Tell me about all-pass filters

[Jafo]

Technically speaking a comb filter is a filter that mixes a signal with a delayed copy of itself (with or without feedback). The result is a regular comb-like pattern in the spectrum, hence the name. Applications include simple physical models, flangers, resonances in some reverbs... and obviously delay lines...

But since an all-pass filter filters nothing -- everything passes -- what does it actually do? How does this differ from just a delay? What gets filtered?

[resynthesis]

It changes the phase of the signal (with different parts of the spectrum undergoing different phase alterations). Loosely, a frequency dependent delay

[MrBeagleton]

Technically speaking a comb filter is a filter that mixes a signal with a delayed copy of itself (with or without feedback). The result is a regular comb-like pattern in the spectrum, hence the name. Applications include simple physical models, flangers, resonances in some reverbs... and obviously delay lines...

But since an all-pass filter filters nothing -- everything passes -- what does it actually do? How does this differ from just a delay? What gets filtered?

When you mix a signal with an allpassed (or delayed) version of itself, you get frequency-dependent phase cancellation. Notice maximum cancellation (notch) when phase-shift = 180-degrees:

4-Pole Allpass Filter:


50/50 Dry + Allpass


1 ms delay


50/50 Dry + Delay