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Old July 2nd, 2003, 06:48 PM   #1
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vx-2000 "strobey" motion video

Hi,

I'm shooting martial arts videos with people swinging sticks. A friend in the video editing business commented that my DVD
footage (MPEG2, 8000 kbs) footage looks "strobey" and
asked if I'm shooting at 30 fps. Since then, I've noticed
that all of my video looks "strobey", even when played directly
from the camcorder into my interlaced TV.

I use a vx-2000, a tripod but with steadyshot=on, and with Autolock selector in the Manual (middle, autolock release) position.
The menu tells
me that I'm not in progressive scan mode. I'm not using
the sports AE mode, although I've switch that on in the past.
Lately, I've been decreasing the shutter speed to 1/60
to 1/180 and playing with the exposure. In the past, I was using
a slower shutter speed which blurred the sticks instead of
strobing them. The strobing is really obvious, almost as if
the frame rate is actually 10 Hz or something.

I've checked just about every setting on the camcorder and
am clueless as to why this happens. Any ideas? And no,
chuckle, I don't have the unit set to record to the Memory stick.
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Old July 7th, 2003, 09:47 PM   #2
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Hi Gints,

I don't think this'll be much help, since I was going to suggest checking that you're not on progressive scan or Golf Dude mode. Those will give you a high strobe effect, which may not be that bad for your martial arts purposes. Think of "Saving Private Ryan"'s opening scene and "Gladiator"'s opening fight scene. Very graphic, very immediate.

Did think of something though. Play back your footage that seems strobey, and press Data Code on your camera twice. It will show the mode you shot in, and the aperture, I think. That might give some insight.

If all else fails, ask your actors to move slower!

Good luck.
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Old July 8th, 2003, 03:25 AM   #3
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Nicky,

Thanks for the Data Code tip. I'll check that out.

As for asking my "actors" to slow down, they're really fighting !
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Old July 8th, 2003, 06:30 AM   #4
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It could be one of two things. If you're filming under fluorescent 'tube' light then that can set up a flicker because there's no element that continues to glow on the AC switchover from neg to pos. Some lamps look like tungsten filament lamps these days but are in fact fluorescents coiled up to make them look more presentable. As this switchover to fluorescents becomes more widespread we video people who use the available light must:

*keep to the default shutter speed!* In your case this means using 1/60th sec. Any higher than this and you risk taking (under exposed) frames out of sync with the 'on' cycle of the fluorescent tubes, and this flicker is often seen as 'strobing'.

If it really *is* strobing then it's only that the shutter speed is too high and you're not capturing all the action. Huh? Yes, if you go faster than 1/60th you only record bits of the action. At 1/60th you record it all.

tom.
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Old July 8th, 2003, 04:11 PM   #5
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Thanks for the advice, Tom. Yes, I'm taping under banks of flourescent lights. I've been recording at 1/120 and higher. Since the motion of swinging sticks is fast, all of my frame grabs and slow motion review yielded blurred stick arcs. So, I'm not sure I want to record all of the action. I'll try various shutter speeds.
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Old July 8th, 2003, 10:46 PM   #6
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Comedy Fighting

Yeah, I suppose asking your actors to slow down would have an unintended comedy effect.

Hey, there's a premise for a movie.

Good luck with that.
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Old July 15th, 2003, 07:18 PM   #7
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http://home.att.net/~gints/download2.htm

I'm still trying to chase down some strobing issues.
The picture above was snapped with a Canon S40
digital camera at a shutter speed of 1/60 , manual
mode for everything. The lighting was "industrial"
8 ft flourescent tubes. Note that both stick fighters
are holding three sticks (chuckle, really, only one).
Is there something about
CCDs that would cause this motion strobing on
still pictures ?
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Old July 16th, 2003, 11:22 AM   #8
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The Canon is a still camera, right?

What you see COULD be caused by the Canon really taking multiple exposures and averaging to create a more noise-free picture. This is just a guess.

There is no way that a single read of a CCD can generate 3 separate images. It would just blur a single image of the stick.

I cannot imagine the 120 Hz flicker from the lights causing this unless the sticks are moving slower than I imagine.
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Old July 16th, 2003, 02:09 PM   #9
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Yes, I'm using the Canon S40 as a "still" camera.
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Old August 1st, 2003, 04:05 PM   #10
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Here's the link to the strobey stix picture:

http://home.att.net/~gints/download2.htm

After talking to some video hardware people, apparently
the issue is simply scanning (analog to digital conversion) the CCD array. Since the cells can't all be scanned at once, they
are scanned in multiple passes using various scan algorithms.
During the Scan, the CCD elements decay. So, a decay correction curve is applied. Of course, this correction is only an approximation. So, apparently, my photo shows a problem with fast moving objects with a reasonable contrast.
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