Thanks for this. It is visible on the rushes so not related to any setting in Premiere. I suspect it is related to the refresh rate of the projector and shutter speed of the camera. The problem was that whatever shutter speed I set on the camera I could not get rid of the vertical roll. I also wonder if it is related to the projector type (DLP/LCD).
I am not sure and have not seen this before.
If you see the lines over the whole image, and not just on the projection screen, then they are not likely caused by a mismatch of scan rates. Something else is going on with your camera.
Jim might be onto something here.
What is the light source in that lecture hall?
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The question is, was the "dark" room lit by something other than the glow from the projection screen?
If it was, your camera frame rate was mismatched to the lighting frequency (e.g. 60Hz mains lighting, 25fps footage - old mag-ballasted fluorescent lighting is worst as it flashes at mains frequency with a wide amplitude, but compact electronic-ballasted fluorescents don't bother DSLRs as they flash at over 10kHz).
If the room lights were off, you're seeing the same effect but due to mismatch between the projector scan rate and the camera frame rate.
There's nothing you can do about it in post, you have to get it right during the shoot.
Thanks for all the responses.
There was daylight from the right side of the room but the lights were down.
I think that as suggested the problem was a mismatch between the projector and camera. Unfortunately whatever shutter speed I used on the DSLR the problem was evident (another "I don't believe it!" moment) . I wonder if the projector was a DLP unit and the problem was the combination of wheel rotation on the projector and shutter speed on the camera. I could adjust the camcorder's shutter speed far more precisely than the DSLR's, so I am glad not to have had to rely on the DSLR as the only camera.
I was hoping that there might be a way to fix short sequences as cutaways, but maybe not.
I think that as suggested the problem was a mismatch between the projector and camera.
If that were so, then the lines would only appear on the screen, not throughout the entire frame. If the lines appear across the whole frame, it's not a scan mismatch. (Assuming, of course, that the screen does not fill the entire frame, that there is other content besides the screen.)