The Voltage-controlled Oscillator (VCO) outputs a continuous stream of alternating current (AC) voltage which may be used as either a signal or control source (or both). The overall shape of the stream may be selected by a waveform selector, or in the case of many early modular synths, multiple waveforms are produced simultaneously and output through separate jacks. In addition, for the pulse waveform, an initial pulse width control is often provided, along with a control input and attenuator for further pulse width modulation, included in the pictured module here. The amplitude of the VCO output is normally fixed at the maximum reference for the instrument (a swing from -5 to +5 volts, for example), with no possibility for amplitude control at this stage. Pictured left is a Behringer Eurorack replica of the legendary Moog 921 oscillator (hover to see image of original), which was one of the more complex oscillators made.
The oscillator's initial frequency offset (the frequency before control voltage is applied) is set for either a wide sweep (±6 octaves), or a much narrower one-octave range, interestingly centered on organ-stop feet-designated octaves via the right knob. This gave the user the choice of a far less precise but huge tessitura, or a more precise fine frequency control. These oscillators were tuned either by ear, or a frequency counter if you were lucky enough to own one, and they often drifted as the electronics warmed up. Some oscillators had a coarse and fine tuning knob. And many, like the Moog, also had a subaudio option, though instruments with LFO's might not.
The Moog 921 did NOT have separate 1 V/oct, linear and exponential frequency control c.v. inputs like many (because the Moog c.v. concept conditioned the c.v. before the module c.v. inputs). But it did have a clamping point control that would offset the phase of it's waveform to a specific phase angle upon a trigger. This might be great when used as a subaudio control signal, to say have a vibrato start at the same point with each note. Or it would act as a sync to force in-phaseness with another oscillator. Sync is still used, even on Eurorack synths and creates some very interesting timbral effects. Finally, the Moog had a separate Aux output at the same frequency as the main waveform, but its waveshape and level could be controlled.