MOTM

Some vocoders, like my old Korg DVP, simply have a highpass filter and pass
that to the output! The assumption is that the only important information
"way up there" is the sibilance and plosives that are difficult to emulate,
so it just passes them. 



-----Original Message-----
From: konkuro <konkuro@...> [mailto:konkuro@...] 

One easy way to extract voiced/unvoiced information (as I recall) is 
to use a high-pass filter with a cutoff at 12 kHz or so.  Plosive and 
fricative sounds have more energy in the higher regions of the 
spectrum, so when an S or F or P or whatever is spoken, the HP filter 
passes the high spectra which is rectified and used to trip a Schmidt 
trigger (threshold detector).  This is used to control an electronic 
switch, which switches from the default carrier to a noise source.  
As soon as the unvoiced sound is over, the switch reverts back.

johnm