could one of you video savants explain this to me once and for all?

I have heard several people say that 540p and 1080i are the same, and I can understand how to the device they are, but I don't see how you could not lose resolution. Here is my understanding visually, and It may be way off base, so if it is please correct me visually if possible, because we are talking video here right. So I understand each frame is two passes so I'll put my understanding here in 5p and 10i

5p frame

10p frame

So there it is from what I've read I thought that in interlaced you got every other line first pass and the remaining lines second pass, whereas in progressive you got every line every time, so I just can't wrap my hands around how 540p = 1080i aren't you actually losing 50% resolution? and how would it determine what scanlines to cut?

Once again I am not claiming to be an expert, just trying to understand since so many people who seem to be experts claim they are the same.

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Xsecrets

* no expert here

but i believe that 1080i is actualy 540p TIMES 2. 1 frame of even lines 540 and 1 frame odd lines 540. Net effect is that at any one time you still only 540.

that is the way i understood it. if im wrong in any way, someone will correct me.

After several discussions I think I finally get this concept. The deal is this: your TV set will interpret the signal as 1080i regardless of whether your computer "thinks" it's 540p or 1080i. Therefore, each 1/60 sec. frame will be shifted by one scan line during playback, regardless of any other setting in your system.

So, if your content comes from an 1080i source, and your processing "converts" it to 540p in the simplest manner, it just takes each 1/60 frame and sends it along to the display. Now if your display were TRULY a 540p system, you'd see a very annoying flicker as the content changed evey other frame. But your TV set now does the "dumb" thing and just displays the frames sequentially. Which gets you right back where you started, 1080i.

The only benefit I can see to setting up a TV as 540p is if there is some reason why the modeline won't support a 1080i mode. Nvidia drivers prior to 6111 were in this boat. (Well, actually, VERY old drivers did it correctly, then it was broken, then "fixed" in 6111. We think.)

The only downside I can see is if your TV and the source get out of sync as to which is the "top" field and which is the "bottom." I can't imagine it would just work perfectly every time -- you'd think that things would sync "upside down" (bottom field on top) about half the time.

I am using a 1080i modeline with my VGA output to my Pioneer PRO-610HD set and original 1080i content looks very good. Actually better than that. Excellent. Maybe even superlative.

720p content is another matter. I think the program is sent to the Xv driver as a 720p stream, but it gets scaled by either Xv or the Nvidia driver into 1080i to match my modeline. It sucks. Not "horrible, unwatchable" bad, but "hey what are all those shimmering artifacts when the camera moves?" bad. And, my favorite, "why did they mark the football field with all those dashed lines?" The lines just alias to dashed lines at certain camera angles and distances.

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I'm trying to get my head around this too liv2cod. Does the modeline allow the video card to produce interlaced output (so that for 1080i it does one scan in 1/60 of a sec with the 1920x540 odd lines, and one in the next 1/60 of a second with the 1920x540 even lines) or does it actually display 60 hz progressive 1920x1080 where the odd lines field and even lines field are interlaced on the display and the display only updates every 2 scans? I suspect it is the second option.

If aovermy sees this, she may know the answer. If she connected the VGA output from her computer to a monitor instead of the scanline converter and the picture showed interlacing artefacts when there is movement, then it probably does it the second way. The monitor may be able to tell you what the resolution and refresh rate are. My monitor currently reports 1280x1024 FH=91.1kHz FV=84.9Hz (this is not my myth box).

The fact that she gets good results using the scan converter suggests that it is somehow correctly extracting the interlaced fields from the 1920x1080 input.

Does any of this make sense?

The interlaced signal is displayed as two "fields" of 1/60 second each, a "top" field followed by a "bottom" field. (I'm not sure what magic the video card and monitor use to synchronize which is the "top" field.) In the case of 1080i this means a field of 1920x540 followed by second field of the same size. Both fields together form the 1920x1080i picture.

If you send this to a progressive display like a computer monitor, you will get a "dog's dinner" on the screen. It will not know how to interpret the two fields and you'll get a flickering mess. More likely, X will switch to one of its built-in modelines which matches the monitor better and reject the 1080i modeline. You can look at the X log and see which modelines it uses and which it rejects.

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So does that mean that from the point of view of X windows, the screen is 1920x540 updated at 60 Hz or is it 1920x1080 updated at 30 Hz and the graphics card extracts every other line to generate two fields?

If it is the former, wouldnt myth need to know this somehow, and what would it do when you paused and stepped forward one frame at a time (there are only 30 frames a second right?).

