Blu-ray and HD-DVD Compared
What is HD?
Let’s start with the basics: High Definition refers to video that has a higher resolution than the sizes that we’re used to. Most HD video has either 720 or 1080 horizontal lines, as opposed to the 480 found on TV or a standard DVD. These lines can be either interlaced (alternate lines in each frame) or progressive (the whole image in each frame), as denoted by an ‘i’ or ‘p’ after the number of lines (for example, 1080i means a 1920×1080 resolution, interlaced so 540 lines show up in each frame). Obviously storing more pixels in each frame means more data, a LOT more data. This is why HD video on either Blu-ray or HD-DVD discs also comes with even better compression methods than those on a DVD, allowing for lower file sizes with minimal loss in quality.
How do Blu-ray and HD-DVD work?
Bits on an optical disc are stored on a thin data layer on the disc. A laser is focused on a certain spot on the disc, where it is either reflected back by the layer, or scattered by a hole in the layer; if it is reflected back, it’s read as a 1, and if it scatters it’s read as a 0. So obviously the way to store more data is to make the holes smaller and the laser good enough to focus on these smaller areas. One way to do this is to use a laser with a shorter wavelength, which decreases the diffraction of the laser once it hits the layer; both Blu-ray and HD-DVD do this. Another is to make the data layer thinner, which also helps with diffraction; Blu-ray’s layer is a sixth of the thickness of a DVD layer, while HD-DVD’s layer is the same thickness as that on a DVD. A third way is to use a laser with a better lens, to help it focus more accurately; the numerical aperture of a lens is an indicator of its ability to focus, with higher numbers being better. Blu-ray’s aperture is 0.85 and HD-DVD’s is 0.65, compared to DVD’s 0.6. As you can see, Blu-ray utilizes more of these than HD-DVD, and thus it can store 25GB per layer, while HD-DVD can only store 15GB per layer. To compensate for this somewhat, HD-DVD has a three-layer specification as well, which brings its total possible size to 45GB, just under Blu-ray’s two-layer 50GB disc.
Video Content
Both formats support the same three video codecs. The first is MPEG-2, the same codec used on a DVD. For a standard high-quality movie, a Blu-ray disc can store about two hours of video per layer in MPEG-2, while an HD-DVD disc can store about an hour and a half per layer. There are also H.264 (a new version of the MPEG-4 codec) and VC-1 (a codec based on Microsoft’s Windows Media 9 standard), both of which offer about twice the compression rate for about twice the video on the same disc.
Audio Content
Additional to PCM, Dolby Digital, and DTS (supported on DVD), Dolby has three new codecs that may be used. Dolby Digital Plus offers 7.1-channel encoding, as do TrueHD and DTS HD, but TrueHD and DTS HD are lossless formats, meaning their compressions do not sacrifice quality to make the files smaller. All three of these formats are mandatory on the HD-DVD, but they are optional on the Blu-ray standard.
Blu-ray and HD-DVD are fairly similar, with only a few differences between them. Most prominently, Blu-ray discs hold more data, but let’s be realistic: an HD-DVD disc is large enough to hold any full-length movie at a high quality. HD-DVD supports better audio standards by default, but you can bet that a good quality Blu-ray player will support them too. In the end, it’s probably just pricing and particular movies that will drive sales of one particular format.
