How to convert period CV to pitch CV

[gstopp at fibermux.com]

     Sounds like a fun thing to experiment with. Here's the way I'd 
     approach it:
     
     Since the pitch VCO puts out frequency F for 10 volts, and frequency 
     2F for 5 volts, this isn't enough data points to conclude that the 
     scale is linear or exponential. However since the voltage halves for a 
     double in frequency, there may be a chance that this curve will do 
     without having to use a liner-to-exponential converter.
     
     First I'd invert the CV. This means that after inversion the range 
     would be from -10 volts to -5 volts, for open string to 12th fret. Now 
     things are traveling in the right direction. Next I'd level shift with 
     a -10 bias into the inverter (same op-amp) so that the range is now 0 
     to +5, open string to 12th fret. Plug this into a standard VCF with a 
     5:1 attenuator on the front end (like the appropriate series resistor 
     or an adjustable attenuator) so that the VCF will see a change of 1 
     volt in the positive direction for the octave change up on the guitar 
     string.
     
     If the curve is not linear, then the VCF will not track the VCO 
     exactly but it will probably be pretty darn close (like the 5th fret 
     on the VCO will track like the 6th fret on the VCF) so that the 
     timbral quality of the filtered VCO's sound may be constant over the 
     playing range. The only time that exact tracking of the VCF would be 
     critical is if you were trying to run the VCF's in self-oscillation 
     mode and the intonation would need to be precise. This is where the 
     above may not be good enough and more experimentation would be needed.
     
     - Gene
     gstopp at fibermux.com


______________________________ Reply Separator _________________________________
Subject: How to convert period CV to pitch CV
Author:  Mark Smart <smart at nn.com> at ccrelayout
Date:    12/19/96 3:24 PM


I need to ask some advice about a project I'm planning to do at some point. 
(When I'll have time, I don't know!)
     
I've wanted for a long time to take my Roland GR-300 guitar synth and 
modify it to make it programable and more flexible in terms of sound 
variety. As it is, it can only generate sawtooth waves, and all six (or 
twelve) oscillators get run through a single VCF at the end. One thing I 
would do to it is add a separate VCF for each pair of voices which would 
track the pitch of the string from which those voices are being played.
     
Herein lies the problem: the GR-300's voltage-controlled oscillators do not 
respond to a 1 V/octave CV, or even a CV proportional to frequency, they 
are driven by a circuit which generates a CV proportional to the PERIOD of 
the guitar string. Each string is set up to put out a CV of 10 volts when 
the string is played open and 5 volts when the string is played at the 12th 
fret.
     
Basically what I'm wondering is if it is possible to convert this CV to a 1 
V/octave CV using analog circuitry so it can drive a Moog-style filter.
     
What is needed is a circuit to perform a 1/x function on the CV (this is 
the part I don't know how to do) and then a linear/exponential converter to 
make it 1 V/octave.
     
Any suggestions on the 1/x part?
     
     
     
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*     Mark Smart                               * 
*     Network Technician                       * 
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