Friday, December 21, 2007

Understanding HDTV Resolution - Video source resolution

The two most common high-def video source resolutions are 720p and 1080i. All HDTV broadcasts, including local over-the-air broadcasts, satellite and cable signals, use one of these formats. 1080i is the most common resolution, but both formats have their benefits and limitations:

  • 1080i has more lines and pixels to show more detail, so it's great for slow-moving programs with lots of close-ups — think Law and Order or nature documentaries on The Discovery Channel. But the "i" tells you that it's an interlaced format, which means fewer video frames per second, so it doesn't handle fast-moving video as well as 720p.
  • The "p" in 720p tells you it's a progressive-scan format, which means it presents fast-moving action much more cleanly. It's ideal for things like sports and action-packed video games.
What about 1080p?

These days, the most talked-about HD format is 1080p, which combines the superior resolution of 1080i with the progressive-scan smoothness of 720p. True 1080p content is still scarce, however; it's mainly available from HD DVD and Blu-ray high-definition disc players and video game consoles such as the Xbox 360™ and PS3. When you hear 1080p mentioned, it's usually referring to a TV's screen resolution rather than a source.

One more thing

Another key thing to understand about video source resolution is that it can also limit how good your HDTV's picture looks. If you give your TV a lower-resolution source, like a fuzzy analog cable channel, that's what you'll see — a high-def TV can't transform a poor picture into a great-looking picture. If you want to see true high-definition images on your HDTV, you'll need to feed it a high-def source — 720p, 1080i, or (in a few cases) 1080p.

What "i" and "p" mean, and how they can affect the level of picture detail

As we mentioned before, "i" stands for interlaced-scan and "p" stands for progressive-scan. These terms originated when all TVs used picture tubes, and images were "scanned" — painted across the screen line by line. Interlaced-scan images required two passes to create a complete video frame, while progressive-scan displayed the entire frame with just one pass (see illustration below). The frame rate for interlaced video is 30 frames per second while progressive-scan video is 60 frames per second.

Interlaced scan splits each video frame into two "fields," displaying all the even horizontal scan lines (2,4,6…) in 1/60th of a second, followed by the odd scan lines (1,3,5…) during the next 1/60th of a second. That means you'll see a complete video frame every 1/30th of a second.

Progressive scan, on the other hand, displays all the lines in a single sweep (1,2,3,4…). You'll see a complete frame every 1/60th of a second.

The bottom line

Today's digital TV displays are nearly all effectively progressive-scan, so interlaced and progressive are mostly relevant when describing video source signals sent to the TV. The main thing to remember is that a progressive signal has twice as much picture information as an equivalent interlaced signal, and generally looks a little more solid and stable, with on-screen motion that's more fluid.

Graph comparing 1080p, 1080i and 720p

This graph shows the total amount of picture information displayed at each resolution, per second. 1080p's combination of high screen resolution and progressive-scan frame rate allow it to deliver twice as much picture information as the other options — which means a clearer, smoother picture. Hopefully, we'll see more 1080p content soon.

What happens if your TV and video source have different resolutions?

This scenario actually happens all the time, and fortunately with today's HDTVs, you don't really need to worry about it. Whether the resolution of your video source material is low (VHS), medium (DVD), or high (HDTV), a fixed-pixel TV will always automatically convert or scale the video signal to fit the screen's native resolution. Scaling lower-quality signals to fit a TV's higher-resolution screen is often called upconversion. Upconversion works great with a good source like DVD, but it can't make snowy analog antenna reception or a noisy cable picture look flawlessly crisp and clear.

Similarly, if the incoming source has more pixels than the screen's native resolution, the video signal has to be "downconverted." It's like trying to pour 10 pounds of sugar into a 5-pound bag: You have to throw away some detail to fit the image on the screen. That's one of the reasons 1080p TVs are so popular — they can display every pixel of every available high-def resolution, so they never have to throw any detail out. But if you don't get a 1080p TV, don't worry — downconverted video can still look great. The best example is 1080i HD broadcasts that are downconverted to be viewed on 768p TVs.

Is 1080p for you?

Despite the fact that there aren't many 1080p sources, a 1080p HDTV may still be the way to go. For one thing, you'll never have to "throw away" any detail from any of your high-def sources. And as we mentioned earlier, a 1080p TV actually has twice the resolution of a 768p TV. So if you want to ensure that you'll see every exquisite detail, a 1080p set is an excellent choice. But there are some other factors to consider. To figure out where resolution fits on your priority list, ask yourself these questions:

  • How large a screen do you want, and how far from your TV will you be sitting? Chances are you won't be able to see much difference between 1080p and non-1080p HDTVs unless their screens are relatively large (46" or bigger). Even then, if you sit at the farther end of our recommended viewing distance range, you might be just as happy with a 768p or 720p TV. But if you plan to get a larger screen and sit closer, you'll appreciate the extra detail 1080p sets can offer.
  • Is 1080p something you're willing to pay extra for? If you want the sharpest picture around, and you don't mind spending another few hundred dollars or so to get it, then the answer is yes. Plus, you usually find 1080p resolution in upper-range models that also offer superior video processing, additional inputs, and more advanced features and conveniences. But you may decide you'd rather put that money toward a larger screen, or a wall-mountable flat-panel TV instead of a less-pricey rear-projection model.

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