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"…modulation effects use slightly different means to manipulate the signal phase, the EQ and / or a time delay. It’s the sweeping of these parameters that earmarks the effects as ‘modulation effects’. While you could move these parameters by hand and achieve the effect, most commonly an LFO (low frequency oscillator) is employed to control the speed and depth of the sweeping. In this instance then, the LFO is the modulator, while the audible signal is the carrier. While flangers and chorus usually rely on delay to create this effect, the phaser is unique because it employs a chain of ‘all pass filters’ to generate frequency notches and peaks. Sweeping (moving) these notches / peaks creates the signature sound of a phaser.
…A phase-shifter (the proper name for a phaser) is made by taking an all-pass filter and automating it’s crossover frequency(the point at which the high and low passes meet, and where the phase-flipping occurs) so that it moves. Usually this movement is controlled by an LFO."
Very good tutorial on phasers: