Plasma vs. LCD vs. LED: Which HDTV Type is Best?

Today's HDTVs can display beautiful, 1,920-by-1,080-pixel video, but the actual quality of what you're viewing depends on the source material. A lot of the time, you're not seeing exactly 1080p. In fact, most TVs today have two modes with similar names: 1080i and 1080p. Both have the same screen resolution, so what's the difference between the two? Here are five things you need to know:

1080i video is "interlaced." 1080i video plays back at 60 frames per second, but that's a bit deceptive, because it's actually broadcast at 30 frames per second. The TV then displays those frames twice, in a way—the first pass is 1,920-by-540 for the even scan line field, and the second pass is 1,920-by-540 for the odd scan line field. The process by which this occurs is called interlacing. It contributes to a sense of motion and reduces perceived flicker.

1080p video is called "progressive scan." In this format, 1,920-by-1,080-pixel high-definition movies are progressively drawn line after line, so they're not interlaced. On paper, that may not seem like a huge deal. But in the real world, what you end up seeing looks sharper and more defined than 1080i, particularly during scenes with a lot of fast motion.

Sometimes 1080p is termed "full HD" or "true HD," to distinguish it from 1080i or 720p video. Blu-ray discs contain 1080p video at 24 frames per second, and then, using a method known as 3:2 pulldown, display it at 30 frames per second on screen.

Data compression can confuse the issue. Sometimes cable companies will deliver a 1080i picture, but then compress the data significantly in order to use up less bandwidth. The result can mean smeared details or pixelated color gradations in certain scenes. It's still technically HD, and still looks better than standard-definition cable, but it's not as good as it could be.

This also happens with 1080p streaming Internet video, but in that case, it's usually dependent on the speed of your data connection. In fact, Blu-ray is currently the only practical format for watching lots of pure 1080p content. Even the latest Apple TV, which supports 1080p streaming, does so in a compressed format that loses a bit of quality (although it still looks quite good).

Both formats look similar on smaller TVs. As a general rule, you need a larger TV to notice the difference between 1080i and 1080p. Depending on your eyesight, you can probably pick up the difference on a 32-inch LCD if you're particular about it. But most consumers don't really see a marked difference until at least a 42-inch screen, if not larger. In fact, many people are perfectly happy with 720p HDTV sets even at higher sizes; we recently named one, the 51-inch Samsung PN51E490B4F, our Editors' Choice for budget large-screen HDTVs.

1080p isn't even the best anymore. Technology never stands still, of course. Five years from now, you'll probably just want Ultra High Definition (aka 4K) video instead. (For a closer look at 4K video, check out What is Ultra HD?.) But for now, if you're a videophile who appreciates a sharper picture, 1080p is definitely the way to go—both in HDTV capability and in the source material you're viewing.

For more, check out Buying an HDTV: Frequently Asked Questions, You're Doing It Wrong: HDTV Setup, and Frame Rates Explained.

For more from Jamie, follow him on Twitter @jlendino.