A phaser, or phase shifter, is similar to a flanger in that it sweeps notches through the spectrum of an input signal. But while a flanger provides only uniformly spaced notches, a phasor can modulate the frequencies of non-uniformly spaced notches.
A phaser is implemented with allpass filters instead of delay lines, as shown in the block diagram of Fig. 3.
Figure 3:
A digital phaser block diagram.
Second-order allpass filters are particularly convenient to use because each can control a separate notch frequency and bandwidth. Second-order allpass filters have a difference equation given by
where
where is the center frequency of the notch, is the sample period, and The closer is to 1.0, the narrower the bandwidth of the notch.
The phaser will have a notch wherever the phase of the allpass chain is at (180 degrees). This happens close to the center frequencies of each allpass section.
The instantaneous frequency response of a phaser created using 4 second-order allpass filters with notch frequencies set at 300, 800, 1000, and 4000 Hz and r = 0.9, 0.98, 0.8, and 0.9 is shown in Fig. 4.
Figure 4:
Instantaneous frequency response of a phaser created with 4 second-order allpass filters and notch frequencies set at 300, 800, 1000, and 4000 Hz.
The depth of the notches can be varied together by changing the feedforward gain parameter .
To achieve the time-varying “phasing” effect, the notch frequencies are modulated with a periodic signal. Note that only a single filter coefficient need be changed in each allpass section to accomplish this.