From: dpbsmith@world.std.com (Daniel P. B. Smith)
Subject: "Strobing" in Triplecast gymnastics
Summary: I think it's just high shutter speed 
Organization: The World Public Access UNIX, Brookline, MA
Date: Fri, 31 Jul 1992 12:07:38 GMT

I've noticed the effect others have called "strobing" or "frames
missing" in the gymnastics coverage, but it looks to me just like
the effect of a high shutter speed. I certainly have the same problems
when I do taping with my l'il old 8mm Sony HandyCam.  If I type fast
action outdoors and set the shutter speed to "1/1000" it looks not
unlike what I see in the gymnastics coverage.  

There's a funny tradeoff here: a low shutter speed looks blurred, but
the motion looks smoother and more natural. A high shutter speed sharpens
the individual frames, but the motion looks jerky and you visually
perceive the individual frame images.  If you think about it, a dismount
off the balance beam can't take more that, what would you say, half a
second? During which time they're doing two or three complete rotations
in how many axes? Obviously fifteen images isn't going to be a very
complete representation of the motion.

It just goes to show that after we get HDTV straightened out, the next
big bandwidth-gobbler oughta be increasing the TEMPORAL resolution.  If
you've ever run a Kodak Super 8 (FILM) projector at 54 fps it's 
impressive how smooth and "real" the motion looks.  Probably some readers
have been fortunate enough to see ShowScan or video running at significantly
higher frame rates.  As you'd expect, frame rates are expensive and ALL
film/video systems tends to reach an economic compromise at the lowest,
jerkiest frame rate people can tolerate.

I remember how thrilling it was the first time I saw skilled jugglers
juggling Indian clubs "live." I'd seen it a million times on TV, but
I hadn't realized that the whole visual appeal of the performance 
is completely lost at television frame rates.  I'll bet there's a big
element of this in gymnastics, too.  I'll bet judges are watching for,
and athletes are FEELING, temporal events with a resolution that's
significantly shorter than 1/30 of a second (BTW don't flame me on
fields versus frames, please).

Certainly film editors claim that a ONE-frame difference in where a cut
is made makes a difference in pace and timing that is meaningful.

Well, anyway, I think it's just a high shutter speed.  It's just barely
possible that the same camera is feeding high-speed video to the slow-
motion equipment and every Nth frame is being extracted to produce the
normal speed signal, I guess. That would mean half as many cameras, and
perfect matches in point of view between normal speed and slow motion
replays.  IF this were true a) the high shutter speed would be a necessity
for the slow-motion, rather than a tradeoff judgement on blur versus
smoothness for the normal speed coverage.  More to the point, b) thinking
about fields versus frames... hmmm... when you use a high shutter speed
does it "open" the shutter every 60th of a second or every 30th?  Anyway,
it's just barely possible that high shutter speeds and/or sampled high
speed video might give you a jerkier image than you'd normally get, when
the second interlaced field is both TEMPORALLY and spatially intermediate
between the first ones.

--Daniel P. B. Smith
dpbsmith@world.std.com

Newsgroups: rec.video