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Discussion Starter · #1 ·
I think I understand the differences in NTSC and the new system. I know what interlaced versus progressive scan is. But why is 480p considered "line-doubled?"


I'm told my Elite 610 takes an NTSC 480i signal and "doubles it to 480p." Shouldn't this be called 960i? Since my nice 610 has a picture without visible scan lines, I'm fairly certain I'm getting 960 lines of vertical resolution.


The term 480p, to me, means there are only 480 lines, but they are displayed in order (at 60 fps?) instead of interlaced at 30 fps. But wouldn't I still see scan lines? Why does everyone call 480p a "line-doubled" 480i?


I know a 480 line picture displayed progressively at 60 frames per second should give me a better picture than 480i, especially with motion on the screen, but I should still see the scan lines. Right? We've only changed their order and the frequency at which they're displayed, not the AMOUNT of lines displayed.


Experts please chime in! I'm fairly good at explaining the new TV system compared to the old when discussing with friends and relatives, but I'm stumped to explain the above when asked.


Thanx,

Barry
 

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Barry,


Technically you are doubling the lines because there's only 240 lines of active video on the screen at a time in 480i mode (even or odd) and when displaying 480P all 480 lines are present on the screen, both even and odd.


Example:


480i - CRT paints lines 1,3,5,7,9,11,.......479; back to top; 2,4,6,8,10,...480; repeat same process.


480P - CRT paints lines 1,2,3,4,5,6,7,8,9,10,11....480; back to top and repeat.


Does that help???


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Discussion Starter · #3 ·
Quote:
Originally posted by DHarp193:


Technically you are doubling the lines because there's only 240 lines of active video on the screen at a time in 480i mode (even or odd) and when displaying 480P all 480 lines are present on the screen, both even and odd.
Are you saying that when I see the lines on an NTSC picture it's because there are only 240 lines visible at a time? After the odd lines are displayed do they disappear and then the even lines are displayed? I had assumed the even lines just got 'painted' in between the odd lines. When I see drawings and descriptions of this process, the latter is the way it's always depicted.


If I had an old style RPTV, I guess I could count the visible scan lines and know for sure. Nothing better to do until my Sony HD100 arrives. http://www.avsforum.com/ubb/smile.gif If I were to count only 240, then it would make sense. I didn't realize that only 480 visible scan lines would be enough to 'disappear' on my 610.


Thanx,

Barry
 

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480i is 240 lines every 30th [correction: 60th] of a second, alternating between even and odd lines. Yes, the scan lines you see on interlaced displays are because you're only seeing half the image at any one time. 480p is 480 lines every 60th of a second. This presents a much more solid image, though you may still be able to see the scan lines if you look closely.



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[This message has been edited by Abdul Jalib (edited 03-13-2001).]
 

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Barry,


What you say is sort of true. The persistence of the phosphors on the CRT face means that when the odd or even lines are painted, the phosphors stay slightly lit while the alternate line is being drawn, so you may see more lines than 240, but those are ones that are dimming from being hit by the electron beam a full frame previous while the acitve ones ae still very bright from being hit by the electron beam during it's pass of the current frame. In 480P, they are all shining brightly from being hit at effectively the same time, so brightness, resolution and sharpness goes up using 480P.


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BKMaggert,

A 480i NSTC frame lasts 1/30th second and is made up of two interlaced 1/60th-second fields, each 240 active lines (carrying picture information). Your eye-brain blends these 1/60th-second fields into 1/30th-second frames (30 each second). Video processors can display NTSC frames in a huge number of ways. -- John


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[This message has been edited by John Mason (edited 03-12-2001).]
 

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Discussion Starter · #7 ·
Quote:
Originally posted by John Mason:


A 480i NSTC frame lasts 1/30th second and is made up of two interlaced 1/60th-second fields, each 240 active lines (carrying picture information).
I understand this but, do the odd lines after 1/30 second go away (fade away), and then the even lines get painted in the same place? Or do the even lines paint in between, in which case we see only 240 at a time but they 'cycle' their posistions on the screen?


The reason I'm getting so detailed about this is because it seems, based on what you guys are saying, that a 1080i picture really only displays 540 lines on the screen at a time. This would yield only a SLIGHTLY better picture than 480p (540/480 = 12.5% more resolution), which I don't believe is the case.


Also, this would mean that 720p would yield a MUCH better picture than 1080i (720 lines/540 lines = 33.3% more lines). But based on all I've read here and elsewhere is that 720p CAN be only marginally better than 1080i on fast motion such as sports because it's 60fps, and 1080i yields a better picture on MOST other things "because it displays more lines than 720p" although only at 30fps.


See my discrepancy? If what you guys say is true, then 720p should be the king of all available formats. A SIGNIFICANTLY better picture--720 visible scan lines, not 540, AND at 60fps. 1080i should NEVER have a better picture.


