It's a flipflop switch, basically.
— The Finn, Neuromancer, 1984
Some oscillators include a sub-oscillator an octave down. This is a simple flipflop switch, which alternates back and forth each time the main shape repeats.
If you wire up an oscillator's output to a flipflop switch'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.
- Doepfer A-115
- Roland Juno-106
- Roland SH-101
- "A study of Sub-Oscillators" Tom Wiltshire, 2017
Synthesis: Envelope generator | Footing | Noise | Oscillator | Program (synthesiser) | Pulsewidth modulation | Sub-oscillator