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Posted by ~P~ on 04/12/06 02:27
Alright, let's begin with material you likely already know...
Movie theaters for years have been showing movies that are shot at 24fps at
48fps because they have had issues with older renditions flickering
obscenely due to the shutter inside the projector. This being an analog
format with individual frames, the shutter had to close, then open, then
close and open again. At 24 times a second, there was not enough image
retention for our eyes to see it as one constant smooth image.
So, they began doubling the amount of time the video was on screen by
showing 2 identical frames back to back. This did not increase the number
of frames per second, so you still see 24 unique frames a second, yet each
frame is shown twice for a little under 1/48th of a second per frame. You
still have a shutter that closes up momentarily between frames.
CRT works with scan lines. At rates below 50 (PAL) or so scan lines a
second, the image begins to flicker pretty bad. This is because the CRT
literally draws the image from top to bottom, one line at a time and then
repeats. The reason your eye sees a solid image is because the phosphors in
a CRT display glow briefly after the CRT passes over it. Faster scan rates
can result in brighter CRT displays. But 60hz (59.97) is the US standard
for video to comply with our electrical system and to produce a decent image
with a CRT.
Now, for a new concept that doesn't apply to the above techologies...
Now, enter the world of digital displays. Digital displays don't operate
the same way as projectors or CRT displays. LCD, plasma, and LCoS displays
have a 'always on' image. When you send 24 frames per second to the display
the display NEVER blanks. It shows one frame for exactly 1/24th of a
second, then immediately moves to the second frame. It does this much
faster and better than the shutter of a movie projector. So, while some say
that you would get flicker, professionals all agree that with digital
displays it is not the case and the only flicker you would have is in
regards to the original video only being shot at 24fps. 1/24th of a
second - exactly. No blanking, no flicker.
You noticed DLP wasn't on that list.
Well, DLP doesn't have a refresh rate - but it operates in the hundreds of
hz for refresh. The MMD inside a DLP projector has mirrors that flicker on
and off to achieve the various shades of red, green, and blue. This
flickering of on/off must happen hundreds of times a second, dozens of times
a frame, plus dealing with a color wheel often spinning with 5x or 6x
rotation speed (5 or 6 groupings of color per frame). This puts DLP refresh
rates way beyond any other technology that has any refresh/shutter/scan
lines.
But, LCoS, LCD, and plasma do not have to deal with blanking at all. Their
digital nature allows one-by-one-by-one frame display. In simple terms, if
you feed a LCD or plasma a 72hz signal comprised of 24fps material. Each
frame is repeated 3 times in a row.
Frame 1: Shown 1/72 of a second
Frame 1 (repeat): Shown 1/72 of a second (plasma does not change)
Frame 1 (repeat): Shown 1/72 of a second (plasma does not change)
Frame 2: Shown 1/72 of a second (plasma image changes to new frame image on
leading edge of frame change)
Frame 2 (repeat): Shown 1/72 of a second (plasma does not change)
Frame 2 (repeat): Shown 1/72 of a second (plasma does not change)
Well, there are two frames shown with a total time on screen for each frame
of 1/24th of a second.
Now, lets look at what happens when we feed 24hz for 24fps material...
Frame 1: Shown 1/24 of a second
Frame 2: Shown 1/24 of a second (image changes on leading edge of frame 2)
Frame 3: etc., etc.
etc.
Hopefully you get the idea that example one above produces the same results
as example 2 below.
Now, throw into the mix that 24hz with digital displays are easier to deal
with than 60hz displays and can lead to faster leading edge changes, more
robust bit depth in colors, and easier/better processing of the incoming
video signal, and you begin to realize that for movies, shot at 24fps,
1080/24p is truly the current holy grail of home theater.
Worthwhile on a 50" plasma at 15 feet? No, not really. But, on a Sony Ruby
and a ten foot screen? Or, the new Optoma, Runco, ProjectionDesign, or Sim2
projectors? This is Blu-rays market. That is exactly who they are going
after with their first generation of products. Upper tier customers who can
make the most of the technology.
~P~
"Jeff Rife" <wevsr@nabs.net> wrote in message
news:MPG.1ea622edb17655c498a45b@news.nabs.net...
~P~ (bmxtrix2005@cox.net) wrote in alt.tv.tech.hdtv:
> Jeff - Flicker is a result of CRT technology, not of digital display
> technology. 24 frames per second could be shown a 72hz, but it won't help
> things at all.
Look, will you *please* stop posting until you do just a little bit of
research. Every theater today uses technology that shows each frame
twice which raises the effective rate to 48fps and solves the flicker
problem. Doing the same sort of thing with *any* display technology will
have the same result.
> 60i HURTS things becuase of 3:2 issues with 24fps not
> playing nicely at 60fps which results in poor video playback.
There are issues similar to 3:2 pulldown with 24fps shown at 60fps as
well, yet *every* single progressive scan DVD player outputs 60fps. Try
and do some research and figure out why.
As for 60i, you can get perfect playback of 24fps material if you can
do the conversion inside the player.
> 24fps... it's what you see in theaters (typically)
No, it is *never* viewed in today's theaters. Do some freaking research.
> Movies SHOT at 24fps should be displayed no other way, and poor
> information
> about flicker - which only pertains to CRT - is not accurate. I will
> explain that further if you would like me to.
Well, if everybody else here agrees that the need a good laugh, too, then
I guess you could go ahead and "explain".
--
Jeff Rife |
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