combfilter - is a filter implemented by adding a delayed version of a signal to itself, causing constructive and destructive interference. The frequency response of a comb filter consists of a series of regularly spaced notches, giving the appearance of a comb. Comb filters exist in two forms, Infinite Impulse Response and Finite Impulse Response; which refer to the direction in which signals are delayed before they are added to the input.

A difference equation describing the feedback comb filter can be written in “direct form 1”:

The feedback comb filter is a special case of an Infinite Impulse Response (IIR) (“recursive”) digital filter, since there is feedback from the delayed output to the input.