Flicker from lights powered by generator

johnmeyer wrote on 10/12/2004, 10:13 AM

I just filmed a football game played on a field lit with portable lights powered by diesel generators. Each bank of lights had its own generator. In person, everything looked great, but the video has a 3 Hz flicker that is quite strong at times. Three times every second the video gets bright and then gets dark. This is especially bad at one end of the field, perhaps because the problem was with one generator.

I have tried Donald Graft's "Deflicker" plugin-in for VirtualDub and a derivative program for AVISynth, but they don't touch the problem at all. I've got to finish off the project today, but if anyone has a quick idea of a tool that could smooth out these luma variations, I am all ears.

Thanks!

Comments

RalphM wrote on 10/12/2004, 10:21 AM

It's surprising that VDubs filter did not have an effect. I assume you played with the settings.......

At the risk of showing myself to be a complete dunce, (I have no video to test this on):

What if you duplicated the video on track 2, then applied the invert effect at about 50% to track 2. Bring the opacity on track 1 down to about 50%. then render the result as a new avi. If the flicker is gone or reduced, use contrast brightness and saturation to bring the image back toward the original look.

Or maybe change to black and white, then apply the invert effect?

rmack350 wrote on 10/13/2004, 9:13 AM

I certainly can't help you fix it but might be able to shed som light on the source of the problem.

I'm going to make a guess that these lights were rented from a tool rental company and that they're the sorts of lights used for night time road construction. If so, they're probably HMI or some other arc light. Did they give you a daylight balance?

Anyway, for any kind of arc light (but specifically HMI, which is used a lot in the film industry) you either need a generator that delivers AC at 60hz-plus or minus a quarter cycle, or you can use a flicker-free ballast, which is more expensive and makes the light buzz loudly. This is why generators for film are crystal controlled to keep the output frequency constant.

If the genny isn't regulated (and construction gennys wouldn't need to be) then you can get a slow flicker or wave in your shots. Even if your camera has an adjustable shutter speed, it's very unlikely you could fix it when shooting because the frequency is so close to camera speed, and it varies, and often it's below 60, which is below clearscan speeds.

So what can you do? Not much in the production phase except to be suspicious of arc lights and shoot a test before the game. Get out that field monitor and look at the shot. I imagine you have no control of the lights they use but you might check with other shooters to see if they have the same problem and then mention it to the field's management while fully expecting that they'll do nothing about it. (That would be dealing with it in preproduction)

Those of you in PAL land can substitute "60Hz" for "50Hz".

I have no idea how to fix it in post production.

Rob Mack

johnmeyer wrote on 10/13/2004, 11:38 AM

rmack350,

Thanks for the reply. You are exactly correct: they were construction lights. I am producing the DVD now. I am certain that I could have fixed it with one of the deflicker plug-ins if I had spent enough time. I think the reasons they didn't want to work is that they operate on the entire frame, and often the pulsing was only on one side of the field (there were four separate generators, and only one seemed to cause a problem).

Sometimes you just have to take it as it comes and get on with it.

rmack350 wrote on 10/13/2004, 9:23 PM

Ugh! one genny was way out from the others probably. That's the worst. it means that you'd have a really hard time correcting it.

To be honest, I've never heard of someone trying to correct an HMI flicker in post. Then again, most of the people I know shoot in more controlled situations.

Generally, if a generator is just a little off it's freak then you'll see a long slow oscillation. If it's far out then you'll see a faster rise and fall of light. Much like tuning a guitar with a tuning fork. when the fork and the guitar don't match you hear a pulse and the pulse slows down the closer you get to matching.

There ought to be a script that could set envelope points on a beat or to marks that you set.

Good luck.

Rob Mack