How to prevent rolling horizontal bands

S

sabbott

Member
Hi there. Apologies if this has been covered 50 times, but couldn't find it here.

Last week I filmed two evenings of presentations. Both involved speakers standing near a screen where images were being projected.

In the first shoot I had bad rolling horizontal bands throughout my footage (these only appeared when the projector was turned on).

In the second shoot there was no problem at all - everything looked fine.

I really need to know how to prevent the rolling band effect and I would like to know:

a) Exactly what causes it (so I can ask venue tech people ahead of time what their set up is and know what to expect)
b) How to prevent it during filming (Change frame rate if I notice it occurring? if so, to what?)

Also, regarding damage control for this issue, I have downloaded and used FlickerFree, which seems to deliver outstanding results, but which slows down my editing process so much as to make it unusable (on trying to render a 7 minute video to which I had applied FlickerFree after 3 hours only 2 percent had rendered!). Is this normal?

HELP! Thanks if you can!

If it's helpful, I was shooting on an AF100. 25p. Shutter speed 50.
 
The banding is caused by the refresh rate of the projector being different than the scan rate or the shutter speed of your camera. (Try pointing you camera at a TV set and upping the shutter speed - you'll see a variety of speed and patterns going on).

Many professional "video" cameras (vs DSLRs) have a control to adjust the shutter speed until the effect goes away - you just eyeball it. Panasonics have had that as far back as the original DVX (I think it was called "synchro scan"??) so check if your camera does, deep in the menus.

If you're in the US, try shooting at 1/60th which aligns with the electrical mains frequency of 60 hz (and use 50 in Europe).

I doubt there are many controls in a projector to address this, since the projector has to be setup to accept the signal it's getting.

As far as rendering, not familiar with that software. The few times I've done these kind of edits, I try to get a copy of the powerpoint/etc and cut away to it so it's actually readable on screen. If the screen is unobstructed by the speaker, you could potentially corner-pin the presentation into it (if you can get it). Yes, that sounds crazy...

You might see if you can find someone with a powerful After Effects setup who could run a deflicker plugin and render the footage out for you?
 
Hi Michael,
Thanks for that. I did try adjusting the shutter speed on the night of the event (both up and down from 50, which is my default because I'm in Australia) but the problem was still easily visible at the different rates I tried.

It seems you're right about my camera having a synchro scan option - I will experiment and make it my new best friend if I can get it working!

Cheers.
 
All the available shutter speed in the normal mode might have the problem of not being in sync, that's why a Synchronization Scan feature is there, it adjusts shutter speed in extremely fine steps as low as 1/1000 to find an exact spot where it matches and flicker disappears.

On Canon DSLRs Magic lantern offers very fine increments in shutter speed control so whenever I used to see flicker I would easily remove it by changing the shutter and eyeballing it.

If it's still not fixed in Synchro Scan, try changing the frame rate slightly and running the process again.

That's about all you can do except for getting a hammer and cracking the aweful light source/projector!
 
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