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Problem videotaping DLP screen
I got a big surprise the other day when I was taping some footage for a program in a room where video projection was happening. Haven’t had many problems in this regard lately, other than the usual balancing of light levels. This day, I experienced a wild flicker on my viewfinder, coming from the powerpoint projection. Even without including the projector screen in the video frame, just the light emanating from the screen and falling on the scene I was shooting was enough to cause the scene to flicker objectionably. No setting on the variable shutter speed would alleviate the problem.
I later found out that the client had just upgraded their projectors from LCD to DLP models. I assume that the color wheel of the DLP projector caused this artifact.
Has anyone else experienced this? Anybody know for sure what was happening here? Anybody figured out a solution?
Thanks!
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3 Replies
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Dave,
Thanks for your thoughts. It’s definitely something similar to the familiar refresh-related issue, only different. As I noted in my original post, no setting of the variable shutter in my camera would elminate it. I’m guessing that since the color-sequential “scanning” of a single-chip DLP projector is very much a different animal that the scanning of a CRT monitor (more like the short-lived CBS field-sequential color system than today’s NTSC dot sequential system), the ClearScan-type adjustments simply weren’t designed to deal with it. We’ve all been spoiled with the near-ubiquity of no-scan LCD panels and projectors which have pretty much relegated the old refresh incompatiblities issue to history. This particular situation was a surprise to me!
I believe that these scanning wheels operate at the nominal 60 hz video rate (or multiples therof), and that might explain why no setting on the variable shutter improved the problem beyond the picture obtained with the “stock” setting. But something unpleasant was happening that I haven’t been able to figure out yet.
Still wondering if anyone else has noticed this effect and possibly figured a workaround?
Thanks!
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Dave,
Depending on what camera you where using (IT chip versus FIT) you may have not had the range of shutter rates to deal with the DLP refresh rate.
The frame rate your camera was set to also affects the shutter range.FYI- IT CCD’s shutter rates can go lower than FIT CCD’s.
However there are some refresh rates that are in between the camera shutter rates which make it impossible to completely dial out any flicker.
What shutter rate did you find came close to dealing with the flicker?
What frame rate where you shooting at?
Another solution is to have the video projectionist change the refresh rate to you 60HZ if you are shooting 60i.
Tony Salgado
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After I read your post, I fired up my DLP (InFocus) video projector and shot its image with a DV camcorder… PERFECT!
Now, I was feeding it with an NTSC DVD player (NTSC camcorder as well).
If you feed a CRT monitor with a COMPUTER, and shoot that CRT, you’ll get the customary “roll-bars/beat-bars/sync-bars” that we all know and love.
You said that the DLP projector at your shoot was being fed by a PowerPoint show (yuck for that, too) so I assume that what you were getting was the same “Computer Flicker” that you’d get with a CRT monitor (just BIGGER and BRIGHTER).
As mentioned above, a camera with a “computer shutter” would probably fix this.
I don’t see why the DLP would be a “unique scan rate” other than what the computer is sending it, if it does not create any special “shutter problems” when projecting NTSC images.
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