Comb Filter

Mixes a signal with a short, delayed copy of itself, stacking evenly-spaced peaks and notches across the spectrum — the metallic, resonant, flange-like ring you hear when sound bounces between close surfaces. With delay times from 1 to 50 ms it sits in the audible range, turning any source into a tuned, hollow-bodied tone.

How it works

The delayed copy interferes with the dry signal, reinforcing some frequencies and cancelling others at regular intervals — the "teeth" of the comb. Shorter delays raise the pitch of that resonance; longer ones lower it. Feeding part of that delayed copy back into the line — set by Reso — turns the flat, flange-like filter into a ringing, pitched resonator that sustains. The delay line is interpolated, so you can sweep the time smoothly without zipper noise or clicks.

Controls

  • Time — the delay length, and so the pitch of the comb: short rings high and bright, long sits low and hollow.
  • Fdbk — how much of the delayed signal recirculates, deepening the resonance toward a sharp, pitched ring. It's bipolar — positive emphasises the evenly-spaced peaks, while negative shifts the pattern for a thinner, more nasal character.
  • Reso — how much of that delayed copy feeds back into the line, setting how resonant and long-ringing the comb is. Near 0 it's a flat, single-notch feedforward comb — hollow and flange-like, no sustain; sweep it up toward 1 and the line starts to ring, blooming into a strongly pitched resonator that sings. The midpoint is the classic, mildly-ringing comb tone.
  • Mix — the wet/dry blend, from 0% bone-dry to 100% fully combed. Pull it back to keep a ringing line present but understated, tucked under the original signal rather than replacing it.

Inputs & outputs

  • In L / In R — the signal to colour; patch audio for tone, or CV for a filtered slope. Mono? Just use L — one input is combed once and sent to both outputs. Patch a stereo pair and each side gets its own delay line through the shared knobs, so a wide source stays wide through the resonance.
  • Time / Fdbk / Reso / Mix CV — voltage control over each knob; patch an LFO into Time for flanging or an envelope for a pitched chirp, sweep Fdbk for rhythmic comb swells, or open up Reso and Mix for evolving resonance. Time tracks a ±5 V signal as a ±50% delay sweep.
  • Out L / Out R — the combed signal. With stereo in but only L cabled, the channels fold to mono automatically so nothing goes missing.

Tips

  • An LFO into Time CV gives classic flanging — slow and shallow for gentle movement, faster for a vibrato-like warble.
  • Push Fdbk toward its extremes for a strong, pitched resonance, then sweep Time (or its CV) to play the comb like a string.