VCO sub-octave outputs add a square wave one or more octaves below the main pitch to fatten bass sound

Many analog VCOs include a sub-octave output: an additional output that produces a square wave at half the frequency (one octave below) or further subdivisions of the main pitch. The sub-octave is derived by dividing the oscillator’s clock, making it always locked in perfect tune with the main output. Mixing the sub-octave with the main oscillator adds low-frequency weight and body. At extreme sub-octave divisions (two or more octaves below), the resulting frequency can fall below the audible range and may produce a low clicking sound rather than a tone. Some modules (e.g., Erica Synths Fusion VCO) offer sub-octaves one and two octaves below; others (Analogue Solutions VCO-Sub) go three octaves down.

Examples

Adding a sub-octave output to a bassline patch immediately thickens the low end without detuning. Sub-octave two octaves below 110 Hz (A2) = 27.5 Hz (A0), which may be below subwoofer range and heard as a click.

Assessment

Explain why the sub-octave output is always in perfect tune with the main oscillator (contrast with two independent oscillators). Identify at what sub-division the output starts to fall below typical subwoofer range for a mid-range musical pitch.