Re: Why interlaced HDTV?

In article <folkg1li6ka4eg139e322nmmc50299m29r@xxxxxxx>, JC <nospam@xxxxxxxxxxx> writes 
On Mon, 22 Aug 2005 09:09:34 +0100, Kennedy McEwen
<rkm@xxxxxxxxxxxxxxxxxx> wrote:

But it is infinitely easier to display interlaced video correctly, and
without a frame store, on a progressive screen than it is to display
progressive video correctly on an interlaced screed.


All current progressive screens are by their nature a frame store. By
displaying interlaced material you are effectively displaying two
fields shot at different times simultaneously. This has the effect of
degrading the apparent resolution of the system.

That is indeed how most of them work, but it doesn't have to be that way. A progressive display can be configured to display interlaced material perfectly correctly.

I have actually done something similar to this on a progressive OLED display where I had access to the raw panel drive, and I can confirm that it does completely eliminate the motion artefacts that you get when doing the conventional frame store reconstruction that most progressive displays use. Actually, what I did was more like the scheme described below, because of the way the OLED interface worked.


This is not however how LCD and Plasma screens work and to try and
replicate this would create all the traditional TV flicker that these
screens are so good at eliminating

You don't get owt for nowt. The price of that flicker elimination has to come from somewhere, and the conventional wisdom of the manufacturers appears to be that it comes at the expense of motion tearing through complete reconstruction of interlaced fields into a single frame for progressive display. That isn't the only possible trade-off. As described, it is possible to design a progressive display that will eliminate motion tearing with interlaced video. That particular trade-off results in 50Hz flicker, but no worse than on a conventional interlaced CRT.

However there is a third trade-off which maintains the flicker elimination of these screens. Instead of blanking the lines which are not present in each field, just repeat the data from the previous field. Again, the latency is consistent on all pixels, however the hold time on each pixel is now doubled, which is the analogous to a very long persistence CRT phosphor. Consequently this time you are trading some motion blur to eliminate both flicker and motion tearing.

You don't get owt for nowt, but the trade space available is multidimensional - the manufacturers just restrict the options available to you. I am sure that there are other options, but these are a couple that I have played with which I know work.

while introducing other artifacts.

What other artefacts are introduced? You eliminate motion artefacts by achieving consistent latency and the flicker is no worse than on a conventional interlaced CRT (its actually a little better because they don't have the emission decay that a CRT phosphor has). Based on the OLED system I did this on, I doubt if you would see any flicker at all, but you can fix that as described. 

You can't do that the other way round, if the display is interlaced by
default, without destroying the latency between adjacent lines.

It's always a compromise. 

It *is* a compromise - it doesn't *have* to be one.

You can display interlaced images on a progressive screen correctly, and I suspect that the reason it isn't done has more to do with manufacturers agendas than a limitation of interlace per se. You can't display progressive images on an interlaced screen though, without artefacts or throwing half the information away, which defeats any advantage the system has.

I was looking at some 720p a couple of days ago, and wasn't overly impressed. Yes, better than 625i, but not dramatically so. When I saw 1080i a while ago, I was completely bowled over by it. That extra horizontal resolution isn't worthless, you know. Equipment was different of course, and perhaps my expectations have changed with time, but that was my impression viewing both options. As always, YMMV.
--
Kennedy
Yes, Socrates himself is particularly missed;
A lovely little thinker, but a bugger when he's pissed.
Python Philosophers (replace 'nospam' with 'kennedym' when replying)
.

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