"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.