Archive of the former Yahoo!Groups mailing list: MOTM

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