This converts midi to A voltage (not necessarily the control voltage you want). using two R/2R ladders on pins 0-6 and 10-16 (each is 7 bit because midi is 7 bit). You could also use some other DAC if you want. It should be pretty easy. It should be approx linear so if you amplify it appropriately, it should be 1v per octave with a massive caveat on the accuracy of the resistor values used.
There might be blips as the values change. They'll be very high frequency, so a low-pass filter should iron them out.
Note -- as yet, this is untested
This currently does saw-tooth output, but it should be fairly trivial to shift this to different wave forms. It's currently outputting over a 7-bit R/2R ladder, but would be good to try it with PWM.