RE: Cinematronics vector generator...

> > From playing with my (fairly) realistic model, it looks like the
> time
> > for the output to react to a change in voltage at the DAC is about
> 1us.
> > Does this sound realistic? Seemed kinda fast to me. (I think this
> is
> > while in "DRAW" mode-- voltage going through the 10K+5Kpot series
> > current limiter. Maybe I screwed up and had it in "INIT" mode...
> Hmmm.)
>
> What do you mean by "react"? I would think the output starts changing
> almost
> immediately - the DAC just feeds an RC circuit, so as soon as there is
> a change at the DAC it should move. Read on.
>
Crap. Major typo. "reach" not "react".

> > Do any of you know what the "clipping" (z blank) timing is? For
> > example-- when watching the output on the "virtual scope" in
> Electronics
> > Workbench it looks like the linear portion of each charging curve
> starts
> > about 50ns after the new voltage is applied. From there it's pretty
> > linear out to about 150ns or so. Is z-blanking software controlled?
> > Seems like 50ns is pretty tight timing to keep, but since it's also
> the
> > period of the 20MHz clock it seemed like a strange number to show up
> > "accidentally". (I'm wondering if the clipping is done in hardware
> and
> > starts automatically one clock tick after the DAC is loaded and
> lasts
> > for two clock ticks after it.)
>
> The Z blanking is controlled in hardware, however vectors may be drawn
> over various amounts of the charging curve. Here's the deal:
> The beam is moved "fast" to the start of a line. Then you load the
> deltaX,Y
> into the accumulators and execute the Normalize instruction. This does
> a
        [...]
> counts UP) it Z-blanks and turns off the 13331 switches so the beam
> doesn't
> keep going to the target point (which was set way beyond the real end
> pt).
>
> Blah. Anyway, for a short line, the "targeted" end point may be say 32
> times
> farther away than the real end point. While a long line, the targeted
> point
> may only be twice as far away. This means the percentage of the
> charging
> curve used can vary quite a bit. Because of the left shifting, there
> are
> only about 9 different amounts of time used for line drawing -
> depending
> of how many shifts it takes to reach "really long".
>
Ahhh. I think that makes sense now. So they actually change the
voltage appearing at the output of the DACs as the line is drawn. (by
powers of two anyway) From my reading of the Cinematronics manual I
thought they just put the potential at the input of the cap/opamp
circuit and clipped that for the line. (Since the cap has to charge,
the curve is pretty non-linear for the first 50ns, then stays pretty
good for the next 150ns, then goes way nonlinear after that.) I take it
they blank Z at first to avoid the non-linear portion, turn beam on
(staying in the linear region), then put new outputs on the DAC and
"elongate" the linear portion until they reach the destination. Funky.

> > Anyway, it just seemed really fast, so I'm hoping for a reality
> check.
> > (This might be explained if I had the thing running with the "INIT"
> > switch closed instead of "DRAW"... Up too late-- don't remember.)
>
> It should respond quite fast. It seems like there is something that
> keeps
> the Z-blank blank for a brief instant while the beam is "stuck" - i.e.
> before it gets going. So I guess movement isn't instantaneous.
>
Right. The charge curve of the cap is a little steep at first...

> > I also put in that weird-ass bridge rectifier-resistor *thing* in
> the
> > feedback loop of the output Op-amp. It looks like it kinda flattens
> out
> > the charge curve of the cap, but it also looked like it was screwing
> up
> > the discharge curve shape. Maybe I had it wired wrong (or it
> confused
> > SPICE, which really shouldn't happen)...
>
> I thought that weird-ass thing was the cinematronics way to compensate
> for nonlinearity in monitor deflection. Notice that X & Y are not
> coupled
> like Atari did.
>
Yeah, that was how I remembered it too, but I sure couldn't "see" how it
was working.

> BTW, I was thinking of running the un-weird-assed CineSignals down to
> my
> space duel board (via X-invert, Y-invert) so they'd go through the
> 1492s
> for pincousioning. I could then use the "invert" signal to select Cine
> or Atari output :-)
>
I was going to try the same thing with the pincushion-corrector I have
bread-boarded up. (Since it's variable and all.) Might be interesting.
 From looking at the Space Fortress DAC board there isn't any correction
in that circuit... (just looks like the Cinematronics circuit, but with
a little integrator on the output.)

Thanks for the explanations-- helps a lot.

        -Clay
Received on Fri Apr 3 17:22:54 1998