True HDTV?
November 29, 2004
Recently while demonstrating the different picture modes on a Sharp LCD with a client, he remarked “why is it in 720p, isn’t 1080i true HDTV?” Somewhere mid-stream through my reply I realized this might make a decent article. I had made the mistake of assuming my client understood both 720p (progressive) lines and 1080i (interlaced) lines of resolution were both considered hi-def.
The Advanced Television Systems Committee (ATSC), which originated the HDTV standard, states both 720p and 1080i qualify as true HDTV. So does the Consumer Electronics Manufacturers Association, But for various reasons other parties in the hi-def debate are choosing sides. Most television manufactures have chosen to display HDTV in 1080i. Among their supporters are CBS, NBC and HBO. While others claim 1080i is significantly flawed and that 720p is better looking, better suited for new display technologies, and therefore more in tune with the future of digital television. On this side are ABC, ESPN HD and Fox among others. Still others state that on ‘real world’ sets the difference is minor.
Let’s consider 1080i vs. 720p. Who’s right, which looks better? And does it really matter? Let’s start by looking at the numbers. Counting scan lines is how 1080i proponents make their case. If you count scan lines, the number 1080 is higher than the number 720, right? But, the more accurate method is to count “pixels,” or picture elements, the dots that make up the picture. It’s mathematical really, multiply the number of vertical pixels in each format by the number of horizontal pixels. In 1080i, 1080 times 1920 equal 2,073,600 dots. In 720p, 720 times 1280 equal 921,600 dots. When you count dots, 1080i seems to have more than twice as many dots as 720p, and therefore a picture that’s more than twice as sharp. However, numbers aren’t everything. As the (i) and (p) imply, there is another distinction, 1080i is an “interlaced” format while 720p is a “progressive” format. Each uses a different method to turn a succession of still images into moving pictures.
Difference between interlaced and progressive:
Interlaced scanning produces a still picture field, or a ‘frame’, by scanning two sets of alternating lines. Progressive scanning creates a frame in one pass. If both are moving at the same rate “refreshing” the screen at the same number of passes per second, that gives progressive scanning the advantage, because it scans a complete picture ‘frame’, not half a picture ‘field’. It produces fewer dots and lines, but at twice the speed. So now it’s a question of timing. As ABC’s FAQ touches on: The number of lines of resolution in progressive and interlaced pictures is not a clear cut comparison. In the time it takes 720p to draw 720 lines, 1080i draws only 540 lines. And by the time 1080i does draw 1080 lines, 720p has drawn 1440 lines.
The truth is that 1080i and 720p each look better in different situations. The 1080i format is better at producing fine detail in still frames and pictures with little or no motion. Regardless of how long it takes to produce a picture, that picture has more lines, more dots. But this works well only as long as nothing moves. Remember how two fields make a frame? If something moves, the path of the motion changes between the alternating fields. That introduces “motion artifacts,” or visible distortion, such as stair-step patterns on a diagonal edge. The 720p format excels at reproducing motion, introducing little visible distortion regardless of the timing of moving objects.
What it all means:
So in the end both 1080i and 720p are high definition, but unless you’re watching a lot of slow moving transitional shots of lilies in a field, 720p for the most part is as hi-def as you need to get. What about 1080p you say? Take all of 1080i and 720p’s attributes and combine them and now you’re really cooking.
Posted by B.Greenway | | Filed Under HDTV
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