Vocoder design

[Jeroen Proveniers]

> -----Original Message-----
> From: jorgen.bergfors at idg.se [mailto:jorgen.bergfors at idg.se]
> Sent: Friday, November 10, 2000 8:15 AM
> To: sbernardi at home.net; synth-diy at node12b53.a2000.nl
> Subject: Re: Vocoder design
> 
> 
> Hi Scott, thanks for the formulas. I'll try to enter them in Excel.
> Last night I did some PCB layout. It seems quite possible to 
> fit eight channels on one 100x160 mm board. That would make 
> it 2 1/2 boards for the entire vocoder.
> One thing missing in the vocoder is a voiced/unvoiced 
> detector. Any of you gurus have any idea how one would work?
> I would imagine that the circuit would crossfade in a certain 
> amount of noise, when the detector exceeds a certain threshold.
> The noise would go through the filter bank too, of course. 
> You could also patch another signal instead of the white 
> noise, if you wanted.
> I think a circuit like this would add really interesting 
> possibilities. The question is how the detector would operate.
> How could you determine if the signal is pitched or unpitced? 
> Maybe it could be considered unpitched if several adjacent 
> bands are "on"? But I doubt that it would work in all situations.
> Let's hear some better ideas!

The Elektor vocoder features a voiced/unvoiced detector. It consists of a
lowpass filter (A) and a highpass filter (B). The output of these are
rectified/lowpassed and then some decision logic follows. Something like
when level of A<B then voiced; if A>B then unvoiced. If A and B are both
zero then last decision remains valid etc.

I found the schematics of the elektor vocoder somewhere on the web, don't
know where.

JJ