A comb filter is a device which acts to cut off a group of frequencies within a range of frequencies. It could be thought of as several notch filters acting together. This filter causes a frequency response curve that resembles a comb--the attenuated frequencies are spaced at regular intervals. Basically, the comb filter is created by attenuating a feedback loop.

A comb filter applied to an audio signal can cause a sort of hollow or ringing sound. Modulating a comb filter causes a flanging effect. Comb filters were used in the early days of stereophonics to give mono recordings a "wider" sound, thus faking stereo separation.

Apparently, comb filters are also used in color video to seperate luminance (Y) signals from chrominance (C) signals. But don't ask me about that. I'm already in over my head goin' on about the use of comb filters in audio. Honestly, I only mention video so I could use the word chrominance.

How to build a comb filter

Comb filtering can be achieved quite easily with the use of a signal delay unit. When a delayed copy of a signal is added to its original, certain frequency components will cancel themselves out because of the phase difference. Others will reinforce themselves in a similar fashion.

When the delay time t is equal to one millisecond the frequency component of the delayed signal that has a period of two milliseconds (that is 500 Hz) will be out of phase by 180 degrees with the corresponding frequency component of the original signal. When the delayed signal is added to the original signal this frequency will disappear completely from the spectrum. In fact, since phase is modular, so will all frequency components for which a one millisecond delay means a shift of 1.5 periods, or 2.5 periods, and so on. Those frequencies are three, five, seven, etc. times the lowest frequency that is cancelled out. In this example 1500 Hz, 2500 Hz, 3500 Hz, etc.

Similarly, positive reinforcement occurs at frequencies that are phase shifted 360 degrees. For a one millisecond delay they are 1000 Hz, 2000 Hz, 3000 Hz, etc. As this pattern suggests the peaks and notches are alternating and they are evenly spaced across the spectrum. A plot of the frequency response of this system will thus look like a comb.

Flangers and chorus devices are constructed like this. They sometimes utilize a feedback loop to 'sharpen' the teeth of the comb. Their delaytime is modulated with a LFO so the teeth of the comb are swept through the spectrum to produce a 'swooshing' effect.

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