Dealing with Flicker

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Dealing with Flicker

On 1/16/95, Patrick Delahanty asked:

Is there any way to reduce flicker when you point a camera at a Mac, PC, or TV?

On Mon, 16 Jan 1995 Michael 'Mickey' Sattler wrote:

Some full-time video geek would know better, but I believe what you're seeing isn't flicker (from intensity variations in the displayed images) but a difference in synchronization: the "top left" of the image being displayed isn't in synce with the "top left" of the camera that's watching. That flicker you're seeing is the dead time between screen draws.

I'm guessing that when professional videographers (camera monkeys) shoot pictures of cameras, they have some hardware way of syncing up the systems.

On Tue, 17 Jan 1995 Michael Wakefield (Dir. of Media & ITV (and camera monkey), Keene State College, Keene, NH) said:

Uh, yeah. Michael is right about the sync problem. The video camera scans its image in at a rate of 30 frames per sec, and each of those "frames" is made up of 2 fields ("screen redraws" for you non camera-monkeys) which are interlaced to make up the whole picture. The flicker that you see when pointing the video cam at the computer screen is caused by the mismatch of that scan rate to the redraw rate of the computer display, which varies considerably from one computer to the next. There is a "dead" time between redraws, as Michael suggests, and as your camera is out of sync with the display, it can "catch" the dead time in the act (as your eye/brain configuration cannot, BTW), and thus the flicker.

We "camera monkeys" use a combination of a scan converter and other devices such as genlock (locking the syncs together) or a frame sync/timebase corrector to do it the "right" way - but if your camcorder has a variable shutter rate it sometimes can be used to lessen the effect (lengthening the time for the scan) as a quick-and-dirty fix. Also, you may want to try changing the position of your zoom lens, as zooming in or out a little sometimes helps - I dunno why...

On Tue, 17 Jan 1995 Craig Miller (Digital Media Lab Coordinator, University of Hawaii) said:

Yes in fact us camera monkeys (I prefer to be called a broadcast engineer) actually have special sync generators which work at 24 fps (normal TV is just under 30 fps) so that when a TV is shot on movie file it looks 'normal'. This is the only place in the world (that I know of) where you will see 24 fps video (NTSC (US & Japan) is 30, and Europe, and most of the rest of the world is 25 fps).

Hope this clearifies the situation. Or atleast brings you in sync. ;-)

On Tue, 17 Jan 1995 said:

Often when professional videographers, like myself, need to shoot off a computer screen they use a camera with a variable sync adjustment on the camera. The variable sync is used to match the display refresh rate of the computer screen, or TV, with the capture rate of the camera. Most consumer level camers do not have that capability. The only way to eliminate or reduce the flicker with a consumer level camcorder is to adjust the screen refresh rate of the computer or computer monitor itself. There is software that will accomplish this and some computer monitors also have a way of adjusting the refresh rate to coincide with the sync capture of the camera. Check out software and find out from the instruction books for the monitor and CPU if you can adjust the sync rate.

Have you found errors nontrivial or marginal, factual, analytical and illogical, arithmetical, temporal, or even typographical? Please let me know; drop me email. Thanks!

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