Vocoder design

[Martin Czech]

:::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.

I wonder why people built additional filters. There are allready
a couple of bandpass filters with envelope detectors in the
analyser, why don't take that information for voiced/unvoiced
detection?

btw.: from looking at spectra of different words, unvoiced/voiced
detection seems to be pretty difficult. I could not find any simple pattern.


m.c.