Video Flicker

Kevin McCoy (KevinMc@starwave.com)
Tue, 17 Jan 1995 13:54:00 -0500


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?

The issue here is a difference in _frame rates_ or _refresh rates_
between the two systems. An analog video system (an if you are using
a video camera, it all starts out that way....) completely redraws the screen
every 30th of a second, so its refresh rate is 30 khz (technically it's
29.97 khz for a color signal, 30 khz for a true black and white signal....).

A computer screen, on the other hand has a refresh rate that varies between
say 65khz to 85khz (that is, every 80th to 60th of a second, it redraws the
screen). The difference between these refresh rates puts them out of phase
with each other. When they are out of phase, parts of the video signal that
are not meant to be seen become visible. The rolling bar that you see is
caused by the electron gun zipping back to the top left edge of the screen to
begin a new screen redraw. This part of the signal is called the _blanking
interval_ . In addition to defining where a frame of video begins and ends,
the blanking interval is also the place on video tape where _Hi-Fi_, or PCM
audio is recorded, as well as certain types of times code.

The way the flicker is removed is by putting the two systems is phase. To do
that you need a video camera with a fully variable shutter speed (not one
with hard, present values like 1/100, 1/500, etc....) Essentially, your
shutter speed is controlled by a knob that you tweek until the flicker is
gone. Unfortunately, this feature is usually only found on cameras starting
at the $7000-$10,000 range! In fact I see flicker on computer screens on TV
news and interviews all the time. It's an easy way to gauge the _quality_ of
the production--if you don't see a flicker when they are interviewing someone
in front of a computer then you know they are using some quality gear!

I realize this e-mail will surely pag me as "some full-time video geek"
(thanks Michael...!), but I thought people might be interested to know.

Even in a fully digital world, analog signals will always rear their
beautifully dirty heads!

Kevin McCoy
video artist
kevinmc@starwave.com

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