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HMI Pulsing

Published : 9th April 2005

I am, as usual, having a disagreement with our engineers. We own our own genny (800 amp, 3 phase Crawford). For some months now I've noticed a pulsing in the video when we use HMI's with the genny. It is difficult to see if there is a lot of detail in the picture but easy to see on a scope if there is an area of solid color. The video level rises and falls about 5ire about every 1 1/2 seconds (that's a guess, I haven't timed it). It is a constant, even pulsing. I believe the genny is out of phase and expect the engineers to fix it. They, as usual, are reluctant to do anything that might be regarded as work.

Last week one of them suggested running the cameras shutter until the pulsing dissappeared. Since I knew this wouldn't work I tried it. Unfortunatelly it did work. Running the shutter at 60 hz eliminated the problem. OK, 60 hz with a genny makes sense but I still think the genny is out of phase. But since the problem is now fixed as far as the engineers are concerned, I need anoth
er argument to get them to work on the genny.

Any suggestions? Or am I wrong?

Pete Roberts
Production photographer, Williamsburg. VA


>The video level rises and falls about 5ire about every 1 1/2 seconds >(that's a guess, I haven't timed it). It is a constant, even pulsing.

Pete,

Firstly, I'm guessing you're using magnetic ["iron"] ballasts? [i.e. NOT "flicker free".]

Rather than out-of-phase I suggest your generator is off speed. Very slightly.

If your estimate of "every 1 1/2 seconds" is close then I'd say the genny is running either at 60 2/3 Hertz or 59 1/3 Hz.

Get your engineers to measure the frequency with an accurate frequency meter. Or set up two similar HMIs, one off mains and the other off the genny. Look for any beating between the two sources!

BTW, at this very moment there is a similar discussion going on on CML-Lighting, although more concerned with "flicker free" [high frequency electronic] ballasts.

Cheers,

Clive Woodward,
Perth, Western Australia.


Pete,

You don't say what HMI's you're using, whether they are magnetic or electronic ballast; what you're shooting with, video or film; what frame rate you're shooting at 24, 23.98, 29.97, 30 or 59.94.

I'll take a stab and guess: you're shooting standard 59.94 interlaced video with magnetic ballasts. If you search the archives of CML you will see several discussions about this issue and you will note that 23.98, 29.97 and 59.94 are not safe shutter speeds for non-flicker free HMI's. The fact that you tested with the camera shutter in 1/60 sec. and the problem went away is more proof that the camera is shooting at something out of sync with your lights/generator. If your camera(s) does true 24P or 30P try that speed and see if your problem changes.

Also, if you're convinced the gene is out of whack, get a freq meter and use it on your lights to determine what frequency the HMI's are running at. (see the other HMI problem thread currently running on cml-lighting)

Again, check the CML archives, this has been discussed many times.

Randy Miller,
DP in LA


Thanks Randy, and your guess was correct, magnetic ballasts and an SD camera, no 24P or 30P. I'll check out the threads you suggested.

Pete Roberts


Pete Roberts writes :

> magnetic ballasts and an SD camera,

Pete,

Just so it's clear, the culprit isn't exactly the SD video camera,
you'll get the same thing with a Sony F900 or Varicam if it's running at 23.98P or interlaced 59.94.

A quick rule of thumb for HMI's is this: if your shutter speed doesn't divide evenly into 120 then you don't have a safe speed - i.e. 120/24 =5 hence a safe shutter speed. 120/60=2 a safe shutter speed.

120/59.94=2.002...... not a safe shutter speed.

Randy Miller


Careful not to confuse frame rate with shutter speed.

59.97 interlaced is indeed not a "safe" frame rate, which is why you need to use an HMI-safe shutter speed, and/or use the clearscan function.

Mark Schlicher
Writer/Producer/DP
Nashville, TN


Sorry, as usual my fingers were going faster than my brain. What I intended to say was - magnetic ballast and a camera operating at 29.97.

By the way Clive,

I think you've nailed it. On occasions that I've had HMI's running off both the genny and mains, only the fixtures on the genny pulse. That's why I thought the problem was with the genny. Now I just have to persuade the engineers to check and fix it.

Pete Roberts


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