Space - the final frontier
Four hours of HD video occupies about 25GB of storage space. Clearly there is a need for higher-capacity discs, but how do we create this extra space? There are three basic options.
First, the disc could physically be made bigger. This is an unlikely path to take, however, in a world of increasing miniaturisation.
Second, a more aggressive compression algorithm could be used to compress digitally the information stored on the disc. This would require increased processing power in the player, potentially increasing the player cost.
Finally, we could make each 'bit' of information take up less disc space, physically compressing the information. This type of compression could require new technology.
When comparing the storage space available on HD-DVD and Blu-ray, the important factor is physical compression (as both formats use the same digital compression algorithms).
The key is the colour of the laser: by shortening the wavelength of light from the 650nm (red laser) used for DVDs to 405nm (blue laser), the illuminated disc area is reduced. As a consequence, the physical size of each bit of information can be reduced too.
However, when we look at the capacities for each disc layer in HD-DVD (15GB) and Blu-ray (25GB), we see a large difference. What accounts for this?
HD-DVD follows the physical model used for DVD: the information is 0.6mm from the disc surface.
Blu-ray, by contrast, has this layer a mere 0.1mm down - this means that the light can be focused far more tightly on a Blu-ray disc, allowing the information to be packed more closely together. (Think about shining a torch onto a flat surface: from close to, the circle of light has about the same diameter as the head of the torch. Move the surface away from the torch, and the circle of light becomes wider.)
The result is that the track pitch of HD-DVD is 0.4µm, much smaller than DVD (0.74µm), but significantly larger than the Blu-ray pitch of 0.32µm.
Incidentally, there is another positive side-effect of having higher data densities: for a given disc rotation speed, a higher data transfer rate can be achieved if the data is packed more closely together.
A constant rotation speed of 10,000 r.p.m., which is about the current upper limit for optical drives, will give 12x speed for Blu-ray, but only 9x for HD-DVD. This means that, for a given data transfer rate, an HD-DVD must rotate more quickly than a Blu-ray disc.
