MOTM

"paulhaneberg" <phaneber@...> schrieb: 
> 1.)  The DVCO is a good idea in the freq shifter if it tracks 
> 1v/oct.  It is my understanding that to do frequency shifting you 
> need a quadrature oscillator and a filter that also does much the 
> same thing (quadrature filter?)  That is you need four sine 
> waveforms (if you are using a sine) which are each offset 90 degrees 
> from each other.  This is difficult to design into an analog 
> oscillator especially over a very big range and you definitely want 
> to be able to generate a very low frequency for subtle shifting 
> like .5 HZ or lower.  You could scan a wavetable with a sawtooth 
> like the MiniWave to generate the four sines using a VCO like 
> Paul's.  The filter is much trickier since it you are trying to 
> shift phase equally across the entire frequency range unlike a so 
> called phase shifter which is really more of a time delay filter.  
> The multiplier part is simple by comparison.   

Doing the whole thing in analogue is possible (I should know (;->)), but
you need a lot of unpleasant trimming and / or component selection.

I've built my own FS in analoge because 
1) I'm not a DSP programmer and
2) It's a one-off unit - no problem to adjust this.

If I would make a digital or hybrid FS, I'd consider the following
(asuming a high quality DSP system):

a)
The dome filter will be easier and more precise in digital. 
No adjustments on the filter required !
Delay time is not an issue, as the dome filter has a long delay
by definition.

b)
The oscillator will be easier and more precise in digital.
No adjustments required, other than the 1V/Oct scale. Even
this can be done in software. No trimming of waveforms, offset,
whatever, needed.

c)
I cannot tell if you can do the multiplications of a FS
without audible artefacts easily. If you can, go for all-digital.
If you cannot, then replace the analogue multipliers with
multiplying DACs: The Audio serving as reference
voltage, and the Q-oscillators coming in form of the digital
code. No trimming, or just minimal trimming required.

The bottom line: Unlike other synthesizer modules, it's not
"warmth", but *precision* you're after. If precision can be better
achieved with a DSP, go for DSP. 
However, you want to avoid artefacts from digital processing
by any means. 
Artefacts mostly come from nonlinear operations. The dome
filter is a perfectly linear circuit. The Q-oscillator is just that:
an oscillator. The multiplication *is* a nonlinear operation,
so you have to be careful here. It may or may not work 
all inside the DSP - if not, you still can do it in analog
(offset adjustment needed) or with a multiplying DAC (watch
out for glitches).

Just some thoughts from a DSP-layman.

JH.