Comb Filter

Filters

1. Frequency Knob Sets the filter’s root frequency.
2. Fine Knob Fine tunes the filter’s root frequency.
3. Amount Knob Controls the effect depth. No effect when the knob is centered.
4. Mode Switch Selects between feedforward or feedback filter topologies.
5. Trim Knob Adjusts output volume.
6. Audio Input Signal to be filtered (stereo).
7. Audio Output Filtered signal (stereo).

Overview ⚓︎

A comb filter alters a sound by adding peaks and notches in a harmonic series determined by the filter frequency. Depending on the Mode and Amount settings, this effect can range from subtle coloration to metallic or flanging textures.

In Depth ⚓︎

Feedforward Comb Filter ⚓︎

A feedforward comb filter works by mixing an audio signal with a delayed version of itself, creating constructive and destructive interference that shapes the sound.

The delay time is adjusted with the Frequency knob. The higher the frequency setting, the shorter the delay time.

The Amount knob controls the gain of the delayed signal.

When the gain is positive (upper range of the Amount knob), frequencies that are multiples of the comb filter’s frequency setting are slightly boosted over a wide band, while frequencies midway between these multiples are strongly reduced around a narrow band, creating a pattern of peaks and notches that look like a comb (hence the name).

When the gain is negative (lower range of the Amount knob), the opposite effect occurs: frequencies that are multiples of the comb filter’s frequency setting are strongly reduced around a narrow band, while frequencies midway between these multiples are slightly boosted over a wide band, creating an inverted pattern of peaks and notches.

Feedback Comb Filter ⚓︎

A feedback comb filter works by feeding a delayed version of the audio signal back into itself, creating resonances that emphasize or suppress specific frequency series.

As in the feedforward topology, the delay time is adjusted with the Frequency knob.

The Amount knob controls the gain of the feedback signal.

When the gain is positive (upper range of the Amount knob), frequencies that are multiples of the comb filter’s frequency settings are strongly boosted over a narrow band, while frequencies midway between these multiples are reduced over a wide band. Higher gain increases the resonance of these frequency peaks.

While the Amount knob can go to 100%, we recommend staying below 90% in feedback mode to avoid excessive resonance and potential distortion.

When the gain is negative (lower range of the Amount knob), frequencies that are multiples of the comb filter’s frequency setting are reduced over a wide band, while frequencies midway between these multiples are strongly boosted over a narrow band.

Gain Staging ⚓︎

Depending on the harmonic content of the source signal and the comb filter settings, the input and output signal levels may vary significantly. Adjust the Trim knob to compensate for these differences.

This is especially important in feedback mode where frequencies in the input signal that align with a comb filter peak can be heavily amplified at high Amount settings.