What is important in a vocoder (Was Vocoder design)

[Bjorn Julin]

>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.

I give you one reason:
Because the unvoice spectrum, for example the "S"
has a lot of high frequency components.

In my vocoder i have the possibility to use any or several
envelope detectors as a envelope detector for the unvoiced part!

When i build my first vocoder in 1986-87 i did some
experiment wiht different detectors and frequencies
for the unvoice part. It all ended to be not to usefull,
on the other hand i found that it was more important
to be able to have the "S" parameters HP filters pole
variable.I allso found that switching between
a wite noise and real voice (straigt mic signal)
for the unvoice detector created real difference
in the final vocoded sound.

Allso one of the most important parameter in a Vocoder
is the attack time in the detectors/envelope followers.
It will set the difference between "distinct" sound
and soft/sluggish sound ".

One of the most difficult things in a vocoder to design
is a unvoiced P and B detector, i tried all sorts of
different shemes, pulse generators ala EMS5000,
low pass filtered noise etc, it ended up that
i scraped the detector and let the BP filters
take care of P's and B's.

To you all who designs a vocoder have to understand
that a channel vocoder is "formant/harmonic" dependent,
that is, the voice used as the modulator will set the performance
of the vocoder in question. To achive the best perfomance
in your DIY vocoder you have to analyse your vocie spectrum
and then design the BP filters after that spectrum!! ;-V

Terrible news! yes i know!

BJ
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