Re: Anybody with an oscilloscope as an Apple II display?

[Charlie Springer <RAM@regnirps.com>]

On Tue, 1 Apr 2008 01:07:15 -0700, Michael J. Mahon wrote
(in article <mJSdnXRw6-2pcGzanZ2dnUVZ_tyknZ2d@comcast.com>):

>> In the late 70's I tried a couple dozen circuits to make a good analog 
>> vector 
>> display using D/A from an AIM-65 (same speed and processor as A II) and an 
>> X-Y scope. I couldn't handle very many vectors in real-time and would need 
>> 8 
>> an MHz Zip or the like to make it worthwhile. memory isn't a problem for a 
>> vector display since any line is defined by two (x,y) pairs. If you use 8 
>> bit 
>> converters (256x256 display) you can go pretty fast. I used 12 bits and had 
>> to do twice as many LDA STA pairs per vector.
> 
> The problem is that when the D/A's are (re-)loaded, the voltage outputs
> will swing rapidly and exponentially damped toward the new values.  This
> results in a trajectory that is not usually a straight line, nor is it
> of constant velocity, so the "vector" is curved and of non-uniform
> intensity.  In fact, if the values of both D/A's cannot be latched
> simultaneously, there will be a "hook" at the start of each vector.
> 
> Of course, short vectors look better.
> 
> Either linear ramp generators must be controlled by the D/A's or they
> must be low-pass filtered and fed redundant points along the desired
> vector to get a straight line (and always to get a curved line).

The A/D outputs went to analog switches to get simultaneous starts for either 
integrators or charging caps and to turn them off at the same time. The caps 
were big enough to get a line that looked straight, though it required some 
gain afterward to match the display scale and on a long line you could see a 
change in brightness as it slowed towards the end, but only if you looked 
very closely. With beam modulation, I didn't use a parking spot.

Once I went to all integrators and had the rather expensive Analog Devices 4 
quadrant multipliers/square rooters I started depth cueing with another 
integrator to control brightness plus the dimmer the line the thinner it 
looks. That part was pretty cool. I did all the experiments on a scope, and 
as you say, electric field deflection is very easy and fast compared to 
magnetic. Vector General had displays with ES deflection and phosphor layers 
that cold be penetrated with varying accelerator voltage to get three colors. 
The games had magnetic and some tricks could be played using the linear 
current input to a magnetic coil.

-- Charlie Springer