Re: Cinematronics > Atari Adapter???

On Mon, 14 Oct 2002 11:53:56 -0500, Rodger Boots <rlboots@cedar-rapids.net>
wrote:

>Tom McClintock wrote:
>
>>Well get right on that Rodger! :)
>>
>>Seriously, I was under the impression that the settling time for the DAC was
>>determined more by the DAC than subsequent Op-Amps. The DAC7541 seems to have a
>>settling time of 30 m sec, while the DAC-80s (or DAC7580) have settling times
>>around 4 m sec.
>>
>>
>
>Not sure where you got the 30 mS spec, the only settling times listed on
>the DAC7541 spec sheet are .6 uS typical for the output CURRENT to
>settle to .5 LSB. The opamp that's doing the current to voltage
>conversion will be the speed limiter. VOLTAGE output DACs are
>notoriously slower than current output DACs.
>
>>Unfortunately, even if the DAC was $11 each, you need two, plus the supporting
>>hardware so your cost is running about $35 in parts alone (gotta include those
>>connectors). A finished PCB would add another $10 or so. This gets into the
>>realm of what a Cine monitor board costs (well, usually on the high end as most
>>operators don't want these monitors at all).
>>
>>But I agree a 'new' Cine->WG conversion board would be really cool. What's the
>>cheapest way to go with this?
>>
>
>The cheapest way? Build your own R-2R array and don't even buy the
>DACs. The way Cinematronics draws vectors (capacitor integration) you
>really don't need an accurate DAC. Probably don't even need 12 bits.

The Cine Vector generator's resolution is 1024 by 768.

You do need 12 bits for the 10 bit vector resolution, using the hardcoded timers
built onto the Cine CPU board. Anything less than 12 bits for 10 bit vectors
and the intensity difference between the start of the vector and the end,
becomes noticeable. That and the timing generator on the CCPU would have to be
changed to compensate for the slower drawing rates of the smaller DACs. And the
instruction set would need to be changed since the Normalization instruction
currently normalizes for 12 bit DACs.

> All you're generating are start points and "end points" (not really end
>points, more like "it goes thisaway" points). A bumpy DAC won't matter
>because it doesn't draw the line in between the points.

The line will be smooth but your endpoints will not line up correctly. The
length of the line is based on very critical timing, and knowing exactly where
the trace will be at any given time. Any voltage offsets will cause the trace
to move faster or slower, and either way it won't be where it's supposed to be
when the Z-axis is turned off.

Ok, that being said, Cinematronics timing was a little sloppy since they didn't
have the luxury of a lookup table to adjust for the non-linear timing needed to
drive the R/C. So it's pretty hard to get all the Cinematronics endpoints to
line up anyways, the line length adjustment is a bit of a compromise as it is.

If you're not real picky about perfect endpoints, the R-2R ladder would work.
Just be sure your R/C circuit and voltage levels match those of the Cine design.
Since you have no control over the timing, you must design to Cine's specs.

-Zonn
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Received on Mon Oct 14 13:04:35 2002