The portion of a voltage controlled oscillator which generates the initial waveform, from which all of the other waveforms that the VCO outputs are derived. The two parts of a basic VCO core are a circuit that charges a capacitor at a rate determined by the input control voltage, and a timing mechanism that "resets" the waveform back to its starting point by discharging the capacitor. There are two basic types, which are named after the waveforms they produce: the sawtooth core and the triangle core. Each has their virtues and disadvantages, and aficionados will argue over which produces the best sounding VCO, but the sawtooth core is more commonly used because it is a simpler circuit.
The timing mechanism usually consists of a comparator that compares the instantaneous waveform voltage with a reference voltage. When the waveform exceeds the reference voltage, the comparator triggers the circuit to reset the waveform to the beginning of the cycle. If the VCO has some digital form on information indicating what note or frequency it should be playing, the comparator can be replaced with a digital timer/counter that triggers the reset; the result is known as a digitally controlled oscillator, or DCO.