Re: Playing Vector Games on the Big Screen - Neil eats crow

On Fri, 31 May 2002 10:11:06 -0700 (PDT), Neil Bradley <nb@synthcom.com> wrote:

>> > what are u talking about the dac's on arcades are 1mhz!!! thats all
>> No, they aren't. Look at Zonn's description:
>> ">and 6mhz? is that the main Processor or the State Machine Processor?
>> This is the speed the DAC is stepped, or more precisely the sample rate of
>> the
>> DAC."
>> Not sure where you are getting the idea that the DACs are stepped at 1Mhz,
>> but that's 6 times slower than it actually is!
>
>Well, I need to eat a nice, big, black, crispy crow on this one.
>
>The S/H circuit is stepped at 3Mhz and the DACs are clocked at 1.5Mhz.
>6Mhz Is what it would take to do all vector games across the board.
>
>Apologies for the error and confusion/noise.

Neil doesn't need to eat anything on this. I was Neil's sole source of
information on the speed of the Asteroids system.

I told him absolutely that Asteroids ran at a 6mhz step rate.

And then when everyone start flaming him (ok maybe not "flaming" but you know
what I mean), I thought, can this many people really not see this? So I went
back to my notes. Duh, of course everyone that was actually looking at the
schematic was right, and those of us (us would be "me" with Neil quoting "me")
that were relying on my memory were wrong.

Sorry guys! (And Sorry Neil!!)

So where did the 6mhz come from?

Easy enough.

When I first looked into a PC->VG I looked into the speeds of the different
games. I wanted a system that would not only play the older, slower games, like
Asteroids, but the high speed analog games, like those designed for the
Amplifone monitors.

Asteroids runs the CRT and at a much slower speed than the monitor is capable
of, the later games come closer to pushing the limits of the monitors.

If you go to the excellent page www.arcademonitors.com you will find a listing
of monitor slew rates.

You will see the G05 monitor can move it's trace a full 14.5 inches in 150 us.

Assuming I want to run the monitor at it's maximum speed, and that I want 1024
pixels in that 14.5 inches, the time for each pixels is:

   150us/1024pixels = 0.146us = 6.8266 mhz per pixel

So using a digital DAC approach, like that used by Asteroids, I would need a
6.8mhz clock. Taking into account possible loose tolerances on monitors, I
lowered that to 6mhz.

So I have a chart of monitors, their slew rates, and at what speed a digital
vector generator would have to run to get the most from that monitor.

So when Neil asked what speed does an Asteroids game run at, I quickly looked up
and saw "Asteroids monitor = 6mhz".

Stepping through the DACs at 1.5mhz, Asteroids does not push the limits of the
G05, but it did push the limits of the DACs of the time.

For a little reality check, the G08 is a very fast monitor, and the Sega systems
use a 7.5mhz clock going into it's DAC clock circuitry. Though I didn't follow
the logic to look for dividers, this is more on the order of clock speeds needed
for DAC clocks to run high speed vector monitors.

Since the G05 monitor is one of the fastest monitors, a DVG capable of driving
it at it's full speed should have no problems with any other Arcade game, so my
DVG designed used a 6mhz DAC clock. But because of the stair stepping of DVG
(like Jed says on his most excellent "Secret life of X/Y" pages about the DVG,
you might as well use a raster monitor), and the complexity of driving DACs and
Sample and Hold circuits at this speed, I didn't go with a DVG approach.

-Zonn "Could you pass the ketchup for this crow please?" Moore.
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Received on Fri May 31 21:58:17 2002