Thanx,

Barry
 

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i have the same problem barry has but just never brought it up. when i look at a display i don't see scan lines flickering, i see scan lines. i understand how deinterlacing creates a more stable image and even a brighter image but i don't see how it makes scan lines go away. you can get an inch from the screen and see the scan lines with space between them and i expect that if barry counts them he will not get 240.


greg


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Think about the times we are talking about here. 1080i may only have 540 just lit lines, on every other line, on the screen in any give 1/60th of a second, but every 1/30th of a second you see 1080 lines on that screen. Progressive scanning can give you 60 frames per second, while interlaced gives you that same resolution but only at 30 frames per second. 720p is still less lines on your screen when you consider what is painted on your screen every 1/30th of a second. So the picture DOES have a higher resolution at 1080i (it just took two passes to do it), but 720p will have less flicker and can appear to be more "stable".


So if I walk up to my screen and count lines from a 1808 source I should see 1080 lines because my brain cannot simply freeze a frame from a point in time as small as 1/60th of a second. Not to mention when you talk about if the line is still "there" you could take that argument to the extreme in saying that as soon as a pixel has been lit up it immediately begins to dim again. So even from the beginning of a scan line to the end of that scan line it is starting to dim. The thing is that the pixels are being lit up at such incredible speeds (and dimming at somewhat slower speeds, but I don't have any numbers to give you on that) we just don't notice it.


Does that help any?


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[This message has been edited by zarlor (edited 03-12-2001).]
 

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Keep in mind that you are only discussing Vert. Resolution. You have to take into account Hort. Resolution. 480P can have two different HR's; 640 for TV and 720 for DVD. HDTV is either 1920 for 1080i or 1280 for 720P.


When you compare resolutions you have to consider both H & V types to accurately measure an image.


Lee
 

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Discussion Starter · #11 ·
Quote:
Originally posted by zarlor:
1080i may only have 540 just lit lines, on every other line, on the screen in any given 1/60th of a second, but every 1/30th of a second you see 1080 lines on that screen.
I agree with you Lenny. On 480i, I should see 480 visible scan lines, regardless of the order they were painted, but Dave and Abdul say differently.


To quote Dave, "there's only 240 lines of active video on the screen at a time in 480i mode." And Abdul said, "Yes, the scan lines you see on interlaced displays are because you're only seeing half the image at any one time." They imply that we are only getting 240 visible lines at 480i and 540 visible lines at 1080i.


Dave, I guess I need more explaination as to what is really going on.
 

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I'm stumped too. When I was in Fry's the other day, they had a DVD feeding all the RPTVs. You can clearly see the scan lines on the NTSC RPTVs but not on the HDTVs . . . why is that? I realize the HDTVs are line-doubling to 480p, but for both the NTSC & HDTVs, there are 480 lines drawn on the screen (regardless of scanning rate & IvP).


Why do you see the scan lines in the NTSC RPTVs but not in the HDTVs?
 

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Quote:
Originally posted by BKMaggert:
I agree with you Lenny. On 480i, I should see 480 visible scan lines, regardless of the order they were painted, but Dave and Abdul say differently.


To quote Dave, "there's only 240 lines of active video on the screen at a time in 480i mode." And Abdul said, "Yes, the scan lines you see on interlaced displays are because you're only seeing half the image at any one time." They imply that we are only getting 240 visible lines at 480i and 540 visible lines at 1080i.


Dave, I guess I need more explaination as to what is really going on.
At any given 1/60th of a second interval only 240 lines of a 480i display have been ACTIVELY painted.


The question I think you are asked later is how fast do those lines dim in the next 1/60th of a second when the next set of lines gets painted in. I don't know the specific answer, but I doubt they dim completely in the 1/30th of a second it takes to repaint them. My guess would be that if you could somehow freeze time at the very moment in that 1/30th of a second when a frame had completed its pass you would notice that on an interlaced display the first set of lines were dimmer (possibly even gone in the upper lines) than the second set of just painted lines and that the upper left hand corner was slightly dimmer than the lower right hand corner, overall. On the progressive frame you would probably notice that the upper left hand corner was dimmer than the lower left hand corner (which just finished being painted), but that's about it. If you could count the "lines" you would be able to count 480 in either frame, but in the interlaced frame every other line would be somewhat dimmer then its neighboring line.


Either way it's not particularly relevant since you are worried about horizontal resolution which is, effectively, 480 completely different lines. The only difference is if those lines get painted every 1/60th of a second or every 1/30th of a second. My argument is that the time length is so miniscule that your brain cannot notice the difference other than to recognize a certain amount of "flicker" or "instability" in the picture. But either way you get the full resolution of 480 lines. The difference comes only in how those lines get displayed.



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Is this what is going on?


HDTV "line doubles" 480i to 480p with some interpolation. Then 480p is drawn on the screen as 960p with every line drawn twice? Thus there are 960 lines even though every other line is identical to its neighbor line.


????
 

