Re: ISA Vector card already done...

On Wed, 25 Mar 1998 16:04:08 -0500 (EST), Mitchell Rohde <bovine@eecs.umich.edu>
wrote:

> Look, I've only been following this discussion in a very cursory manner,
>but there is NO WAY you need 144 MHz anything to generate the vector stuff
>for one of these games. Consider: the game boards have roughly 10 MHz
>clocks... these boards use D/As.. and slow-ass ram, etc etc... so I guess
>I better read these designs being flung around a little closer because
>that 144 MHz rate is ridiculous!

Wow, settle down there Mitch! Whew! Here drink this, that's better!! ;^) ;^)

What were trying to do is simulate the analog vector generators on most games
with a digital one.

Take the Cinematronics for example. It doesn't take much of a system clock to
start a capacitor charging, then using circuitry similar to an oscilloscope you
watch the capacitor charge for a bit, and walla a vector. The board electronics
must only be fast enough to start and stop the vectors. They set a DAC to a
start value, then set another one (or mux the same one) to another value, then
go away for awhile while the vector is drawn.

Now to do this digitally you must be able to move the trace along the same path
the charging capacitor takes. So you must now divide that trace up into as
small as pieces needed to create the appearance of a smooth line using digital
jumps. 1024 positions across a 19" monitor is the minimum that looks decent.
Even at a 1024 positions you can still see the "pixels" that make up the line if
you look carefully.

Now in order to draw the lines fast enough to keep up with the number of times
the lines must be drawn in a second, to fool the eye into thinking they're
always there, we must draw them pretty fast. How fast depends on the spec of
the monitor. And for our purpose, how fast the original hardware drew them
since we must keep up with it.

What it gets down to, is for us to draw a line digitally from one side of the
screen to the other we have to update the DAC 1024 times, in the same amount of
time the game has to update the DAC once (assuming the trace is already on one
side of the screen). So now we're already 1024 times faster than the
Cinematronics (worst case). The DAC will have to be updated at a 6mhz rate to
keep up. And this is exactly what the original Asteroids (which is digital)
does. It used parallel input DACs so that all 20 data lines were loaded in one
6mhz clock pulse (10 X data lines, and 10 Y).

Clay was talking about using a dual serial 12 bits DAC that loaded both DACs
with a single data line, using one clock transition per bit. Assuming no
overhead (start bits, stop bits, etc.) 24 bits of data would have to be sent
through the serial port per update. A 6mhz update rate multiplied by 24 bits
per update = Data being clock at a 144mhz rate. Real fast!

> Just spouting off,

No problem, sometimes the email starts flying back and forth and there is a lot
of unspoken information assumed on both parts -- since Clay and I have already
done some research into this we make a lot of assumptions, someone coming in the
middle can be completely lost.

Spouting off'll keep us honest!

-Zonn

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Received on Wed Mar 25 14:09:05 1998