[sdiy] Digitally controlled VCO core

[mikko.a.helin at nokia.com]

Hello!

Has anybody tried to use a microcontroller to control the current source and
capacitor decharging circuit for ASM-1 style VCO core? I thought it would
simplify the design as the CV to exp. current converter doesn't have to be
that precise - so no expensive transistor pairs or T.C. resistors were
needed and you could also use a 8 bit DAC - if the uC sets the time when the
cap is decharged and the exp converter only takes care of the amplitude of
the waveform. 

I see (only) some problems with this design. One is the difficulty to keep
the output amplitude constant and the other is that (software) exponential
frequency modulation and portamento are little bit tricky to implement, and
you can't easily (without A/D) use your analog modifiers. Good thing is that
you could build the thing into your MIDI-2-CV converter.

-Mikko