How do all pass filters work

Can someone explain how all pass filters work? And maybe give an explanation or link to a video on how to use them practically.

Thanks

https://lmgtfy.com/?q=https%3A%2F%2Fwww.youtube.com%2Fwatch%3Fv%3D5ka_14DkoYQ

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An all - pass filter is a signal processing filter that passes all frequencies equally in gain, but changes the phase relationship among various frequencies.

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I don’t think I can get passed 30 minutes… how do you use it in a practical application for music?

The math is too much for me

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I’ve never used one tbh, and i didn’t watch the lecture lol. It seems to relate to phase, for correcting phase irregularities in your mix for example. Probably worth starting by googling ‘phase cancellation’ if you don’t know how phase problems can affect your mixdowns.

all pass filters are very rarely used on their own when it comes to production afaik, i think their only real practical use is to adjust phase in things like crossovers since they don’t change frequencies amplitude but just the time of a signal

they are the foundation of most phaser effects as well

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Also, very important when designing reverb algorithms. An all pass filter plus a delay is the fundamental building block of most non-impulse based reverbs

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i did this at uni and made a reverb that sounded like a fart in a tin can but it was gratifying to a degree

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Even once you set up a good algorithm, a lot of the time in making a reverb sound good is spent in tweaking the constants behind the scenes. I’m sure there is a bunch of academic research on delay spacing for a good decay, but I’ve largely done it through trial and error.

The other component is feedback.