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 [notch|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].

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