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Monitor flicker on film


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#1 Peter Egan

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Posted 02 July 2006 - 12:15 PM

Can somebody explain the phenomenon of flicker on monitors? Why it happens, and how it can be solved?

Thanks.
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#2 David Mullen ASC

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Posted 02 July 2006 - 12:28 PM

Can somebody explain the phenomenon of flicker on monitors? Why it happens, and how it can be solved?

Thanks.


If you are talking about a video tap image from a film camera, it's because there's a mirror shutter spinning while the camera is rolling. More expensive taps have a flickerless mode to smooth that out using some sort of frame buffer. I kinda like the flickering taps because at least I know when the camera is starting and stopping, but the flickerless taps are generally brighter -- instead of the image going darker once the mirror starts spinning.
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#3 Peter Egan

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Posted 02 July 2006 - 01:18 PM

Oh, sorry, I didn't put it rightly. I meant those lines, bars you get accros a TV or a computer monitor when in shot. Is 'flicker' the wrong word for that? Because I saw it in a friend's film and it looked like it was flickering. Anyway, it looks really distracting, and I was just wondering why it happens and how one would go about getting rid of it?

Edited by Peter E., 02 July 2006 - 01:18 PM.

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#4 Stephen Williams

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Posted 02 July 2006 - 01:32 PM

Oh, sorry, I didn't put it rightly. I meant those lines, bars you get accros a TV or a computer monitor when in shot. Is 'flicker' the wrong word for that? Because I saw it in a friend's film and it looked like it was flickering. Anyway, it looks really distracting, and I was just wondering why it happens and how one would go about getting rid of it?


Hi,

The camera has to be in phase with the TV and running at the right speed. If you have the right tools it's not an issue.

Stephen
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#5 Peter Egan

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Posted 02 July 2006 - 01:42 PM

OK, I'll be more specific. Say I shoot in future with a 16mm MOS camera, probably Scoopic or an Arri S, what tools would I need to make the bars go away?
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#6 Hal Smith

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Posted 02 July 2006 - 02:43 PM

If you ever took a science course that had a stroboscope lab then you know why the flicker. The monitor screen is being scanned at one rate, and the film camera is taking at a different frame rate so there is a visual "beat" between the two. The solutions are either a monitor with a "memory" AKA LCD's, an instance where "smear" is good. With LCD's the monitor stores successive frames on top of each other and there's a continous image for the camera to shoot. Alternately have the camera and monitor running at the same rate - usually some form of crystal control on the camera. I think Clive Tobin's website,
http://www.tobincinemasystems.com has some pretty good information on framing rates. Phasing a sync motor is to time the camera so that it starts taking a frame at the same point in time that the monitor is starting to scan a new video field.
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#7 Peter Egan

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Posted 02 July 2006 - 03:27 PM

If you ever took a science course that had a stroboscope lab then you know why the flicker. The monitor screen is being scanned at one rate, and the film camera is taking at a different frame rate so there is a visual "beat" between the two. The solutions are either a monitor with a "memory" AKA LCD's, an instance where "smear" is good. With LCD's the monitor stores successive frames on top of each other and there's a continous image for the camera to shoot. Alternately have the camera and monitor running at the same rate - usually some form of crystal control on the camera. I think Clive Tobin's website,
http://www.tobincinemasystems.com has some pretty good information on framing rates. Phasing a sync motor is to time the camera so that it starts taking a frame at the same point in time that the monitor is starting to scan a new video field.


Thanks, it's a bit over my head, but I'll get it eventually... I hope :)

Another example/question: so say you were doing just an insert of a TV, no surroundings, and if you shot at a different frame rate, 20, 40, or whatever, would that do the trick? Or is getting a sync motor and phasing the only way?
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#8 Michael Nash

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Posted 02 July 2006 - 05:03 PM

Thanks, it's a bit over my head, but I'll get it eventually... I hope :)

Another example/question: so say you were doing just an insert of a TV, no surroundings, and if you shot at a different frame rate, 20, 40, or whatever, would that do the trick? Or is getting a sync motor and phasing the only way?


The camera has to run at a constant speed (which usually requires a crystal motor or electrically governed motor), and the frame rate and shutter angle have to be at a setting that captures a complete display scan of the monitor.

If you're shooting with a wild-motor 16mm camera, your best bet is probably to shoot at 30fps with a 180 shutter and hope for the best. The nice thing about cameras like the Arri S is that you can dial in the frame rate by hand while the camera is running (if I recall correctly), so you'll be able to see the results in the viewfinder. When you project or transfer the footage at 24fps though, motion on the TV screen will be a little slo-mo and out of synch with any audio that accompanies the TV image.

If you can get a hold of an LCD TV, you won't have flicker.
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#9 David Mullen ASC

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Posted 02 July 2006 - 06:40 PM

You're talking about eliminating the roll bar, not flicker.

Yes, make life easy on yourself and shoot an LCD screen.

To be in sync with an NTSC monitor, you need to be able to shoot at 29.97 fps and phase the camera to the monitor. You'd need a sync box and a camera that allowed you to hook it up to the sync box. Now you may get lucky without the sync box just running the camera at 29.97 fps or even 30 fps -- you might just get a thin line that drifts a little.

If you need to have sync-sound during the shot and have to play it back at standard 24 fps, then you'd need to shoot at 23.976 fps with a 144 degree shutter, again with the sync box to phase the camera to the monitor.

If you used a camera with an adjustable shutter, just shooting at 24 fps, by setting the shutter angle to 144 degrees you can reduce the size of the roll bar to a thin line, which won't be too distracting in wider shots as it rolls.

But as well as the 144 degree shutter, if you can shoot at 23.976 fps crystal-sync, you can stop the line from rolling at all and with a phase box, move the line to some less distracting area of the TV. But you can't remove it completely unless you shoot at 29.97 fps with a normal 180 degree shutter. Or get a 24 fps playback company to come in with converted tapes, decks, monitors.

In other words, try using an LCD screen (not a Plasma screen).
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#10 Indrajith Ramesh

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Posted 06 July 2006 - 07:28 AM

For ordinary monitors, you can reduce the "flickering" to a limit by adjusting its display resolution
i tried this method when the same problem came while shooting with arri 35III...

jus try this

regards

INDRAJITH
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