD has another drawback, in that the initial players are priced around twice as much as those for HD-DVD. Nevertheless, many consider BD to be technically superior to HD-DVD due to its greater capacity.
The final consideration with HD-DVD and BD is how the discs are encrypted. This is clearly important, since the visual quality is so high. To combat the problem, an Advanced Access Content System (AACS) has been developed. Rather than using static encryption, with one key for every player model, AACS has different keys for every player. If a key is compromised, all future content can be produced without support for that key, and that particular player will no longer be able to play the discs. The end user experiences HD-DVD and BD content protection via HDCP, which ensures that the digital connection between the player and screen is encrypted. The HDCP system involves a handshaking system where the player checks the keys of the screen as well as its own against the collection of keys stored on the disc.
The physical formats may differ, but the content formats are essentially the same. We’ve already detailed the three possible compression schemes used in each format. Both formats are also able to support video at all the main HD video resolutions, including 720p, 1080i and 1080p. Although the first HD-DVD players can’t actually output 1080p, they can still read discs in this format.
Whatever the technical merits of either HD-DVD and Blu-ray, the
most likely factor to decide which one prevails will be how many
movie titles are available, and how popular they are. HD-DVD has
built up a clear initial lead in the US in terms of the number of
movie discs available and their sales rankings, but Blu-ray has
the support of more major studios. So the tables could turn as these
studios release more discs. This makes choosing between the two
difficult for early adopters, with the only possible solution being
if multi-format devices do actually arrive capable of playing both
types of disc.