Some ideas, questions re: pitch shifting

Don Tillman don at till.com
Sun Mar 17 23:55:04 CET 1996


   Date: Sun, 17 Mar 1996 12:38:04 -0800
   From: chordman at ix.netcom.com (Scott Gravenhorst, Synthaholic)

   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 
   signal.  

(Nomenclature issue: "Pitch Shift" means changing the frequency of the
input signal by a specified ratio, effectectively transposing by, say,
a minor third.  "Frequency Shift" means changing the frequency by a
specified number of Hz.)

1. You could add a frequency offset current to a given VCO, by
literally adding the current to the capacitor.  

2. CV -> exponential amp -> add offset voltage -> log amp (yick!)

3. CV -+------------------------\
       |                         sum --->  offset CV to VCO
       |                     +--/ 
       +---> invert ---+     |
                       |     |
   offset V --------> VCA ---+


I'm not sure what you want exactly though, because a frequency shifter
takes an audio signal as an input and the example you gave above take
a control voltage as an input, so 1,2, and 3 are along those lines.
If your input is an audio signal...

4. The Bode frequency shifter.

5. If your input is a square wave, do a digital version of the bode,
xor gates for multipliers, rc-low-pass/comparator for the phase
shifter.





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