[sdiy] Digitally controlled VCO core

[Jay Schwichtenberg]

Here's another idea:

If this is done in the digital domain an aspect that should be looked into
is using a very high freq. clock to control things. If the freq. is high
enought you could drop or add a few clock cycles to tweak the freq a few
fractions of a cent. You could implement something like bit depth dithering
when down sizing bit depths (adding/subracting a small amount of randomness
(1/2-1 bits usally)). This would give you some deviation between the osc and
get rid of some of the steralness.

Also a wavetable or algorithmic based waveform could be used to give
non-perfect signals like many of the osc we use.

Now what DSP had that 3 DAC interface on it..........................

Jay

> -----Original Message-----
> From: owner-synth-diy at node12b53.a2000.nl
> [mailto:owner-synth-diy at node12b53.a2000.nl]On Behalf Of Jim Patchell
> Sent: Thursday, June 07, 2001 10:47 AM
> To: synth-diy at node12b53.a2000.nl
> Subject: Re: [sdiy] Digitally controlled VCO core
>
>
>
>
> 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
>
>
>
>