Some ideas, questions re: pitch shifting

Scott Gravenhorst, Synthaholic chordman at
Sun Mar 17 21:38:04 CET 1996

First a question:

Does anyone know of a circuit that will reliably add a constant 
shift of the pitch of an input signal?  I know that it can be 
done with multiple VCOs, run from the same CV with an offset for 
one, but I was thinking more along the lines of a sideband type 
of system.  Like taking a 6000 hz VCO output and somehow 
combining it with a, say, 30 hz signal to produce a 6030 hz 

An idea I had was to use a phase locked loop to track a VCO 
signal.  Then, inserted in the loop is a circuit that upon 
receiving a clock from a second, lower frequency signal, causes 
one whole pulse to be 'dropped' from the PLL VCO output.  This 
would cause the phase comparator to see a 'late' phase coming 
from it's oscillator and would force it to speed up a little to 
compensate.  The faster the input clock, the more off frequency 
the loop would operate.  I think this idea may only work for a 
limited range depending on the paramaters of the loop filter.  It 
also may have a very long portamento effect.  I also thought it 
might work better if a large counter, like divide by 32 were 
inserted in the loop and then another divide by 32 at the output 
( to raise the loop frequency, giving better signal processing 
resolution and then dividing the output by the same amount to 
bring it back to scale. ) 

Another, different idea would be a fixed input/output ratio PLL.
I thought of putting a large counter in the loop, like 
divide by 33.  This would multiply the input frequency by 33.  
Then the output could be divided by 32 giving a final output of 
33/32 F, or 1.03125 F.  The problem here is that the frequency 
would be shifted up a constant 3.125 percent and could not be 
changed without modifying the circuit ie changing the divide 
ratio of loop freq to output freq.

Any thoughts?

-- Scott G., Synthaholic

There is no 12 step program for synthaholics.  Thank your Superior Being.

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