[sdiy] Hybrid (analog/digital) vocoder?

[John Mahoney]

At 10:56 PM 2/14/2010, Paul Perry wrote:
>Just saw mention in the CMOS switch as VCA thread that some famous
>vocoders use PWM for the VCA.
>So I thought - not to complicate matters - to start a new thread, on the
>question of  a 'hybrid' vocoder, in which the  filters are analog, but the
>VCAs are digital, and the PWM is controlled by DSP.
>The idea is, the carrier remains analog throughout, with a PWM VCA.
>But the analysing is done in a DSP or micro, digital outputs from which
>control the various filters in a PWM fashion.
>
>This would have the advantage that
>1. the filters are all analog, giving possiblility of resonance knobs on each.
>2, the PWM - which can be tricky - can do the difficult parts - no
>comparators needed - plus the analysing filtering can be in software
>without affecting 'musical' quality.

Interesting.

Regarding point #1, digital filters can also have resonance knobs, can't they?

A benefit of analog circuitry is easier access to each band's output, 
for special effects purposes. Then again, multiple-output DACs are 
not so expensive, anymore, so maybe this is a wash.

The analysis (digital) filters should have the same characteristics 
as the analog ones, so a bit of profiling and model tweaking may need 
to be done, I guess.

Analog vocoders have far fewer bands than digital ones. More bands 
sound more accurate. Seems that using analog circuits is the limiting 
factor, presumably because of cost and/or size. Are the benefits of 
"pure analog sound" worth the trade-offs?

There are some pretty good analog filter models, these days, as well 
as reasonably convincing models of tube circuits and other analog 
components. Therefore, using DSP doesn't have to mean that the sound 
has no character. In fact, a DSP-based unit can offer a variety of 
models and modes. "Bode mode," varieties of clean and dirty sounds, 
reverse mapping, formant shifting, etc.

I'm thinking that it's a neat idea but not worth doing. Just my opinion.

Curious to see more comments,
John