Steve Baker (steve++at++mred.bgm.link.com)
Sat, 16 Nov 96 09:24:26 -0500
I'm afraid you are confusing two issues here.
1) Can you see flicker at 50Hz, 60Hz and 72Hz?
2) Do you see beat frequency artifacts when the video
rate is different from the video rate?
There is nothing inherently bad about 50Hz - 50 Million people
from my mother country (UK) watch 50Hz Television every day without
complaint. It depends in part (as you point out) on the decay rate
of the phosphor - and in part on conditioning. I have heard Americans
complaining that British TV seems flickery to them. Another factor is
the field-of-view. You eye is MUCH more sensitive to flicker at
the edges of the retina than in the center - so wide field-of-view
displays appear to flicker more than narrow ones.
You can choose displays with long or short persistance phosphor - if
it's too short you get flicker - if it's too long, the image smears
as it moves.
However, even people who have become acclimated to 50Hz video will
see flicker in 60Hz room lighting. I can personally attest to this.
(Although I can't ever recollect using incandescent lamps instead
of flourescents).
I don't hear people complaining about 72Hz video in 60Hz lighting,
but it's evident that the limit of human flicker perception is
somewhere between 60 and 72Hz - so perhaps this isn't a surprise.
Operating the graphics pipeline at a lower update rate than the video
rate causes yet another artifact. If you run 30Hz updates on a
60Hz screen, you don't get flickering - you get double-imaging.
If you run 24Hz on a 72Hz screen, you get triple-imaging. As the
update rate drops, the eye/brain suddenly stops seeing smooth
motion and multiple-images and starts seeing images that 'step'
or 'jump'. The rate at which that happens has been shown to
vary widely between individuals - but in my experience, the
rate is somewhere between 10Hz and 20Hz.
The human eye/brain is an incredibly complex mechanism.
Peopple need to understand some of the more obvious
things that it's sensitive to if they are going to
produce good simulation.
Getting back to the very original question though - if
you want to use 50Hz video, you'd better use 50Hz lighting
(or no room lights at all).
Steve Baker 817-619-1361 (Vox-Lab)
Hughes Training Inc. 817-619-8776 (Vox-Office/Vox-Mail)
2200 Arlington Downs Road 817-619-4028 (Fax)
Arlington, Texas. TX 76005-6171 Steve++at++MrEd.bgm.link.com (eMail)
http://www.hti.com (external) http://MrEd.bgm.link.com/staff/steve (intranet)
http://web2.airmail.net/sjbaker1 (external)
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