VCDO

[mbartkow at ET.PUT.Poznan.PL]

>     I have another idea, and there is probably somebody that can implement it.  Use
> a simple voltage controlled high frequency oscillator and divide it down by the
> number of samples you want to have per waveform, say 64.  Use a phase-locked loop to
> lock the 64 times oscillator to the audio frequency you want to create.  The audio
> frequency is taken from an audio exponential front-end VCO.  The result will have
> the same accuracy and stability of the your audio VCO.  The audio VCO is not
> intended to generate tones, just a square wave for phase locking and can be
> consequently as simple as possible.  Any comments?

That remainds me about my idea for vco bank to be implemented in my future polysynth.
Each voice employs few PLLs driven by a single DCO (a programmable frequency divider).
Each PLL contains its own programmable divider and frequency shifter (e.g. the one
designed by Scott Gravenhorst) within the loop. The loop filter is voltage controlled 
to introduce a controlable lag (aka portamento). The phase detector is an advanced
design (I like that one from Tietze & Schenk based on bidirectionl counter).

This structure is aimed at perfect longterm stability of the VCO frequency ratios, yet 
it allows the short-term microdrifts to achieve that organic feel of analogue oscilators. 
I have already experimented with 4046. Yes, that's true the useful range is 
too limited to provide full tracking over 10 octaves, but my idea is 
to insert a programable prescaler:


                                   PORTAMENTO
                                      |
  DCO ---> /2                         V
   ^       /4 ---> PHASE DET. ---> FILTER --> VCO ------------> WAVE SHAPER
   |       /8           ^                                |
 NOTE      ...          |                                |
            ^           |   /2                           |
            |           --- /4 <--- DIVIDER <- SHIFTER <--
            |               /8         ^          ^
            |               ...        |          |
            |                ^      DETUNE     DETUNE OFF.    
            |--------|-------|
                     |
                   OCTAVE SEL.


I think the best results might be achievable using a typical widerange synth VCO,
but I also consider 8038. Another point is if exp VCO is suitable here (or even 
better than linear). Did anybody tried anything similar ?


regards,

--

Maciej Bartkowiak
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