Understanding HD DVD Resolutions

With HD DVD players already on the market and the HD DVD accessory set to ship for Xbox 360™ this fall, it is more important than ever to understand how this hi-def thing works. How exactly do the movies you want to watch, the HD DVD player, and your HDTV all relate to each other?

Many questions and much confusion surround which resolution and frame rate are best suited for viewing HD DVDs, but this basic primer should at least give you a proper understanding of the situation as it relates to you, the viewer.

Film First
Unlike games or television programs, movies are filmed at 24 frames per second (FPS), including digital movies. For example, the special digital cameras used to film Star Wars Episodes II and III were built to utilize a standard 24 FPS rate.

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Native Resolution
HD DVD discs contain the movie as it was meant to be viewed, maintaining the original FPS in which they were filmed. As such, the film can be shown on your TV or monitor in what is referred to as its 'native' format. It is when this image is transmitted from the player, and then received, translated, and projected onto your screen that the different resolutions begin to get involved.

1080i-30 Resolution
First of all, the "i" at the end of the 1080 stands for interlace, and it's important to understand what that means. Imagine the following:

• Take a picture and cut it into 1080 lines, with each line being numbered.
• The first thing you do, is to draw all the even numbered lines on the screen. The collection of all the even-numbered lines is called a "field".
• In the next sixtieth of a second, you then go back to the top and draw all the odd-numbered lines. This is the second field.
• Two fields make one frame, so the process has to happen twice as fast as the number of frames shown per second.
• Repeat the process indefinitely and you'll understand exactly how your TV handles projecting a 1080 interlace image.

Historically, interlacing was first used in TV signals because CRT displays built in the 1940s could simply not work fast enough to draw every line in one-sixtieth of a second.

Understanding exactly what each resolution
means to your HD DVD experience
is an important foundation to have.

This technique—while still maintaining a great picture—creates problems because of the frame rate conversion.

3:2 Pulldown
The number 30 after 1080i reflects the number of frames being shown per second. Note that this doesn't match the 24 FPS your movies are being filmed in. This signal format causes the film to be transferred to your display in an artificial sequence as the DVD player needs to translate a 24 FPS source into a 30 FPS video signal. This process is called "3:2 pulldown," which repeats 12 fields of the 24 FPS source to generate a 30 FPS signal. Pulldown causes an odd "judder" movement in the image as it shows one frame for two fields, then three fields for the next frame, and so on and so forth.

No actual information is lost—the entire normal film frame is still present, it has just been transferred to the display with 12 repeated fields. A modern display can remove those 12 fields and recover the original 24 FPS signal. In other words, with a proper display, a 1080i picture will look identical to a 1080p picture when you begin with film material source like that from HD DVD.

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Incidentally, the 1080i input resolution is one you find on all HDTVs, and it is the most common HD broadcast format. For example, if you watch a movie or show broadcast in HD, it's usually presented in 1080i30. Look for the HD logo at the beginning of the show to be sure.

1080p-60 Resolution
The "p" at the end of 1080 stands for progressive, and the distinction between it and an interlaced image is important to understand, if only to know that it makes little difference to films on HD DVD that start at 24 frames per second.

Instead of capturing only half the lines at a time, like an interlace image, a progressive image captures the whole picture—all 1080 lines—at once. This allows for a much sharper image, because all pixels are present in every frame. This 60p signal can be beneficial to a display, but only when starting with something filmed in 60p. Film material, which is shot in 24p, will not show any advantage on a modern display.

A modern display can perform an operation on the video called "inverse telecine" or "inverse 3:2 pulldown." It recognizes and removes the 12 extra fields, reassembles the remaining 48 fields into the original 24 frames, and then repeats those frames at whatever frequency the display selects (60 Hz, 72 Hz, etc.).

However, there is still a problem if a display updates at 60 FPS because it doesn't match the native 24 FPS of film—a 60 Hz refresh in the display will produce "judder" just as a 1080p-60 interconnect signal does because it also employs the 3:2 pulldown technique by showing two copies of the first frame, three copies of the next, etc.

1080p-24 Resolution
Here we have the optimum resolution, so it's valuable to note just how it works. Though called 1080p-24, it is typically displayed at 1080p-72 since this frame rate takes the 24 FPS native to the film and the HD DVD and shows each frame three times. Incidentally, this is also how most theatre projectors work using three-bladed shutters.

This resolution is progressive so you don't have to worry about visual artifacts appearing, and because 72 is a multiple of 24, you also don't have to worry about 3:2 pulldown causing an unnatural judder in movement. This is therefore the ideal framerate to display movies. PC playback can typically provide this frame rate, but today very few displays support this, though we expect to see more in the near future.

If you have a 720p TV, a 1080i30 or 1080p24 input is also optimal because the display can use the original 1080P images to resample to its display resolution (usually something like 768 lines, not 720), and repeat those frames at one of its available refresh rates (most often 60Hz). Internal scalers and filters in the display can do the best job of downscaling from 1080 to native resolution while matching the characteristics of the particular display technology.

When the input arrives as 24p or 30i with 3:2, the display knows the actual capture rate was 24p and can refresh frames accordingly. If it gets a 720p60 signal, it is less likely to detect that it was a 24p film that was somehow format converted, and probably won't be able to restore it to 24P before processing it for display.

Since HD DVD is stored on the disc at the natural frame rate of 24 frames per second, it can be displayed at any of these rates (1080i30, 1080p60, 1080p24). As you upgrade your HD display, you can take full advantage of your new display with all the HD DVDs in your collection.

There is no doubt that HD DVD will provide a significantly better viewing experience over standard DVDs, but understanding exactly what resolution means to your HD DVD experience can help you cut through the nonsense.

Article by Ryan Treit