[sdiy] Digitally controlled VCO core

[Jim Patchell]

mikko.a.helin at nokia.com wrote:

> 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.
>

    Using a DAC to control the threshold of the comparator, I think, would be
making things a little overly complicated.

    I can think of other overly complicated ways that may or may not solve some
of the problems you might be trying to solve.

    If you use a DAC to control the pitch, you can eliminate the TC resistor by
having the reference voltage for the DAC track at 3300ppm (can't remember if you
need positive or negative TC on the ref voltage).  This would give the pitch
setting the Zero TC you need, and the other Exponential control inputs that you
might use for modulation would drift, but you might not care.

    Then again, somehow I think I might have missed the point of your post.

>
> 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

    -Jim