It's a flipflop switch, basically.
— The Finn, Neuromancer, 1984
Some oscillators include a sub-oscillator an octave down. This is a simple flipflop, which alternates back and forth each time the main shape repeats. The sub-oscillator is a cunning, cheap way of making a single oscillator's waveform slightly richer, adding in harmonics from an octave down.
If you wire up an oscillator's output to a flipflop's input, then when the oscillator's shape rises, the flipflop also rises; when the oscillator lowers, the flopflop ignores it. When the oscillator rises again, the flipflop finally lowers; when the oscillator lowers again, the flipflop ignores it.
The flipflop therefore generates a shape at exactly half the oscillator's frequency, but that shape is only a simple square wave. Electronic music is often based on which sounds are easiest to make on a synthesiser, which in turn are based on what's easiest to make electronically.
As the sub-oscillator is in perfect sync with its oscillator, it sounds quite different to having two whole oscillators tuned an octave apart, as they will be at least everso slightly out of tune with each other, making a richer sound.
Several single-oscillator monosynths by Roland, namely the SH-1, SH-09, and SH-101, feature an especially versatile sub-oscillator. They have a second flipflop switch in turn connected to the first one, offering a two-octave-down square wave in addition to the regular one-octave-down square wave.
They also combine the two signals together with diodes, giving an additional option of a narrower (technically wider, but it sounds the same flipped) two-octave-down pulse wave. I would have thought an AND logic gate would be required to ensure there's no audible difference between one or both signals being high, but looking at their circuit diagrams, that's not the case.
- "SH-1 service notes" Roland, Jul 1978, p. 8
- "SH-09 service notes" Roland, Mar 1980, p. 4
- "SH-101 service notes" Roland, Nov 1982, p. 7
- "A study of Sub-Oscillators" Tom Wiltshire, 2017