HD TV resolution explained

Is there such a thing as high def and higher def? CNET helps cut through the hype surrounding the various formats of HD resolution.

Resolution is the main reason why HD TV looks so much better than standard television. On a high-def TV displaying a high-def source, a million or more pixels combine to create images that appear sharper and more realistic than TV ever has before. Resolution isn't the be-all and end-all of picture quality, however, and its numerous, well, numbers, can be incredibly intimidating at first. In this article we'll try to demystify HD TV resolution and help you cut through the hype that surrounds all of those numbers.

How important is resolution?
Not as important as you might think. According to the Imaging Science Foundation, a group that consults for home-theatre maufacturers and trains professional video calibrators, the most important aspect of picture quality is contrast ratio, the second most important is colour saturation and the third is colour accuracy. Resolution comes in a distant fourth, despite being easily the most-talked-about HD TV spec today.

In other words, once you get to high-definition, most people are perfectly satisfied with the sharpness of the picture. All other things being equal -- namely contrast and colour -- HD TV looks more or less spectacular on just about any high-def television regardless of its size or the HD TV signal's resolution itself. The leap from normal TV to HD TV is so big that additional leaps in resolution -- from high-def to higher-def, let's say -- are tiny by comparison.

Nonetheless the HD TV landscape is littered with resolution discussions, in regard to both sources and displays, so a little knowledge of how they interact is a good thing.

The fix is in
For the rest of this article, we'll be talking about fixed-pixel displays. A fixed-pixel display is any HD TV or monitor that uses pixels to produce an image, including flat-panel LCD and plasma screens as well as rear-projection microdisplays and front projectors that use DLP, LCD, or LCoS technology. We'll ignore non-fixed-pixel displays; namely, direct-view and rear-projection CRTs, because they treat incoming resolutions differently than their fixed-pixel cousins do -- since they don't use discrete pixels, their specs are much more difficult to pin down.

All fixed-pixel displays have a native resolution spec that tells you how many pixels the display actually has. Native resolution is the absolute limit on the amount of detail you'll see.

Fixed-pixel displays follow a few basic rules:

• No matter the resolution of the source material, whether VHS, DVD or HD TV, a fixed-pixel display will always convert, or scale, it to fit its native resolution.
• If the incoming source has more pixels than the display's native resolution, you will lose some visible detail and sharpness, though often what you're left with still looks great.
• If the incoming source has fewer pixels than the native resolution, you're not getting any extra sharpness from the television's pixels.

HDTV source resolutions
If you read those three axioms closely, you'll see that source is everything with HD TV. Or, as some unknown wag once said, "Garbage in, garbage out." There are two main HD resolutions in use today by Australian broadcasters: 1080i and 576p (although 576p would not be considered an HD format in many other countries). One is not necessarily better than the other; 1080i has more lines and pixels, but 576p is a progressive-scan format that should deliver a smoother image that stays sharper during motion.

Another format is also becoming better known: 1080p, which combines the superior resolution of 1080i with the progressive-scan smoothness of 576p. True 1080p content is extremely scarce, however, and it will probably stay that way until more Blu-ray content is available here. The term 1080p today appears mostly in reference to the displays' native resolution, not the source.

Despite the obvious difference in pixel count, 576p and 1080i both look great. In fact, unless you have a very large television and excellent source material, you'll have a hard time telling the difference between any of the HD TV resolutions. It's especially difficult to tell the difference between 1080i and 1080p sources. The difference between DVD and HD TV should be visible on most HD TVs, but especially on smaller sets, it's not nearly as drastic as the difference between standard TV and HD TV.

HD TV display resolution
Now that we've considered the source, let's look at the televisions. As we mentioned above, all fixed-pixel HDTVs scale the incoming resolutions to fit the available pixels, throwing away information if they have fewer pixels and interpolating information if they have more pixels than the source.

Technically speaking, all of these numbers are accurate and useful, but don't put too much stock in them. In the real world, it's difficult to tell the difference between native resolutions once you get into high-def. For example, despite the fact that a 37-inch LCD with "only" 1,366x768 pixels has to throw away a good deal of information to display a 1080i broadcast, you'd be hard-pressed to see more detail on a similar 37-inch LCD with 1,920x1,080 resolution.

The truth about 1080p
In Australia, we're finally starting to see HD TVs with 1080p native resolution, which typically cost a good deal more than their lower-resolution counterparts. But as we've been saying, once you get to high-def, the difference between resolutions becomes much more difficult to appreciate. We've done side-by-side tests between a 50-inch 720p HD TV and a 50-inch 1080p HD TV using the same 1080i source material, and it was extremely difficult for us to see any difference. It becomes even more difficult at smaller screen sizes or farther seating distances -- say, more than 1.5 times the diagonal measurement of the screen.

¹ This is the number of physical pixels the television uses to produce a picture. You may notice that few of the resolutions in the table match the HD TV source resolutions exactly. That's mainly because TV makers find it more cost efficient to make panels with the pixel resolutions in the table and then scale the incoming sources to fit the screen. It's true that ideally you'd like to exactly match the incoming source with the display's native resolution, but it's much less important in HDTV than in, say, computer monitors. That's because scalers in HD TVs generally do a good job of converting the signals, and because most HD TV is in motion and seen from a distance, as opposed to static text seen up close.

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