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the net issue is, how can a line doubler make scan lines disapear as many say it does? there are still 480 lines on the screen, the screen is the same size and the lines are not any thicker. i get how it could make it flicker less but i don't see how it can make scan lines disapear unless you scaled the result up to higher than 480 lines.


greg


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Quote:
Originally posted by BKMaggert:
I understand this but, do the odd lines after 1/30 second go away (fade away), and then the even lines get painted in the same place?
Your eyes merge the two 1/60-second fields creating a 1/30-second frame, the phosphors decay, and the process starts all over again for the next frame. Phosphor decay is very fast and doesn't play that much of a role; it's our 'sluggish' eye-brain system.


On the other points raised, hope you don't mind if I cite a few earlier threads. I went into some of these points at length, and stand by what I contributed earlier unless someone points out where I've gone off track.


But I'll have to edit these threads in later since the search server, I've discovered, has been overloaded for some time. Back a while... - John




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Argh. Why is this so hard to convey? indygreg, you basically have got it right. A close examination of a large HDTV showing 480i upscaled to 480p will usually allow you to see the scanlines. On an analog TV that's showing 480i as 480i, the scanlines are extremely evident, due to the flicker. You don't see the flicker as a flicker really, because it's so fast, but nevertheless, the fact that only 240 lines at a time are displayed at full intensity at any 1/30th [correction: 1/60th] of a second interval creates visible transitions between each scan line.


Sony's DRC upscales 480i to 960i instead of 480p. At 960i, the scanlines are so thin that it's extremely difficult to see them, despite the flicker caused by only 480 lines being drawn every 1/30th [correction: 1/60th] of a second. Same thing goes for 1080i.


It may help if you understand that 1/60th of a second is kind of a magic number - a bit above it, and your eye sees successive pictures as smoothly as reality - a bit below it, and your eye sees successive pictures as distinct. 1/30th of a second causes a severe flicker. [And each line is drawn only once every 1/30th of a second in an interlaced display.]



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[This message has been edited by Abdul Jalib (edited 03-13-2001).]
 

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I had the exact same question... both 480i and 480p are putting 480 distinct lines on the screen in any given 1/30 of a second, so it would seem that they should be equivalent in terms of how much visibility the scan lines have.


But yet there is a real visual difference, and I can personally vouch that the scan lines are much less visible with 480p (Iscan) than 480i (NTSC) on my Sony 1272 projector. I think it ends up being a psychovisual effect, that the eye tends to see whatever is brightest. In the 480i case, 240 of the lines are always _much_ brighter than the other 240 lines at any given instant of time. That's just a guess.


Another psychovisual effect that I do understand however is as follows. Staring at the middle of the screeen, move your gaze upward or downward at a rate 60 lines per second (4 seconds from middle to top or bottom). It helps to move your hand at that rate and watch the scan lines behind your hand. You will see what looks like "double spaced" scan lines, where your eye fuses alternate scan lines and thinks they are the same. This is a good way to observe that the scan line phosphors have dimmed down significantly by the time the second line paints; you can see the dimming afterimage just barely between the bright lines.


Anyway, perhaps the effect above is what causes the perceived difference in vertical lines. I think that the quick dimming of the scan lines must be a root cause; simpler line doublers simply repeat the interlaced fields twice, and yet they give a higher perceived vertical resolution. It all must boil down to the fact that human vision isn't quite as simple as we expect, and we can somehow detect (and reject) the 30 fps field rate.



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Discussion Starter · #19 ·
Quote:
Originally posted by Abdul Jalib:
Argh. Why is this so hard to convey?
Because you keep talking about frame rate, and I'm talking about vertcal resolution. I understand that 480i is 30fps, and that 480p is 60fps, hence less (or none) flicker. I also understand that the reason we interlace 30fps is to reduce flicker to imperceptable levels.


But BOTH show 480 lines on the screen, just displayed in a different order. So why is 480p called "line-doubled" when supposably it's not doubling the number of lines? But my Elite 610 in '480p' does not reveal scan lines at 6-8 feet viewing distance, but a 480i display does.


Sorry Abdul, but you (and Dave, and John) haven't explained this at all. And it doesn't look like I'm alone here. The closest explaination I've received is that 480i is really showing only 240 lines at time with 'empty' lines in between, which is why I SEE lines at 480i, but not at 480p. But you, and others, contradict this when its said that our eye can not see phospers 'fade out' at 30fps, which makes very much sense to me. So we should still have 480 lines VISIBLE with 480i. Hence, I'm still confused why 480p is considered "line-doubled."


Barry
 

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BKMaggert, this thread ,not so far back, goes into some of these topics. I recall outlining many of them at several points. Also went over some similar points in this December thread . On your last point, doubler is sometimes considered the wrong term to use;it's better to use deinterlace since that's what you're doing. The Kell factor, mentioned briefly in both threads, is key to comparing interlaced versus progressive vertical resolutions, and you can find a fair amount of discussion about it on the web. -- John




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