Analog Tracking Generator

aka Control Voltage "Twister"

The Oberheim Xpander contains a module called a "Tracking Generator" that allows you to program a 5 section non-linear curve.

This is my attempt at implementing the idea in analog. It can be used to "twist" control voltage signals into various shapes, and has a 10 section curve.

The implementation makes us of the LM3914 bargraph chip with a high frequency "dither" oscillator. The dither oscillator pulse width modulates the comparator outputs. When the PWM is integrated, it produces reasonably straight line segments between programable voltage points.

Note that multiple functions can be available simultaneously by paralleling potentiometers. Each group being summed by an individual op-amp. Not all stages need to be summed either. So functions involving only certain stages are possible, such as crossfading multiple sound sources.

 

0-10 volt input sweep, with "Interpolation control set to zero, produces stepped output voltages usable for sequences, or as a scale quantizer.

Vert = 5 volt/div
Horz = 10 ms/div

10 Hertz sawtooth input

 

0-10 volt input sweep with alternating zero and full scale settings.

"Interpolate" control set to zero.

Vert = 5 volt/div
Horz = 1 ms/div

100 Hertz sawtooth input

 

0-10 volt input sweep with alternating zero and full scale settings (same knob settings as above).

"Interpolate" control set to maximum.

Vert = 5 volt/div
Horz = 1 ms/div

100 Hertz sawtooth input

 

+/- 5 volt sine wave input with alternating zero and full scale settings (same knob settings as above). Notice similarity to bottom section of Serge Wave Multipliers (multiple full wave rectifications)

"Interpolate" control set to maximum.

Range witch set to +/5 volts with sine wave input

Vert = 5 volt/div
Horz = 1 ms/div

100 Hertz sinewave input

 

+/- 5 volt sine wave input sweep with alternating zero and full scale settings (same knob settings as above).

"Interpolate" control set to zero.

Range witch set to +/5 volts with sine wave input

Vert = 5 volt/div
Horz = 1 ms/div

100 Hertz sinewave input

 

10-0 volt input sweep showing how a single AR envelope can be turned into two ADSR shapes.

"Interpolate" control set to maximum.

Vert = 5 volt/div
Horz = 1 ms/div

100 Hertz sawtooth input

 

 

Another arbitrary shape

 

Schematic Diagram of Analog Tracking Generator

Contents Copyright 2001 Grant Richter