A couple of years ago Toshiba was talking about 45GB/3-layer HD DVD as a growth option, but that faded. Then at CES this year they were touting a 51GB/3-layer HD DVD growth option – same thing with the layers tweaked to 17GB instead of 15GB. But things have been fairly quiet since then. And the HD DVD camp has made a lot of noise about how you don’t need more than 30GB (the max capacity of HD DVD), and Blu-ray’s 50GB capacity doesn’t mean anything.

But I guess maybe it wasn’t so minor after all if new reports are correct. It looks like Toshiba has taken the 51GB version through approval. While it is being reported that the DVD Forum has approved 51GB discs as an update to the HD DVD specification there are also reports that it has not been voted on by the Steering Committee and thus has not been fully approved at this point. Which makes sense, as if it were approved in August as claimed Toshiba and the HD DVD camp would’ve been touting it at CEDIA, I’m sure. If this is approved, it would put HD DVD on par with BD in capacity (51GB vs. 50GB), at least unless and until the BD camp decides to standardize one if their higher capacity discs – we’ve seen 100GB and even 200GB prototypes.

The question is, if this is true, when will the first 51GB discs come to market? And the natural follow-up is – will they work in all existing HD DVD players? There is some debate over that. In any case it would probably require a firmware update, which isn’t that big of a deal. I’d expect the forthcoming 3rd generation players to support it, and probably the 2nd generation players out now, since Toshiba had the design for the 51GB disc before they shipped. So there was probably some consideration given to the upgrade. But first generation players shipped well before the disc was designed, so the drives in those may not handle it. I’m sure we’ll find out in time. Also reportedly approved, another hybrid disc with one standard 4.7GB DVD layer, and two 17GB HD DVD layers. Which is kind of pointless, IMHO, since 5GB is pretty small for most content. Especially something that would need 34GB for the HD version.

While the BD camp has shown hybrid format discs – both single-sided (Two BD50 layers over two DVD9 layers – giving full capacity for both) and dual-sider (BD50 one side, DVD9 the other), neither have been commercialized. Hybrid discs have more issues in manufacturing, the more layers you press the more changes there are for something to go wrong, and lower the yields. That raises the costs. It is often more cost effective to simply package two discs (one high-def, one DVD) in one case than to mess around with hybrids. The HD DVD camp has already brought discs to market – single-sided discs with one 15GB HD DVD layer and one 4.7GB DVD layer as well as ‘flippy’ dual-sided discs wit HD DVD30 on one side and DVD9 on the other – but these cost more. Flippy discs are unpopular with both studios and consumers as they cost more to make, have a lower yield, don’t provide a place for printing a label (other than a ring around the hub), and both sides are vulnerable to scratching. These factors killed DVD18 – which is DVD9 on two sides – and studios begin shipping two DVD9s in one box when they needed the capacity. It seems some studios are repeating the same mistakes with flippy high-def discs, but I expect that to die off for the same reasons DVD18 did.

Even with the higher capacity, HD DVD is still handicapped by a lower transfer bandwidth. Both Blu-ray and HD DVD specified their 1x speeds as 36Mbps. The transfer rate is the maximum rate authors can rely on for streaming data off the disc to be decoded. And that data stream includes the video, and all of the audio tracks, plus subtitles, etc. While HD DVD stuck with the 36Mbps rate, Blu-ray actually mandated a 1.5x, 54Mbps rate for BD-ROM video. And more of the stream is reserved for audio and video – 48Mbps for BD, 30.2Mbps for HD DVD. And of that, 40Mbps is availble for video on BD, and just 29.4Mbps on HD DVD. Which means Blu-ray films can encode at a higher bit-rate, providing for a higher image quality. And there is more room for audio – allowing for higher bit-rates on audio as well as the ability to support more codecs at once without as much concern for restricting other bandwidth. For example, Blu-ray supports DTS-HD High Resoltion up to 6Mps, HD DVD is 3Mbps, and BD does DTS-HD Master Audio up to 24Mbps, HD DVD is 18Mbps. (By comparison, DVD has a raw bit-rate of 11Mbps, video & audio 10Mbps, and 9.8Mbps available for video.)

From EngadgetHD, Screen Digest, and AVSForum.