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FEATURE
Format Wars - Blu-ray vs HD DVD
On a fundamental level, both operate in the same way as CD – a laser scans the reflective surface of a disc, reading or writing data stored in the form of microscopic pits moulded into a substrate layer. What’s more, both are strictly speaking ‘blue ray’ products: each uses a blue/violet laser with a wavelength of 450nm, the advantage over the old 650nm red laser being that it’s possible to focus the laser spot with greater precision. You can fit more pits – and therefore more data – within a given area of disc, thus capacity and data rates increase.
But here the two formats diverge. The physical format of the HD DVD media is close to that of DVD; it’s the blue laser and some advanced data-processing techniques that give it the increased capacity. Blu-ray, however, uses an increased aperture lens on the laser to concentrate the beam into a cone, allowing for even smaller pits and so even more data. While this improves capacity, it also necessitates a thinner disc and smaller layer spacing – where DVD had a cover layer 0.6mm thick, Blu-ray has one of only 0.1mm. Originally, this meant the more fragile Blu-ray disc had to be protected by a cartridge, but this need has since been obviated by a specially developed coating.The difference in capacity is significant. An HD DVD-ROM or -R disc can hold 15GB of data per layer, meaning 30GB on a dual-layer disc or 60GB on a double-sided, dual-layer disc. HD DVD-RW, meanwhile, allows for only 20GB or 40GB total.
Blu-ray handles 25GB per layer, or 50GB on a dual-layer disc, whether BD-ROM, BD-R (write once) or BD-RE (rewritable). What’s more, the format is extensible – if you want more capacity, just add more layers. TDK has already demonstrated a four-layer 100GB disc and claimed that 200GB discs might eventually be possible. According to Sony’s digital development manager Adrian Northover-Smith, size always mattered in the Blu-ray camp. ‘With Blu-ray, we wanted to build a technology that was a lot more future-proof in terms of the length of the product cycle, he said. ‘By adding layers, we can match the market’s demand for capacity.’
In terms of speed, there’s no real difference. HD DVD has a base read/write speed of 36.5Mb/s; Blu-ray runs at 36Mb/sec, with movies streaming at 1.5x (54Mb/sec) and most early players and writers supporting 2x speeds (72Mb/sec). Early HD DVD drives are expected to match. So what about video quality? Again, there’s little to choose between the two formats, for the simple reason that both use the same three video codecs – MPEG4 AVC (also known as H.264), MPEG2 and Microsoft’s VC-1 (a variation on Windows Media Video 9). MPEG2 has a disadvantage in terms of larger file sizes, needing a bit-rate of 12 to 20Mb/s for a high-quality, high-definition picture that MPEG4 and VC-1 can achieve with 8Mb/sec or less. However, the studios and DVD-authoring houses are already familiar with MPEG2, and Sony has selected it for its own early Blu-ray releases. Other studios are free to make their own choices.
The other key factor is output resolution. All high-definition screens offer a base standard of 720p (1280 x 720, progressive scan), but some also offer 1080i (1920 x 1080, interlaced) or 1080p (the same resolution, but progressive scan). Whether 1080i is better than 720p is a moot point – you get more horizontal lines, but with the interlaced ‘flicker’ found on conventional TVs – but 1080p is rightly seen as the premium HD resolution, and nobody is promoting this harder than Sony. Where HD DVD stops at 1080i, Blu-ray goes up to 1080p, as does Sony’s forthcoming PlayStation 3 Blu-ray games console. 1080p sets are currently rare and expensive, but this could easily change during the product’s lifespan and add to Blu-ray’s allure in the high-end home cinema market.