True HDTV?

November 29, 2004

colorbarsRecently 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 Bryan Greenway | | Filed Under HDTV


Comments

  • Chas

    Is there a risk the 720p will shortly be outdated or nonfunctional in 2009?

  • Chas

    Is there a risk the 720p will shortly be outdated or nonfunctional in 2009?

  • ricardo

    i have a 32 in sharp aquous and it 1080i looks really good specially watching discovery hd i havent tried 720p but ill try and ill see wich one looks better .

  • ricardo

    i have a 32 in sharp aquous and it 1080i looks really good specially watching discovery hd i havent tried 720p but ill try and ill see wich one looks better .

  • devo

    How do you know if a display is capable of full 1080?

    Mine gives me the option for both when I hit the format button. How do I know if it’s 1080i or 1080p?…I thought that tha 720 was “P” and the 1080 was “I” ???

    If there is a 1080 P, wouldn’t that be the best since the P thing is all about frames and motion-which most people watch TV for anyway?

    as I see it-720 was greater than 1080 just because of the “P” to the “I” (and also cuz I’m not watching stills–I’m playing games and watching REAL tv)….but if both were P….it would be the 1080, right?

    how do I know if my tv has this? does it say it on tha box?

  • devo

    How do you know if a display is capable of full 1080?

    Mine gives me the option for both when I hit the format button. How do I know if it’s 1080i or 1080p?…I thought that tha 720 was “P” and the 1080 was “I” ???

    If there is a 1080 P, wouldn’t that be the best since the P thing is all about frames and motion-which most people watch TV for anyway?

    as I see it-720 was greater than 1080 just because of the “P” to the “I” (and also cuz I’m not watching stills–I’m playing games and watching REAL tv)….but if both were P….it would be the 1080, right?

    how do I know if my tv has this? does it say it on tha box?

  • B.Greenway

    Hi Richard, this post is a bit dated and as such things have changed wrt to available displays, if your display is capable of full 1080p (or 1080i for that matter) by all means output the games at 1080i/p if they were rendered as such.

  • B.Greenway

    Hi Richard, this post is a bit dated and as such things have changed wrt to available displays, if your display is capable of full 1080p (or 1080i for that matter) by all means output the games at 1080i/p if they were rendered as such.

  • Richard

    What about video games, wouldn’t you like to use 1080i than 720p? I have an Xbox 360 and have it set at 1080i all the time. Should I change it to 720p.

  • Richard

    What about video games, wouldn’t you like to use 1080i than 720p? I have an Xbox 360 and have it set at 1080i all the time. Should I change it to 720p.

  • B.Greenway

    Very true, however I was speaking more to mid-res 1024×768 consumer sets. I should have clarified.

    But if you’ll notice I did mention:

    “1080 times 1920 equal 2,073,600 dots. In 720p, 720 times 1280 equal 921,600 dots”

    and

    “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.”

    So again it isnt (in my opnion) always about total resolution.

    Your right, pure 1080i has more raw horizontal and vertical resolution, than 720p. but so many of today’s displays cant make full use of that resolution, so 720p winds up winning on that fact alone.

  • B.Greenway

    Very true, however I was speaking more to mid-res 1024×768 consumer sets. I should have clarified.

    But if you’ll notice I did mention:

    “1080 times 1920 equal 2,073,600 dots. In 720p, 720 times 1280 equal 921,600 dots”

    and

    “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.”

    So again it isnt (in my opnion) always about total resolution.

    Your right, pure 1080i has more raw horizontal and vertical resolution, than 720p. but so many of today’s displays cant make full use of that resolution, so 720p winds up winning on that fact alone.

  • Michel

    Interesting reading, but 10801 also has more horizontal pixels than 720P. Which means higher resolution horizontal.
    I don’t see any comment about in the comparison.

  • Michel

    Interesting reading, but 10801 also has more horizontal pixels than 720P. Which means higher resolution horizontal.
    I don’t see any comment about in the comparison.