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projection flicker

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Stewart Bourke
projection flicker
on May 27, 2011 at 11:31:54 pm

Forgive me if this is off-topic, but not sure where to start.

I recently did a three-screen/projector show using vegas-rendered footage playing on qlab into a matrox 3-head. The images projected were a mixture of live footage and still images. Everything worked fine in the show, but when I saw the DVD of the show I noticed flicker on all the screens regardless of whether I was displaying still images or video. The still images were png files displayed directly by qlab and the videos were all .mp4 rendered in vegas in progressive mode and displayed in qlab.

The projectors are standard commercial projectors.

The video for the DVD was shot in SD, standard PAL DV on a Sony DCR-HC1000 - old by now, but still an OK camera for amateur use...

Would anybody have any suggestions as to what could cause this flicker? Could it be something to do with the camera picking up on the 50Hz mains flicker (I am in Ireland) and not being in sync with the projectors?

Any help/pointers appreciated.

Thank you.


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Thomas Leong
Re: projection flicker
on May 28, 2011 at 6:27:48 pm

My 2 cents is I think the flicker is caused by using a high shutter rate at the video camera end that does not quite 'match' the progressive scan rate of the projectors. Slowing down the shutter rate usually solves the problem but it is very much a trial and error thing, i.e. you need to visually check the video cam's output to see the result as you change the shutter rate-and-aperture (or gain) to find a balanced result.



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Stewart Bourke
Re: projection flicker
on May 29, 2011 at 10:44:43 am

Thomas,

Thanks for the info - I will look into setting the camera appropriately, but will also maybe ask on the Vegas forum if there are any specific render settings I should be using in conjunction with this.


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Walter Soyka
Re: projection flicker
on May 31, 2011 at 1:25:53 pm

I'm with Thomas -- flickering like that happens when your cameras and your projection are not running on the same refresh rate. This is the same thing that happens when shooting computer monitors. Most professional video cameras have a "clear scan" feature that lets you dial in very precise shutter speeds. You adjust this until the on-screen flicker disappears.

There is nothing you can do on the video production side in Vegas -- this is a physical issue involving the imaging elements on the projector and the camera.

Walter Soyka
Principal & Designer at Keen Live
Motion Graphics, Widescreen Events, Presentation Design, and Consulting
RenderBreak Blog - What I'm thinking when my workstation's thinking
Creative Cow Forum Host: Live & Stage Events


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Tom Sefton
Re: projection flicker
on Jun 14, 2011 at 1:38:59 pm

Hi Stewart,

We had a recent show where we experienced this problem. We found it came from earth hum. After removing the earth on our supply feed the problem vanished.

Best
Tom


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Stewart Bourke
Re: projection flicker
on Jun 14, 2011 at 1:41:47 pm

Tom,

Very interesting - I'll talk to the electricians about this.. When you say you removed the earth from the supply feed where exactly do you mean? I assume the mains side is still earthed?

Thanks,

Stewart


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Tom Sefton
Re: projection flicker
on Jun 14, 2011 at 1:47:25 pm

We had a single phase supply which went to various 13amp 4 ways. When we removed the earth from all of the 4 way supplies we found that it dropped the earth hum and the flickering.


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Stewart Bourke
Re: projection flicker
on Jun 15, 2011 at 1:32:57 pm

Tom,

Does this mean that all your equipment had no earth connection, or was it earthed at one end only? I spoke to the electrician and he raised the question of using unearthed equipment...

Thanks,


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Walter Soyka
Re: projection flicker
on Jun 15, 2011 at 2:47:58 pm

Ground loops look like rolling bars over an entire display. Are you saying that projection looked fine in the room, and video of content other than the projection screens looked fine on tape, but that the projection screens seemed to flicker on tape -- and this was resolved by lifting ground?

Walter Soyka
Principal & Designer at Keen Live
Motion Graphics, Widescreen Events, Presentation Design, and Consulting
RenderBreak Blog - What I'm thinking when my workstation's thinking
Creative Cow Forum Host: Live & Stage Events


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Tom Sefton
Re: projection flicker
on Jun 17, 2011 at 2:51:28 pm

No, all the equipment; PC's, vision switchers, cameras, DVD players, sound system and lights was drawn from one single phase supply. The projection had earth noise on all inputs. When we lifted the earth from the feed problem was solved.


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Walter Soyka
Re: projection flicker
on Jun 17, 2011 at 3:00:30 pm

[Tom Sefton] "No, all the equipment; PC's, vision switchers, cameras, DVD players, sound system and lights was drawn from one single phase supply. The projection had earth noise on all inputs. When we lifted the earth from the feed problem was solved."

Ok. Unless I'm reading something wrong, then, this sounds like a different problem than the one Stewart is experiencing -- since all his video appears fine to the naked eye in the room (I think).

My guess is still on the shutter speed of the cameras. I'm really surprised that no one noticed or said anything while they were shooting. This should have been obvious in the viewfinders or record monitors.

Walter Soyka
Principal & Designer at Keen Live
Motion Graphics, Widescreen Events, Presentation Design, and Consulting
RenderBreak Blog - What I'm thinking when my workstation's thinking
Creative Cow Forum Host: Live & Stage Events


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Stewart Bourke
Re: projection flicker
on Jun 17, 2011 at 3:31:43 pm

Yes, it is a different one, and from what I have been reading here and elsewhere it certainly seems like shutter-sync issues.

I was not on the cameras that night - I was manning qlab for the projectors - and there was no flicker on the screens - I would have noticed flicker(I hope!!)

I will need to start looking at what needs to be done here, as we will be using multiple projectors more and more.

Thanks all for the very useful input - very much appreciated.


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