This effect splits a signal into two copies. One copy is fed through an “all pass filter” which does not attenuate any of the original harmonics like a low pass or high pass filter does, but which does alter the phase of the signal, causing those harmonics to have varying amounts of phase shift in relation to the original depending on their frequency. Mix these two copies back together, and different harmonic components of the original sound cancel each other out (see Phase), resulting in a notch filter effect. Each “stage” – all-pass filter section – of a phase shifter creates one of these notches. More stages create more notches, and a deeper effect.
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