[Open-graphics] Interlaced vs. progressive scan HDTV

[James Richard Tyrer]

Dieter wrote:
>> It appears that many HDTV sets display 1080p which they generate 
>> internally (those with so-called 3:2 pull down have to do it) and all 
>> flat panels are progressive (although most of them are currently 720 
>> lines).  Strange thing about 3:2 is that they don't display it at 48 fps 
>> as theaters do (24 fps movie projectors have a two blade shutter to 
>> avoid flicker)
> 
> If you display a 24 fps movie on a LCD, how do you avoid flicker?
> Once a pixel settles into a new value, it just stays there constantly,
> unlike a CRT where the phospher is constantly fading and being refreshed.
> LCDs aren't fast enough (yet?) to simulate the double shutter they do
> with film.

An LCD is on all the time so there is no flicker just the slight delay 
when they are refreshed.  The issue is to avoid the artifacts from 
interlaced conversion of 24 fps film.

IIUC, an LCD TV is progressive scan and refreshed at 60 fps.  So you 
show one frame three times, the next one two times, the next one three 
times, etc. and this adds up to 60 fps with a slight amount of stutter. 
  The easiest way to do this would seem to be exchange buffering.

The question that I have is how an LCD is updated.  Does it update with 
a (progressive) scan like a CRT or is it double buffered and updates the 
whole screen at the vertical sync pulse?  This is going to make some 
difference in how a movie looks but there isn't anything that the 
graphics board can do about it.

LCD computer monitors would present different frame rate issues.  Many 
of them are only 60 fps and would be treated just like TV, but there are 
some that support higher vertical refresh rates.  I would suggest that 
we support 72 fps since this is 3 times the movie frame rate which would 
display movies a little better than 60 fps since there would be no 
stutter (each progressive frame shown 3 times).

This would also be good for CRTs and projection TVs.  When you are 
showing a small image on your computer monitor it doesn't make much 
difference, but if somebody hooks up a HD DVD to their 42 inch (or 
larger) 1080 HD set, we should be able to optimize this for the best 
viewing.

With a computer monitor (that supported higher refresh rates), it would 
be best for the user to select either 60 fps or 72 fps depending on 
whether they were watching 24 fps filmed movies or 30 fps video media 
(or the original print of Oklahoma).

-- 
JRT