The Retailing of HD-DVD and Blu-ray
September 1, 2006,
Do Blu-ray discs have more storage capacity, and is it relevant?
The present myth is that Blu-ray discs hold 50 GB of data, and HD-DVDs hold 30 GB. The truth is that storage capacity on both formats is evolving. The issues are how many layers can be encoded on a single surface, and whether one or two sides of the disc can be brought into practical use.
Blu-ray stores more information in a single layer than does HD-DVD, the difference being 25 GB for Blu-ray vs. 15 GB on HD-DVD. However, at the moment, Blu-ray can only produce single-layer discs. So the total storage capacity on all Blu-ray discs that have been brought to market thus far is 25 GB. Meanwhile, from the outset HD-DVD has been delivering dual-layer discs that hold a total of 30 GB on a single side of the disc. In addition, HD-DVD can presently encode dual-layer information on both sides of the disc, for a total of 60 GB per disc. Dual-sided encoding has been used so far in HD-DVD "combo" discs to put the HD version of a movie on one side, and the standard def version on the other. But both sides of the disc can be encoded in HD if the space is needed. So as of this writing, HD-DVD is capable of delivering 60 GB of storage on a two-sided disc compared to Blu-ray's 25 GB single-side, single-layer limit.
This of course is not the end of the story. The Blu-ray folks claim that they will begin to deliver dual-layer 50 GB Blu-ray discs by the end of the year. This may or may not happen. One cannot be faulted for some skepticism, since Blu-ray was supposed to be delivering dual-layer discs long before now. But if they do, then at the end of the year Blu-ray will have 50 GB of storage on a single-sided disc, and HD-DVD will have 60 GB on a dual-sided disc, with 30 GB on each side.
Beyond this, there are the futures being touted by both groups. The HD-DVD group has already developed but not yet released a triple-layer encoding capability that disc replicators have certified as good to go for mass production. This would boost HD-DVD capacity to 45 GB per side, for a total of 90 GB per disc. Not to be outdone, Blu-ray claims that it will one day deliver four-layer encoding capability for a total of 100 GB on a single side, and possibly even an eight-layer disc with 200 GB.
The question for the consumer is: Why should I care? How much storage is required to deliver an HD 1080p movie? Those who are enthused by the potential 50 GB limit of Blu-ray over the actual 30 GB capacity of HD-DVD are arguing that 30 GB is not sufficient to encode full length films in 1080p, and have space left over for all the special features that consumers enjoy.
If 30 GB of storage per side of the disc represented a practical limitation on quality, run time, or special features, we would be concerned too. But with the newer and much more efficient video codecs VC-1 and MPEG-4, as well as the advanced HD audio codecs, there is really no problem here. What we need is enough storage to hold a full length movie in 1080p/24 resolution, and to deliver it to the screen without noticeable compression artifacts. Once that is achieved, we could double or quadruple the storage space available with no perceptible change in quality, because storage is no longer a limiting factor.
Practically speaking, what the home theater market needs is a disc that can deliver superb high definition video at run times longer than most people would want to sit in a chair without moving. In our experience, that is usually about three hours. The large majority of films ever made do not run anywhere near that long. But there are some that can run up to four hours. One of them is Lawrence of Arabia which I screened a while back with some friends. That movie comes on two DVDs. When it came time to switch discs, everyone leapt from their chairs, grateful for the intermission break. In reality, people rarely want to sit for four hours straight.
Currently Blu-ray is using the inefficient MPEG-2 video codec, which compared to the advanced codecs takes up a lot of storage space. Blu-ray is also using uncompressed PCM audio on many of its discs. The audio quality is superb, but uncompressed PCM chews up a lot of storage as well. Despite these inefficiencies Blu-ray is still delivering 1080p movies that run over two hours on 25 GB discs.
HD-DVD is currently shipping 30GB/side discs. But more importantly they are providing authoring tools for VC-1 and MPEG-4, and the HD audio codecs Dolby Digital Plus, Dolby TrueHD, and DTS-HD. With the incremental storage space and much better technical efficiency, one can achieve run times of four hours or more on a single-sided HD-DVD. In addition, since HD-DVDs can be encoded on both sides of the disc, studios can put a movie on one side and an extensive collection of HD special features on the other if the extra space was needed.
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