Crazy CPS-1 DAC recreation

mikejmoffitt

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This isn't a huge deal, and not a huge restoration, but it was a lot of work probing, documenting, and trying new things, and I'm pretty pleased with the result after hours of work.

I received a CPS-1 JAMMA board with Street Fighter 2' HF loaded on it. Game played blind. Visual inspection returned this:

cps1dac.JPG


That's one burnt to hell and back DAC section! Likely some chump miswired a JAMMA connector and sent power down the video lines.

Anyway, by probing around with a scope (and later a test-bench CRT) I figured out the following digital RGB bits:

dacpins.png


I've figured out now that the ?? bits are ANDed with the RGB values to allow for screen fades in hardware (instead of palette manipulation in software).

I built a really ugly 4-bit DAC on the red channel just to verify that my theory was correct in how the video works:

dac.JPG


And here's the red channel represented in greyscale:

reddac.JPG


Cool, it works!
 
After I knew that that worked, I made a much nicer 3 x 4-bit DAC proto board:

tridac.JPG


My monitor was turned down a little so the camera could deal with the brightness, so I promise the image looks correct on the real thing:

itworks.JPG


This video really shows that it looks good:

http://www.youtube.com/watch?feature=player_embedded&v=Od0gXmsvZ28

You can see that since I didn't implement the fade bits yet there are no screenfades, it just jumps between screens.

Anyway, if I were to do this again, I'd want to re-implement the LS07 hex buffers that I've bypassed for this, since not all of the color bits come out of the CPS1 VDP at the same time, and as a result some digital displays (my projector, some LCDs, scaler boards...) show a tiny bit of color channel separation.
 
I'm no electrical engineer but i know this is cool. Nice job, I can do simple repairs but nothing along this line, maybe someday.
 
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