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Re: [Phys-l] 240 and 600 Hz



Interpolation will not get rid of the wagon wheel effect. It will only make
it clearer. The interpolated frames will still show them going backwards
because interpolation can not tell which direction it should be
interpolating. There is no directional information in the picture of the
wheels, unless you have very sophisticated software that can parse the
picture the way our brain does. At present I don't think that is possible.

A real world test is available at:
http://reviews.cnet.com/240hz-lcd-tvs-what-you-need-to-know/

Aliasing definition: The appearance of jagged distortions in curves and
diagonal lines in computer graphics because the resolution is limited or
diminished.

So aliasing depends on the resolution and not the frame rate. It is most
notably reduced by having interplated pixels which give the appearance of
smooth lines.

The higher frame rate with interpolation should reduce "judder" or the
herky-jerky appearance of some videos and films. On the other hand one
might want the "film" experience which needs to have the material shown at
exactly 24Hz rate. Actually just having 120Hz available allows accurate
reproduction of 24Hz movies which is a large decrease in judder.

Some viewers have complained that films shown on a 240Hz TV often look like
TV rather than film. Some film producers wanted to have the grain and
jerkiness which can be characteristic of film, so the 240Hz screens may
defeat that. The highest frame rate in HD is designed to reproduce at 24Hz
and is supposed to be for actual movies. So the 240Hz would just be the
refresh rate and not the interplated frame rate. Supposedly there should
not be any interpolation when showing a film. But there should be an
advantage for some things like fast moving real-time sports.

The reviewers in the reference above did not seem to think that 240 was
significantly better than 120Hz. Part of this may be that motion blurring
is actually already in the individual frames, so interpolation just creates
interpolated blurring. There is also the problem of mixed telecined films
where instead of true 3-2 pulldown, they mixed frames so you have an
original frame plus a ghost image of the other one. Telecining is where
they take a 24Hz movie and make it into a 25 or 30Hz TV picture using
appropriate pull-down.

How on earth can digital filters run above the audible range? The
processing certainly has to be faster than the bitrate, but the results must
be in the audible range. These are mixed concepts. Actually the earliest
Dolby decoders were analog switches. The current system is a digital
version of the older analog system. In the case of audio there is no
interpolation above the audio frequencies. Certainly the processor always
runs at a higher rate than the real time information is presented.

At one time there was a proposal to double the frame rate of regular movies.
Apparently this gave the appearance of smoother motion and better
resolution, but the theaters would not go along with it. Many of the big
enhancements came when the film companies owned their own chains. This
meant they could unilaterally put in enhancements as a competitive advantage
for their films. But once this was severed, the theaters had to
independently decide what they would tolerate. Actually the double frame
rate would have been a change in the gearing of the existing projectors,
which was very possible to do without huge expense. And it would probably
have been cheaper than the 70mm films which ate up huge amounts of expensive
film stock.

John M. Clement
Houston, TX



There actually are advantages to the higher refresh rate.

The issue is aliasing. A familiar example is the "wagon wheel
effect".

It could be argued that the aliasing depends on the frame rate
not the refresh rate, and that's partly true, but that's not
the whole story, for a couple of reasons:

-- If the screen is used as a computer monitor (rather than as
a TV), the frame rate could be very much higher than 60 Hz,
whereupon the wagon wheel effect in your computer simulation
is much less of a problem.

-- Even when you have a movie coming across the wire with a
60 Hz frame rate, there is actually a lot of interpolation
going on in the MPEG decoder. You're better off doing the
interpolation using a digital filter running well above
frame rate. That is to say, given a 60 Hz frame rate and
a 600 Hz refresh rate, you are not obliged to paint the
same scene 10 times; you can interpolate.

This is analogous to the fact that your audio system does
a lot of filtering (Dolby etc.) using digital filters that
run well above audio frequencies.

The idea is that even though there are some irreducible problems
associated with the frame rate, but you don't want the digital
filter rate to add to your problems.