vibrato

[Mike I.]

It should be possible to build a pitch shifter (harmonizer) using a pair 
of long BBD's. A steady pitch shift will result for the length of time 
that a "sample rate differential" can be maintained between the input 
and output of the delay. eg. for a one octave up transposition, the 
sample rate for signals at the input is R KHz, while the sample rate for 
signals coming out is 2R KHz. A voltage controlled 1/x clock would 
probably be best for this, with the clock being modulated by an upramp 
(upshift) or a downramp (downshift). One way of doing this would be to 
use a variable amplitude ramp generator with a period of about 20 mS, 
another way would be with a fixed amplitude ramp generator with a 
variable period (faster LFO gives larger shift). A problem is the 
"retrace glitch" which occurs when the ramp resets. A possible solution 
for this is to use 2 parallel BBD's, modulated by ramps that are 90 
degrees out of phase. A "crossfade" circuit alternates between the 2 
outputs, so the BBD output glitches are alternately blanked. Doing it 
with DSP is the most sensible way to go, since there are better methods 
of avoiding glitches. I think there were some attempts to make BBD 
pitch shifters in the 1970's. Mike