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From: grr@cbm.UUCP (George Robbins)
Newsgroups: net.micro.amiga
Subject: Re: Technically speaking.. Flicker
Message-ID: <19@cbm.UUCP>
Date: Wed, 29-Jan-86 10:25:56 EST
Article-I.D.: cbm.19
Posted: Wed Jan 29 10:25:56 1986
Date-Received: Sat, 1-Feb-86 03:37:47 EST
References: <187@oberon.UUCP>
Organization: Commodore Engineering, West Chester, PA
Lines: 47

> Randal Spencer      Student DEC Consulting - University of Southern California

> I really need a good description of exactly why is it that the Amiga will
> flicker in the 400 mode.  I understand Television reasonably well, enough
> to understand that television is a set of 60 fields of 262 lines of video
> which extend from the above the top of the visible portion of the screen
> to just below the bottom. There is 1/60th of a second worth of motion and
> then the next frame is drawn, this time the lines are drawn between the
> previous lines.  This process is done so as to keep the screen from fading
> before the electron beam gets to the bottom of 512 lines after a 1/30th
> of a second.  When the Amiga is in 200 mode I assume that it is drawing
> on the first field and then not on the second?  

In 200 mode, the same image is displayed twice during the 1/30 period on
the very same lines.  The size of the scan line is large enough that you
do not notice any gaps.

> Perhaps the real problem that I could not figure out in my own head is,
> if the Amiga had to flicker to remain compatible with NTSC, why is it
> that even the highest resolution video of the day will not flicker like
> the Amiga?

The basic flicker is not really the fault of the amiga, it is caused by the
relatively short persistance of the color phosphors.  Monochrome phosphors
are available in varying persistences - from very long like used on medical
heartbeat displays to very short for oscilloscope photography.

Since only standard color phosphors are readily available, when two alternate
scan lines are very different they are each fading out 30 times a second,
which is perceived to be an alternating or flickering effect.  Because a
normal TV image has, on the average, very little contrast between scan lines,
you do not notice this while watching the A-team.

As has been pointed out before, you can minimize the effect by dimming the
ambient lighting and turning down the brightness.  Many of the commercial
color graphics displays run at 30 Hertz, non-interlaced, and must be used
in darkened rooms to minimize eyestrain.

> I run Setlace it sure seems to start shaking the formerly rock steady
> lines around on the screen.

I'm not sure what the problem is here, but anyway if you find a source of cheap
long persistence color monitors, please let Amiga and the world know about it!!!
-- 
George Robbins - now working with,	uucp: {ihnp4|seismo|caip}!cbm!grr
but no way officially representing	arpa: cbm!grr@seismo.css.GOV
Commodore, Engineering Department	fone: 215-431-9255 (only by moonlite)