A low-pass filter swept by its own LFO. It gives any sound a rhythmic, wah-like movement that keeps time with its own clock rather than with what you play.
The idea
The filter's cutoff swings around a centre frequency on an exponential curve, so each step of the sweep covers the same musical interval whether it sits low in the bass or up in the treble. The low-pass is maximally flat, with no resonant peak, so the sweep shapes the tone without making the signal louder than it came in.
Because the LFO runs freely, the effect does not follow your dynamics. That is the difference from an envelope-driven auto-wah: the movement stays steady whether you play softly or hard.
Controls
- Rate sets how fast the LFO sweeps, from 0.1 to 10 Hz.
- Depth sets how much of the octave range the sweep uses. At zero the filter stops moving and stays at the base frequency.
- Base Frequency is the centre of the sweep, from 20 Hz to 2 kHz. The cutoff moves the same distance above and below it.
- Octaves sets how wide the sweep is, from half an octave to eight.
- Waveform chooses the LFO shape: sine for a smooth swell, sawtooth for a ramp, square for a jump between two tones, triangle for an even back and forth.
- Mix blends the filtered sound with the dry one.
The sweep picture in the panel draws one cycle of the cutoff path from the current settings, with the lowest and highest cutoff it reaches. It shows the control law, not a measurement of the audio.
Getting started
Auto Filter is a mono audio effect: send it a guitar, a synth pad, a drum loop or any sustained sound. Start from the default settings, a one-hertz sine sweep of about two and a half octaves around 200 Hz, then try Square for a choppier, two-step pattern or raise Base Frequency to keep the movement off the low end.
