CDC 6600 display character generation

[Paul Koning]

> On Jun 7, 2018, at 3:21 AM, Lars Brinkhoff via cctalk <cctalk at classiccmp.org> wrote:
> 
> Paul Koning wrote:
>> I have converted the "chassis tabs 12" wire lists to a VHDL model,
>> which you can find on my Subversion server.  Run on GHDL, it
>> demonstrates the behavior of the circuit and reproduces the documented
>> waveforms.
>> 
>> I have also attempted to create a SPICE model of the DD60 deflection
>> signal path.
> 
> I wish more terminal emulators had this attention to detail!

Thanks!

The SPICE model isn't in the display emulator.  I was thinking that if I could make it look correct, I'd measure the impulse response and use that for an FIR filter that would go in the emulator, replacing the current hand-hacked IIR filters.

The emulator does have some attention to detail.  It uses the controller's stroke shapes, adjusted by those IIR filters.  Then I apply a Gaussian distribution to mimic the CRT spot (width controlled by the focus control).  The resulting shapes are saved.  Then to plot a character, I add that into the pixel map, with saturation and decay.  The idea of simulating the decay was lifted from Phil Budne's work in SIMH.

Yes, that pretty nicely simulates the actual display, about as accurately as you might hope to do in software.  It could be done better in an FPGA, that would be an interesting project for LCM (6000 channel in, HDMI out).

	paul