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## Making and using all-pass filters

Figure 8.33: All-pass filters: (a) making an all-pass filter from elementary filters; (b) using four all-pass filters to build a phaser.

Example H14.all.pass.pd (Figure 8.33, part a) shows how to make an all-pass filter out of a non-recirculating filter, second form (`rzero_rev~`) and a recirculating filter (`rpole~`). The coefficient, ranging from -1 to 1, is controlled in hundredths.

Example H15.phaser.pd (part b of the figure) shows how to use four all-pass filters to make a classic phaser. The phaser works by summing the input signal with a phase-altered version of it, making interference effects. The amount of phase change is varied in time by varying the (shared) coefficient of the all-pass filters. The overall effect is somewhat similar to a flanger (time-varying comb filter) but the phaser does not impose a pitch as the comb filter does.