[Coco] replacing main rom

[Arthur Flexser]

Now that I think about it further, I see more clearly what you are saying,
though I still disagree with some aspects of your analysis.  Since the blink
durations for the "on" and "off" periods are presumably equal, there will be a
50/50 chance for the blink to be in the on state when the relevant range of scan
lines for a certain character is reached in each frame.  (Sometimes the blink
will be in the on state for only part of the character, resulting in a portion
of the upper or lower part of the character not being drawn.)  The horizontal
scan time is around a 16th of a msec per line, vs. an on period, at the fast
rate, of about one msec, so whether the character is drawn as on or off at the
fast rate will depend on which 16-line strip of screen it happens to fall into
in each frame.  That is, at the fast rate, characters with the blink attribute
are drawn as on in one 16-line strip, off in the 16-line strip just below, etc.,
alternating over the height of the screen.  Since I believe that the GIME timer
is not synched with the vertical scan, whether a particular character will be on
or off in a given frame will effectively be random, with 50% probability.  On
average, the character will be in the on state in half the frames, so its
physical blink rate will average out to one "on" per two 60th of a second
frames, or a 30th of a sec (33 msec).  This is true regardless of phosphor
persistence.  The effect of persistence will be to lessen the difference in
character luminance between frames where the character is present and those
where it is absent, reducing the amplitude of the flicker, but the flicker rate
will still appear as a 30th of a second, on average, though the flickering will
be irregular.  Which pretty much corresponds to what I remember observing when I
tried kicking the rate up to high.

Art

On Fri, 13 May 2005 jdaggett at gate.net wrote:

> Art
> 
> we are talking about the same thing from different perspective. 
> 
> What I am saying is that the "apparant" blink rate and not the actual 
> blink rate. Due to the persistance, only about every 10th blink is 
> seen on the screen. The phosphur cannot change fast enough to 
> see the change in every field of the raster scan. 
> 
> james
> 
> 
> On 13 May 2005 at 14:09, Arthur Flexser wrote:
> 
> Date sent:      	Fri, 13 May 2005 14:09:17 -0400 (EDT)
> From:           	Arthur Flexser <flexser at fiu.edu>
> To:             	CoCoList for Color Computer Enthusiasts 
> <coco at maltedmedia.com>
> Subject:        	Re: [Coco] replacing main rom
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> 
> > I disagree.  Putting it in other terms, the persistence should
> > decrease the amplitude of the variation in luminance, without
> > affecting its frequency.
> > 
> > Art
> > 
> 
> 
> 
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