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A: By law, HD must have two characteristics, revolving around "aspect ratio" and "screen resolution."

First, HD broadcasts must use a widescreen, 16x9 "aspect ratio," which mimics modern movie theater screens. That is, for every 16 inches of width, it must have 9 inches of height. A traditional TV, by contrast, uses a "full screen" aspect ratio of 4x3, similar to the dimensions of most movie theater screens before the 1950s. The difference between these dimensions is the reason you see horizontal black bars when you watch a widescreen movie on a standard TV and vertical black bars when watching an analog signal on an HD set.

Second, where standard TV use a screen resoltuion of 640x480 -- displaying pictures through 640 horizontal lines and 480 vertical lines -- HD must use a minumum of 1280x720; true HD is defined as 1920x1080. As a shorthand, only the second number -- 480, 720, 1080 -- is used to define screen resolution.

Q: How does this offer a better picture?

A: To visualize this, turn on your TV and get real close. See all the little illuminated dots. Those are called pixels. Now do some math. A standard 640x480 TV has 307,000 of them. A 1280x720 picture uses about 1 million pixels; a 1920x1080, more than 2 million. The more pixels you have, the sharper the picture.

Now take one more look at your TV. Notice all the tiny black lines between the pixels -- this is dead space that blurs the picture. More pixels means smaller dead spaces, and sharper pictures.

Q: What does the term "native resolution" mean?

A: That's the number associated with the TV -- 480, 720, 1,080. TVs can only display images in their native resolution. For example, 720 and 1,080 TVs will upscale 480 broadcasts to their higher resolutions, turning those 480 lines into 720 or 1,080 lines. This is why standard DVDs look better on HD TVs. A 720 TV will downscale a 1,080 signal to its native resolution.

Upscaling and downscaling are tricky -- some TVs are better at it than others. This is the reason you should ask your salesperson to display various types of signals on the set you might buy.

Q: What's the difference between 720i and 720p, or 1080i and 1080p?

A: The "i" stands for interlaced, which means your TV is showing only half the picture every 1/60th of a second -- first the odd numbered lines, then the even numbered ones. The human brain is an amazing thing; it is able to merge these pictures into a single image so that it's hard to notice your TV's flickering dance.

The "p" stand for progressive scan, which means that your set is displaying all 720 (or 1080) lines all the time.

In most cases the naked eye will be hard pressed to notice much difference between interlaced or progressive scan signals except in some fast-moving live action broadcasts -- especially sporting events and concerts, which can seem slightly blurry in particularly intense periods.

Q: Does that mean that a 1080 TV is better than a 720?

In the abstract yes. As a practical matter, the naked eye is hard pressed to discern differences in screens 50 inches and less. As you move up into larger screens, a 1080's advantages become clearer.

However, no one broadcasts in 1080p. The standard HD signals now are 720p and 1080i. Again, in theory, the 1080p -- which is known as "true HD" -- provides a better picture as it upscales those lesser signals. But only the most discerning viewers will be able to appreciate them.

Q: So the more expensive 1080p sets are a waste of money?

A: Not exactly. Blu-ray players and the recently discontinued HD DVD format offer 1080p resolution, allowing you to experience all the advantages of true HD.

Q: Blu-ray discs look like my regular DVDs, what's the difference?

A: Size matters. Just as a digital signal can carry more information than analog, and an HD TV can display more of it, a Blu-ray disc can store more it.

DVDs are encoded by a red laser that draws relatively fat lines on the disc. The disc can only hold about 8 gigabytes of information. Blu-ray discs are encoded by a blue-violet laser; it draws much finer lines, allowing the discs to store up to 50 gigabytes.

Blu-ray discs can meet the larger storage requirements of the 1080p signal and also have room for advanced audio formats.

Q: Can it play my old DVDs?

A: Yes. And they will look better than ever on an HD TV.

Q: Can I play Blu-ray discs on my standard DVD player?

A: No.

Q: Are the prices of Blu-ray players expected to fall?

A: Yes. Industry analysts expect a $199 model to be available by the holidays.

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