Re: vocoder pitch-shifting

[Robin.Bussell@axa-sunlife.co.uk]

>(I know not much about vocoders), but how can you use a vocoder for
pitchshifting?


It can be done by changing the relationship between the analysing filters 
and
the synthesising ones:

Think of a vocoder like this: (physically impossible thought experiment 
follows)
You feed your voice into a spectrum analyser  and the sound that you want 
to
make 'talk' into a
graphic equaliser with the same frequency bands as the spectrum analyser.

Now rig things so that as the columns of lights on the spectrum analyser 
go up
and down they
wiggle the corresponding graphic eq slider up and down.. this is how a 
vocoder
works.

Now if you shift one of the components off to the side so that whereas 
before
the
100Hz->200Hz band on the analyser was wiggling the 100Hz-200Hz slider on 
the
graphic eq it now wiggles the 600->700Hz slider you've ended up with a 
pitch
shift
of sorts... even weirder things can happen if you flip one unit round so 
all the
high
tones in the 'voice' signal are modulating the low tones on the synth 
signal,
IIRC some early analogue vocoders used to have a patch bay to allow this 
sort
of experimentation. nowadays when it would be really easy to do this sort 
of
thing digitally no one seems to go beyond the norm.. pity :-)


Hope that helps a bit,
                                       Robin.




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