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HD DVD - is it the better format?

Over at IFA 2006, we sat down with Jim Armour, from Toshiba’s Storage Device Division, to talk to him about the HD DVD format and how it compares to Blu-ray.

Jim believes that HD DVD will be success because the technology used in its creation is firmly rooted in the tried and tested technology we all use now, though there are a few fundamental differences, starting with the way the actual discs are made.

Jim Armour: A normal CD disc is made up of a polycarbonate layer, which is the main body of the disc, followed by a recording layer and then a reflective metal layer. DVDs and HD DVDs are made in a similar way to each other, using two substrates glued together. The problem with the single-layer method as used in CDs - and possibly Blu-ray discs as they’re constructed in the same way - is that the disc can warp when it gets hot, which is why you get that ‘whooming’ noise as the warped disc spins at high speed in the drive.

HEXUS.lifestyle: Ok, so what difference does this actually make?

Jim Armour: Under exposure to excessive heat (e.g. 50 deg C and above) a disc made of one polycarbonate layer is much more likely to warp that one made of two layers. With the BD lens having a smaller working distance (0.1mm) this could cause problems.

HEXUS.lifestyle: So the warping becomes an issue?

Jim Armour: It’s actually a combination of factors. Using a longer wavelength light meant that you have a larger area on the DVD that the lens was reading. With the blue light we use now, at a shorter wavelength, we can focus the beam far more, meaning the lens reads a smaller area and so makes the tolerances far smaller too. Of course, that means we can pack more information onto the disc but the disc itself needs to be more robust.

HEXUS.lifestyle: So how does an HD DVD disc differ from a standard DVD disc then?

Jim Armour: There’s actually very little difference between an HD DVD and standard red DVD disc, that’s the beauty of it. We’ve taken proven technology and just advanced it a step forward with the two-layer polycarbonate substrate technique, the disc is less susceptible to warping or bowing, making it ideal for the fine tolerances required for HD DVD.

HEXUS.lifestyle: So this two-layer of substrate method cancels out the warping?

Jim Armour: Yes, what happens now is that the two layers, in trying to expand due to heat, will push and pull against each other in equal amounts, reducing the amount of warping or bowing.

HEXUS.lifestyle: Ok, so an HD DVD disc will hold up to 15GB of data per side, giving us a maximum of a 60GB disc if you do a dual-layer, double-sided disc. The big question is why would I want that if Blu-ray gives me 25GB per layer and a theoretical 100GB using dual-layer, double-sided discs?

Jim Armour: Well the first thing to understand is just what performance you can expect from HD DVD and Blu-ray given a set capacity. What I mean is, what’s the quality of the video like from a 15GB HD DVD disc compared to a 25GB Blu-ray disc? This comes down to the video Codecs being used by each system. This is where HD DVD, though being a lower capacity than Blu-ray, can achieve the same results, purely through sensible use of video Codecs. In fact, because HD DVD focuses more on AVC (H.264) and VC1, it can achieve the same perceived quality as a 25GB Blu-ray disc. It’s worth noting that Blu-ray focuses more on MPEG-2, which needs more space than video compressed with VC1, even though both HD DVD and Blu-ray are capable of using the same three video Codecs (VC1, AVC and MPEG-2).

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