From: shoppa@erin.caltech.edu (TIM SHOPPA)
Subject: Multiplex displays (Was Re: You know you've been hacking too long...)
News-Software: VAX/VMS VNEWS 1.41    
Organization: California Institute of Technology
Date: Thu, 13 Aug 1992 18:48:00 GMT

In article <1992Aug13.143452.18092@kth.se>, kjell@elixir.e.kth.se (Kjell Rilbe) writes...
>In article <BswnIE.Dpt@world.std.com> bmarcum@world.std.com (Bill Marcum) writes:
>[Referenced articles deleted]
>>Speaking of screen color, I've noticed somthing interesting on a computer
>>where I work.  The screen display has alternating lines of red and light-blue
>>text on a dark-blue background.  I wear very thick glasses, and as I move my
>>head, the red letters seem to move around while the light blue letters stand
>>still.
> 
>This has got nothing to do with computer folklore, but I just have to tell
>you all about this. The other night me and some friends were out to watch
>the fireworks they have here at "Stockholm Water Festival" every other night.
>One of us had a walkman. It was dark, and the walkman had this little red
>LED to indicate power is on. When he moved the walkman, this little LED
>seemed to kind of float around, like it was not attached at all to the
>walkman!!!!!
>It was nothing wrong with our eyes, we all saw it, and one of us did NOT
>wear glasses. Really freaky!!!
> 
>/Kjell Rilbe
Strobed (rapidly blinking) LED's can play such tricks on your eyes.  Most
LED calculators and digital watches didn't actually display all of the digits
at the same time - the display was multiplexed.  This saved some circuitry
(you only needed one BCD to 7-segment decoder) and also saved power (By
running a higher peak current to your displays, you lost less power in
the current-limiting resistors, while keeping the average brightness
constant.)  It's possible that the walkman's LED was strobed in this way:
the 5 or 10 mA (constant) it takes to light it up is not a ridiculously small
amount of the total current draw (especially if radio instead of tape is
being used.)  I've seen the insides of lots of walkmans (walkmen?) but have
never seen this done, actually.

In any event, these strobed displays can appear to jump up and down if
you watch them.  I think that what happens is that the feedback system
(i.e. your brain) that keeps your eye pointed in one direction constantly
makes adjustment to where your eye is pointing.  This is particularly true
when your eye is tracking an moving object.  Now if your eye
is following some sort of smooth average motion, but the object you are
watching actually moving irregularly and jumpily, the blinking of the LED will
occasionally happen during a jump your eye doesn't follow.  The image you
see is the integrated image over the last twentieth of a second or so looks
smooth, except for the LED which blinked out of place!

Another neat trick: take a multiplexed LED display and wave it back and forth
rather quickly.  You'll see the digits, but not necessarily where you might
usually expect them!

This doesn't necessarily explain the original posting regarding the CRT
display, but it might, if the time constants for the different phosphors
were radically different.  I don't think this is the case.

Oh, I remember my first day-long session in front of a green CRT.  When
I went outside into the snow, I discovered that everything that should be
white looked pink!  This never seemed to happen later - all the damage
to my eyes must have been done that first day :-).

				Tim (Shoppa@erin.caltech.edu)
Newsgroups: alt.folklore.computers,comp.unix.misc