[sdiy] Hybrid (analog/digital) vocoder?

[Naoki Iwakami]

Hello John,

I'm not sure which of analog or digital processing is superior for a vocoder.
Mostly I agree with you but there should be a lot of other opinions here.
But when you are working on analog and want an analog VCA, I think the
PWM type has some advantages against traditional OTA type VCA.
PWM VCA is simple and linear.

You can create PWM using micro processor but actually you can also obtain it
using a triangular generator and a comparator, just as usual VCO does.
If the VCAs are multi-channel, they can share the trianglar generator.
So you just need to add a comparator and a switch to increase a VCA channel.
Roland VP-330 uses a single transistor for a comparator.  So its VCAs are quite
simple compared to OTA-type.  I thinks this is an advantage in "analog domain".

Also, the PWM type is quite linear as long as the switch is fast enough and
the rectangular is linear enough.  It is hard to obtain for a
transconductance type
VCA with such a small set of parts.

So other than vocoder, the technique might be interesting to be used for some
analog application like multi-channel compressor or a mixer.

Thank you,
-- Gan


On Mon, Feb 15, 2010 at 1:35 PM, John Mahoney <jmahoney at gate.net> wrote:
> 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
>
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