Subject: Re: why 30hz flickers and movies don't

From: Dave Stampe-Psy+Eng <[email-masked]>
Date: Fri, 25 Oct 91 17:38:14 -0400

> From [email-masked] Fri Oct 25 17:01:23 1991
> From: [email-masked] (Christian Ludwig - CATS)
> Subject: Re: why 30hz flickers and movies don't
> To: dstamp@watserv1 (Dave Stampe-Psy+Eng)
> Cc: [email-masked]
> 
> > >Been following the thread on Sega LCD glasses.
> > >
> > >Why does 30hz to each eye cause a flicker, when 24hz (Film) does not?
> > >
> > 
> > I believe the reason is that the Sega glsses give you 50% duty cycle
> > on the images to each eye, and a movie projector gives over 90%.
> > This results in 5 times the 24 Hz/30Hz flicker (at least).  You can
> > see the flicker in movies if you have a fast-moving white object on
> > a dark background-- just look at the edges.
> > 
> 
> Also...  movies are filmed at 24fps, but when projected, each frame is shown
> TWICE in that 1/24sec bit of time.  This approximates the effect of 48fps,
> hence, no (or at least far less) flicker.
> 
> Chris Ludwig
> 
Depends on HOW you're showing the movie!  The film has to move SOMETIME,
and be blanked during the motion.  So you've got a MINIMUM blanking interval.
Adding TWICE the amount of TWICE the frequency (do a Fourier analysis of the
brightness versus time) will result in the same level of error thru a
first-order lowpass filter, which the human eye is.  Showing the same 
frame twice won't help that much.

- Dave Stampe