If it is the latter then I could understand (and it would explain why 720p looks bad on a 1080i modeline, because extracting every other line from a 720p rescaled to 1080 is not going to be what you want).

The short answer is... I dunno.

I'm also curious where the dividing lines are drawn in the Myth output architecture. For example, I think that Myth sends a 1280x720 signal to Xv and it rescales the output to 1920x1080. Or is the rescaling done by the nVidia driver? I just don't know yet.

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To try to definitively answer Xsecret's original question:

1080i and 540p are definitely not the same thing. They only seem the same if you don't worry at all about the horizontal resolution or the fact that the scan lines are different in successive phases. In a 16x9 aspect ration as in HDTV, 1080i = 1920x1080 = 60 phases per second each of which are 1920x540. By contrast 540p would be 960x540 = 60 frames per second each of which are 960x540. And one should also keep in mind that on a real 1080i display, the lines alternate from phase to phase, so it can also be looked at as 30 frames of 1920x1080, which 540p cannot.

I think the reason for the misconception is because 1) the vertical syncing signal is the same for 1080i as it would be for 540p 2) people are overly fixated on the number of scan lines. The second is probably largely historical. In NTSC and PAL, the number of scan lines (horizontal lines) is measurable, but each scan line was an analog signal and as such the number of pixels in it is really just sort of a guess. Even with a digital signal on the other end, it's not likely that all of the pixels from the other end make it through an NTSC broadcast. Particular TVs divide the scan lines into pixels by using a mask, either a set of vertical bars with gaps between them or more of a lattice as in trinitron TVs. But there's no guarantee that the number of horizontal pixels on an NTSC or PAL TV is the same as any other one. I've also seen several people say that NTSC is 720x480. The number of horizontal pixels, although fuzzy, is definitely not as high as that, that's just what DVDs use, largely because they want to be able to support widescreen.

So, I think that some of the people who say that 540p = 1080i are just fixated on number of scan lines per second. Others are 720p advocates who are willing to say anything which puts down 1080i without worrying about the accuracy of their statements. 720p and 1080i are both good formats with their own pluses and minuses. There's no point in arguing over which one is best, but that hasn't stopped some people.

So, to do the 10i vs 5p analogy, the difference is this (assuming a 16x10 aspect ratio to make things work out well):

5p frame


10i frame


Keith

I think you have assumed that we're talking about 960x540p -- we're not. We're contrasting 1920x540p vs. 1920x1080i. The horizontal resolution does not change, only the interpretation of the vertical information.

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I don't know I've never seen a 1920x540p modeline anywhere. all the 540p modelines I've ever seen are 960x540, and that would make sense cause other wise you are skewing your aspect ratio and I would think that would reall screw with the picture.

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-You- can talk about 1920x540p with it's 32:9 aspect ratio, but I don't know that anyone else is. And usually when people say "1080i is just 540p" they are not talking about how the vertical sync signals for the two look the same, they're trying to imply that the resolution of 1080i is less than 720p. If we were to take a poll, do you think that if we asked people what the horizontal resolution of 540p is, that most of them would guess 1920 or something vaguely near it?

Keith

hey I like watching at 32:9 makes me feel like I'm thiner than all the starts.

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Look, Keith, I'm not trying to be argumentative. The posts I've seen on Gossamer Threads have been about 1920x540p modelines. The scenario is, you send a 540p signal at 60 frames/sec to your HDTV instead of a 1080i signal at 30 frames/sec. Don't take my word for it. Search for yourself. You will find posts like:

http://gossamer-threads.com/lists/mythtv/users/110196
http://gossamer-threads.com/lists/mythtv/users/60902
http://gossamer-threads.com/lists/mythtv/users/75394

Personally, I'm not keen on the idea. I think it trades off vertical resolution, i.e. the 1080i picture actually carries more information. But some disagree with me and claim no loss of vertical resolution. The only way I can see this being true is if the two successive frames of 540p are actually different, and are interlaced by the TV set to form a picture with the full resolution. But I think the XV driver will scale the 1080i picture into 540 lines when told to output to a 540p modeline.

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True, but if you use bob deinterlacing for 1080i content, the 540 lines for a field would not need to be scaled and it should look sweet. If you are not using bob it would screw things up as you say. I was at a friends house last night looking at his hdtv and trying to tell the difference between his 720p and 1080i modeline watching 1080i content (1440x1080i in this case) He preferred the 720p and I preferred the 1080i. To me the 720p looked a bit softer, presumably because of the scaling.