Re: Flicker?

Michael Wakefield (mwakefie@keene.edu)
Tue, 17 Jan 1995 11:42:37 -0500


On Mon, 16 Jan 1995 msattler@jungle.com wrote:

> At 10:49 1/16/95, Patrick Delahanty wrote:
>
> >Is there any way to reduce flicker when you point a camera at a Mac, PC, or
> >TV?

>
> 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.

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.

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

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...

==Mike Wakefield <mwakefield@keene.edu>
Dir. of Media & ITV (and camera monkey) Keene State College, Keene, NH