# 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.