[sdiy] Anyone built the ELEKTOR VOCODER?

[jbv]

> > These can be tuned nicely, but the parts count increases dramatically (3
> > opamps per filter stage). I was even looking into doing third order
> > filters (3 cascaded state variables) and whereas it looks like it would
> > work quite nicely (I've done some computer simulations), we're talking
> > about 100's of opamps here.
>
> Very, very true.
> It's easy to build a small vocoder, but the difficulties are dramatically
> increasing with size. There's a reason that the classic big vocoders were
> so much more expensive than the simple ones.

Although the above is quite true, I found a way to reduce the number of
op-amps, and still have 30 bands with 1/3 octave filters.
I actually found the trick in another Elektor article (spectrum analyzer),
and it consist in detuning slightly the central freq of each filter, so that
Fc of filter 1 is the central freq of the band, and the central freq of the
2 other filters respectively the 2 cut freqs (high & low) of the band.
Carefully adjusting gain & Q allows to obtain a rather linear curve (all
filters
have Q slightly > 4, "central" filters have a gain of 1, and filters for Fh &
Fl
have a gain of 1.4).
Furthermore, 2 adjacent bands share the same filter.

Schemos can be found here :
http://perso.wanadoo.fr/jbv.silences/DIY/Vocoder1.jpg
http://perso.wanadoo.fr/jbv.silences/DIY/Vocoder2.jpg

The total amount of op-amps for the analyzer part is reduced from 90
to 75. The same is true for the synthesizer part which is built around
the same structure, which means a total of 150 instead of 180.

JB