PHASER

This is similar to the flanger but with the phaser it takes part of the signal it then shifts the phase and then remixes it self back in to the original signal, When these two signals are mixed frequencies are cancelled out this creating a unique effect.

A all pass filter is a filter that have a flat frequency response, they don't effect any part of the frequency spectrum.

The character of the phaser is shaped by the filter stage, the phaser shares a lot of similar controls as the flanger.