High definition DVD. It's the Holy Grail for home theater fans. And now the folks at Artisan Home Entertainment claim to have it. Their June 3, 2003 release of T2: Extreme will contain not only a regular DVD of Terminator 2: Judgment Day; it will also include a second disc containing a high definition transfer of the same film. Accordingly, Artisan is calling this second disc an "HD-DVD."
What Does "HD" Mean?
On a normal, standard-definition television set, each frame of video is displayed by a trio of electron guns shooting beams of electrons onto a screen of colored phosphor dots which are arranged in clusters of three: one red, one green, and one blue. By combining these three primary colors in varying relative intensities, colors like yellow (a mixture of red and green with no blue) can be displayed. The electron guns sweep across the screen from left to right, exciting the trio of phosphors in each tiny area of the screen. Then they turn off, move back to the left and begin the process again, sweeping a new line across the screen just a little below the previous line. This happens so rapidly that after 1/30th of a second, your TV has drawn about 480 such lines across the screen, filling it from top to bottom with one complete frame of video. We say, therefore, that standard definition television has a resolution of 480 lines.
Standard definition television sets in the US are designed to display "interlaced" video. In interlaced video, each video frame is split into two video fields -- one containing only the even lines, and the other containing only the odd lines.
A new field is displayed every 60th of a second. That means, every 30th of a second, a complete frame is displayed.
While interlacing solved some technical problems related to limited bandwidth (an important accomplishment at the time this broadcast standard was being introduced), it creates some unfortunate problems too.
One problem is that, because only half the lines of a given frame are onscreen at one time, there is a tendecy for diagonal lines to appear to have a jagged "stair-step" quality, due to the lack of vertical resolution.
Another problem is "line twitter." Imagine an image with a horizontal detail that is small -- say, the top of a chain-link fence surrounding a schoolyard. If the top of the chainlink fence is only one scan-line tall in the TV image, it will only be onscreen every other field. As a result, it will appear to flicker on and off.
Unfortunately, when TV broadcasting standards were being developed in the 1950s, beaming 480-line TV images to people's TV sets required too much bandwidth for the available technology of the day to handle. To get around this problem, each frame was divided in to two parts. Each part contained only every other line of the frame. This even/odd separation has stayed with American broadcast TV signals to this day.
Each such half-frame of the image is called a video "field", and each video field is displayed on your TV set for 1/60th of a second. So in a 30th of a second, you still get the full 480 lines of resolution, even though only half are actually on-screen at any one time. When a video frame is displayed in this way - as a sequence of even-line/odd-line video fields - we say the video is "interlaced."
The "aspect ratio" of an image describes the relationship between its width and height. Standard definition TV is composed of video whose frames are 1.33 times as wide as they are tall, so they have an aspect ratio of 1.33:1 (sometimes written as 4:3, which is equivalent). Knowing that the video frame is made up of 480 lines stacked on top of each other - i.e., the image is 480 lines "tall" - and knowing the aspect ratio is 1.33:1, we can compute that the horizontal resolution is 1.33 * 480 = 640. So normal broadcast TV has a resolution of about 480 x 640 picture elements per frame.
By contrast, HDTV images are made up of thousands of small dots, called "pixels" (a term derived from a bizarre contraction of "picture elements"), arranged in a rectangle to make up an image for each frame of video. US television sets that can display high-definition content are generally designed to support video resolutions designated as 480i, 480p, 720p, and 1080i. These designations indicate the vertical resolution in one frame of video. Since the aspect ratio for HDTV video frames is 1.78:1 (or, equivalently, 16:9), we can derive the horizontal resolution for any of these formats from the vertical resolution. For example, at 720p, each frame of video will have the resolution 720 x 1280. The letters "p" and "i" at the end of the designations indicate whether or not each video frame is interlaced. Hence, in 1080i HDTV broadcasts, each frame is composed of 1080 x 1920 pixels, but only 540 x 1920 are on screen at any one time because this format is interlaced. Designations that end in "p" are "progressively scanned." Hence, in the 720p example, all 720 x 1280 pixels are on screen at the same time for each frame.
But Artisan's new "HD-DVD" disc will actually contain the movie at an even higher resolution, 1080p.
Issues With "HD-DVD"
This is interesting on several levels. First, the maximum resolution that most consumer HDTV sets can display is 1080i. Since the new "HD-DVD" disc's video image is not interlaced, most HDTV sets will be unable to display it! Some video front-projectors can handle a 1080p signal, but virtually no direct-view, rear-projection, or plasma sets can.
Another hurdle that Artisan will have to overcome in order for this new "HD-DVD" format to become popular with consumers is that existing consumer DVD players cannot play the disc. Instead, it can only be played on DVD-ROM drives attached to computers running the latest version of Microsoft's Windows XP operating system.
Also, Artisan's "HD-DVD" is not utilizing a format sanctioned by the DVD Forum. The DVD Forum is the official standards body charged with defining various DVD specifications. The Forum has its own group trying to come up with a formal specification for high definition DVD. One possibility being investigated by the DVD Forum is to utilize a new optical disc technology based on blue lasers.
The reason that the DVD Forum's Working Group 11 considers blue lasers to be a promising approach is because such lasers have a shorter wavelength than the red lasers used in today's standard-definition DVD players. This shorter wavelength will allow blue-laser-based DVD players to read smaller pits on the optical disc, which in turn allows those pits to be packed more densely on the disc, increasing the disc's data capacity enough to allow it hold a high definition transfer of a feature length film. If such discs spin at the same speed as today's red-laser DVDs, more data will pass over the read head in the same amount of time (because there are more pits in the same amount of space), allowing for a higher data-transfer rate from the new discs. Of course, since we'd need completely new players for these discs, the designers of such players wouldn't necessarily be limited by the rotational speed used by current red-laser DVD players.
But this new Artisan "HD-DVD" will not require a blue-laser HD-DVD player. Instead, Artisan has found a way to cram a high definition video onto a regular, standard-definition DVD disc.
This approach has a significant advantage over the Working Group 11's blue-laser-based technologies, an advantage that can summed up in two words: manufacturing costs. Artisan's "HD-DVDs" can be manufactured right alongside their standard-definition brethren, at the same DVD replication factories, whereas the manufacture of blue-laser-based high-definition DVDs would require significant (and expensive) retooling of the replication factories.
Of course, the Artisan approach would seem to have a major disadvantage as well. Namely, without increasing the data capacity of the disc, how can one fit the higher-resolution video transfer in the standard DVD's limited amount of data storage space? The answer, it turns out, is to use much more aggressive compression on the video.
And that's where Microsoft comes in. Standard DVDs contain video that has been compressed using a method developed by the Moving Picture Experts Group, called "MPEG-2." Our DVD players contain MPEG-2 decoders which know how to uncompress the video during playback. MPEG-2, which allows video to be both encoded and then decoded, is an example of a "codec." Unlike regular DVDs, Artisan's "HD-DVD" discs will not use the MPEG-2 codec. Instead, they will use a codec from Microsoft called Windows Media 9 (WM9).