The architecture of the allpass-based notch filter is shown in Fig. 3.14. It consists of a series connection of allpass filters with a feed-around. Thus, the delay line of the flanger is replaced by a string of allpass filters. (A delay line is of course an allpass filter itself.) The phaser will have a notch wherever the phase of the allpass chain is at (180 degrees). It can be shown that these frequencies occur very close to the resonant frequencies of the allpass chain. It is therefore convenient to use a single conjugate pole pair in each allpass section, i.e., use second-order allpass sections of the form
and is the radius of each pole in the complex-conjugate pole pair, and pole angles are . The pole angle can be interpreted as where is the resonant frequency and is the sampling interval.