An All-Pass Filter is a type of electronic filter that alters the phase of the frequencies of an audio signal without altering its volume.

While normal filters (such as high-pass or low-pass filters) attenuate or remove certain frequencies, the All-Pass Filter leaves the amplitude of the entire sound spectrum intact.

Its only effect is to delay different frequencies by different amounts.

Imagine that all the frequencies enter a race together.

The All-Pass Filter acts like a course that causes each "runner" (frequency) to reach the finish line at a slightly different time, changing its phase relationship with the others. Main Uses Phaser Effect: This is its most famous use. The phaser effect is created by mixing the original signal with a copy of itself that has passed through one or more all-pass filters. The phase changes cause cancellations and sums at certain frequencies, creating the characteristic "swirling" and "psychedelic" sound of the effect. Broadcast Processing: It is used in audio processors for radio and TV to correct asymmetries in the waveform of a signal. By "rotating" the phase of certain frequencies, it can make the wave more symmetrical, which allows you to slightly increase the overall volume of the audio before it starts to clip (distort).