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How to make a "differential pitch wheel"

Background: why build a better vibrato

Here is an interesting idea for a quick home project — I just modified a DX7 myself this way so it's still a bit early to tell if it's useful or not. The idea is to make a realtime pitch control which is "relative". That is, the pitch output of the control is the derivative wrt time of some physical movement. This is to get around the fact that most if not all pitch controls on commercial MIDI controllers have a "dead spot" in the transfer curve around zero. This is of course to prevent pitch drift due to physical or electrical inaccuracies. Without this mechanism the pitch of the center position would not be rock-solid "normal pitch".

The drawback is that if you try to make a vibrato with such a pitch wheel, the vibrato will become "deformed" due to the non-linear nature of the wheel.

Incidentally this is actually the idea behind the Nordlead's odd "wood stick" pitch controller, to eliminate this dead spot.

The first idea I had was to just add a control which then feeds a differentiated voltage into the pitch wheel input of the synth. The easiest way to accomplish this is simply drill a hole somewhere, mount a potentiometer and feed the signal from its center tap thru a capacitor and then to the center tap of the "real" pitch wheel. That's basically all there is to it.

On the DX7 I had to adapt the circuit a little, because one additional way that the pitch is centered on zero in the DX7 is by feeding a voltage to the center of the potentiometer (the other way is with springs in the pitch wheel assembly that returns the wheel automatically to center position.) This voltage is stabilized ( = from a low-impedance source ) and the op-amp in the stabilizer, like it should, completely absorbs any output from the capacitor. The quick fix is to put a variable resistor in series with this voltage. Thus, when the pitch wheel is in the center position, current from the capacitor can flow thru this resistor and affect the pitch voltage.

Technical details

Herewith some related pictures and drawings! The circuit:

How it looks

Where to get to the +15/-15 rails easily

Experimental force translation device (note high-tech spring mechanism <g> )


Page updated Feb 12, 2016 at 10:06 Email: jens@panix.com

All content copyright © Jens Johansson 2017. No unathorized duplication, copying, mirroring, pilllaging, archival, or redistribution/retransmission allowed! Any offensively categorical statements passed off as facts herein should only be construed as my very opinionated opinions.