Surge XT's comb filter delays the signal by a pitch-tuned amount to create resonant coloration and physical-model waveguides

The Comb filter types (Comb + positive feedback, Comb - negative feedback) in Surge XT play back a delayed version of the signal. The delay time is set by the Cutoff frequency control, with sub-sample precision. At resonance 0% and subtype 100% Wet, the comb filter acts as a pure delay unit. The negative feedback variant (Comb -) produces an output an octave lower than the positive version. Combined with the other filter unit and feedback from the Serial 2/3 filter configuration, comb filters enable simple physical modeling of wind instruments and plucked strings. Comb filters sit in the Effect category of filter types.

Examples

In Serial 2 filter config, set Filter 1 to Ladder LP, Filter 2 to Comb +, and add feedback — the comb creates resonant modes producing a waveguide-like flute or clarinet character.

Assessment

What determines the perceived pitch of a comb filter’s resonance? What is the sonic difference between Comb + and Comb - at the same cutoff setting?