A filter whose response curve contains a series of notches in a repeating pattern. Such filters produce a characteristic modification of an input audio signal, resulting in a sound similar to what might result if one were to listen with a long cardboard tube placed to one’s ear.Comb filters are seldom produced using discrete notch filters for each notch; the easy way to do such filtering is to either delay the input signal or alter its phase, and then mix this in some proportion with the original signal. If such a filter is varied over time, so that the notches move up and down the frequency spectrum as a group, an effect known as flanging or phasing results. Comb filters are also sometimes used to create formant effects.

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