# Some ideas, questions re: pitch shifting

Scott Gravenhorst, Synthaholic chordman at ix.netcom.com
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
signal.

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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