From the looks of it, this is actually not a 3dfx Card.

It should be a :

STB Velocity 4400
(NVidia RivaTNT Chipset)

www.nvidia.com would be the right place to look for a suitable Nvdia Detonator Driver.

(STB Systems produced NVidia based cards as well, mostly using the Riva128 and later RivaTNT Chips if my memory serves me right)

thanks a lot for these answers . I should be very happy to find schemes of this board , the goal is to try to do a converter SCART ( video composite PAL and SECAM) to RGB for watching the TV on my LCD monitor without my PC . the first thing is to digitalize the video with DAC and obtain digital signals then  to use the VGA card with the bus and the memory .