The first synth circuit I ever built was a VCO consisting of an ultrasonic core followed by a pair of waveform generators where the waveform was "drawn" by setting the positions of ten slide potentiometers (ELECTRONOTES #112). A recent improved version of this oscillator is called the DoubleDeka VCO. Its main documentation is in this file:  doc link
Here are some sound clips demonstrating a few of the many capabilities of the DD:
This is a very basic demo of the DoubleDeka VCO. First switching through the octaves and then moving the sliders around.
Here is another very basic demo. With fixed waveforms, switching through all the combinations of the octave switches.
This demo has four sections using a patch with the two waveforms from the DoubleDeka fed through two VCFs.
Here is a clip from a patch using four waveforms from two units. These are raw sounds without filtering.
Here are some very basic demos of the wide variety of sounds available using the DoubleDeka's unique synchronization and (pseudo) ring modulator circuitry. Sweeps of a Tri VCO fed into the the sync input (for each of the two sync modes) and then into the RM input.
Here is a demo of the "Harmonic Sync" mode. A few seconds of the unsynced sequence, then the sync master is plugged in, then the sliders are shoved around.
This demo of the "Anharmonic Sync" mode has truly hideous sounds, with a brief respite when switched over to "Harmonic" mode and back. Again, with different slider positions.
Here is a demonstration of a DoubleDeka FM patch, similar to other VCOs. My ADAR EG is used in the monostable drive mode, with different settings of Mono Width and Decay Time.
The digital ring modulation input DRM) used to make some bell sounds.