The core of the VCO-1 produces a clean triangle wave which is "shaped" into a sine from which a pulse/rectangle with variable width can be derived. The core functions by the principle of a Schmitt Trigger Integrator. This is a feedback loop between a Schmitt Trigger and an Integrator with an LM13700 OTA as a "variable resistor" to change the frequency of the oscillation.
A Schmitt Trigger is a circuit which turns an discrete input signal (=analog voltage) into sort-of-digital On-Off voltage. The VCo-1 Schmitt Trigger (on the right side of the schematic) is a built up with two 2N3904 Transistors and switches on with a rising and off with a falling voltage (see oscilloscope traces).
The output of this Schmitt Trigger is fed back into the Integrator (TL072 chip in the middle) via the LM13700. The Integrator integrates the current coming out of the LM13700 and creates an Triangle which is again used to feed the discrete Schmitt Trigger: a classical Feedback loop which produces a very nice and clean triangle wave.
The C3 1nF capacitor can be changed to achieve lower or higher frequencies (10nF would turn the VCO into an LFO etc.). The Schmitt Trigger could potentially be replaced with an IC or an opamp based Schmitt Trigger, however the discrete Schmitt Trigger is claimed to have superior speed (which is important to produce accurate tones at higher frequencies). In simulation this VCO achieved an output frequency up to 80kHz without any degrading of the waveforms (~20kHz is the upper limit of human hearing). Also note that the discrete Schmitt trigger is driven by stabilized ±10V from the voltage reference and not from the ±12V rails, to further improve pitch stability.