[sdiy] vocoder filters

Mattias Rickardsson mr at analogue.org
Sat Aug 31 11:00:18 CEST 2019


Den lör 31 aug. 2019 06:46David G Dixon <dixon at mail.ubc.ca> skrev:

> Well, I think I might have answered my own question.  Looking again at the
> JH Living Vocoder, since Jurgen Haible’s filter responses overlap at about
> the 8dB mark, it really should not matter at all what’s going on around the
> skirt of the response, and higher-Q filters with two 2-pole stages should
> give very similar results to low-Q filters with four 4-pole stages (and be
> much much cheaper to build).
>

Between those two alternatives I'd expect audible differences due to
ringing, and practical differences due to sensitivities to component
tolerances.

 I’d still appreciate if anyone has any specific insights into this
> problem.  Cheers.
>

Paul's viewpoint is one of many when it comes to the filterbank design.
Some aspects have conflicting optima. Figuring out these strategic choices
is the charm in designing a vocoder, I guess. 8-)

/mr



> ------------------------------
>
> *From:* Synth-diy [mailto:synth-diy-bounces at synth-diy.org] *On Behalf Of *David
> G Dixon
> *Sent:* Friday, August 30, 2019 9:05 PM
> *To:* synth-diy at synth-diy.org
> *Subject:* [sdiy] vocoder filters
>
>
>
> Hey SDIY Team!
>
>
>
> I’m thinking about building a vocoder, and I have a general question about
> the bandpass filters.
>
>
>
> I’ve looked at Jurgen Haible’s Living Vocoder, and he used 8-pole filters
> with low Q.  These give a reasonably broad band with fairly steep slopes.
> He makes the filters from two pairs of LP and HP.
>
>
>
> I was thinking about using BP filter sections, but just 4-pole, and with
> higher Q (around 10).  This gives a somewhat narrower band, and the slope
> is steep near the corner, but fairly shallow around the skirt.  This idea
> uses a lot fewer components (about half as many).
>
>
>
> What I’m asking is, does anybody here have any insight into what the
> “proper” approach to vocoder filters would be?  What is the design goal?
> Do you want significant overlap from one band to the next, or should they
> be fairly distinct?  I guess I’m just looking for some general guidelines
> and conventional wisdom.
>
>
>
> Cheers,
>
> Dave Dixon
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