I'm admittedly not very big on electronic things, but couldn't you just get an 8-bit DAC chip and hoof it to the parallel port (possibly via opto-couplers - or whatever they are) or is that not what we are on about here?
Dan. P.S. Arne - I'll upload that sample disc ASAP, I've already got the disc image, it's just getting the time to do it (or getting someone else to do it rather :) )
