Comb Filter

A comb filter is an effect that occurs when you join two different audio signals together with a tiny delay between them. Funny story: the term originated because its magnitude response looks like the teeth of a comb. It is very common when microphones pick up the same audio signal and end at the same output, for instance. This filter results in various arrival times for the same exact signal at the precise point of the audio circuit. 

Comb Filter: the Basics

The interval of the impulse response of any comb filter is either finite or infinite. For this reason, experts divide comb filters into two different categories. The first one is Finite Impulse Response (FIR); the second, Infinite Impulse Response (IIR). 

A comb filter is a Linear Time-Invariant digital filter, also known as LTI. To explain this, we will break the previous statement into sections. Firstly, the linear part means that the output to a scaled sum of input digital signals is equivalent to the scaled sum of the outputs to every one of these input signals. Secondly, the Time-Invariant part means that for any input signal, the output signal experiences the same delay as the input.

Comb Filters: the Audio Filters

Comb filters are a type of audio filters. Their response curve involves a series of notches that form a specific pattern. They generate unique alterations of input audio signals. These produce sounds that are nearly identical to the noise we can hear when we place a long cardboard tube near our ears. These filters are capable of creating diverse format effects. They, too, achieve the desired filtering process by delaying the input signal or by altering its phase. After this delay, you must mix the new signal with the original signal. A flanging effect or a phasing effect results when the comb filter suffers a modification, so that the notches are able to move through the frequency spectrum.

An Extra Piece of Information

The feedback comb filter is a type of comb filter. It’s a specific kind of IIR digital filter because feedback generates from the delayed output directly to the input.