Tell me about all-pass filters

[DJ Warmonger]

Can you just tell me, or recommend some resources about all-pass filters?

• How they are made
• How they can be tuned / modulated
• Plugins which nicely demonstrate all-pass behavior, or just the sound?

Or anything else that comes up.

[rd2play]

Check this out for some good info https://thewolfsound.com/allpass-filter/

[mystran]

[*] How they are made

A filter is all-pass when it's magnitude response is unity. If we place the poles and zeroes of a continuous time filter as mirror images across the Laplace imaginary axis (ie. they differ only by the sign of their real components) or the poles and zeroes of a discrete time filter as reciprocals of each other (ie. essentially mirror images across the unit circle on the Riemann sphere), then their magnitude response cancels out, but the phase response does not.

[*] How they can be tuned / modulated

The same way any other filter really.

[*] Plugins which nicely demonstrate all-pass behavior, or just the sound?

The sound depends a lot on what kind of all-pass filter we are talking about, but in general they "smear" or "disperse" the sound in one way or another without changing the spectral balance.

[dmbaer]

Can you just tell me, or recommend some resources about all-pass filters?

• How they are made
• How they can be tuned / modulated
• Plugins which nicely demonstrate all-pass behavior, or just the sound?

Or anything else that comes up.

Answering in reverse order:

Phaser effects are the purest example of all-pass filter application. Multiple all-pass filters with spaced center frequencies are applied to a signal and the result is mixed with the original signal result in comb filtering.

The all pass filters in the phaser normally will then have their frequencies modulated, producing the spacey movement in the sound.

You don't want to know how they are implemented unless you've learned beaucoup amounts of fairly advanced mathematics.

[Full Bucket]

The blooo (and ModulAir too) features allpass filters. Checkout preset 7 "Allpass Magic".

Cheers, Björn

[MrBeagleton]

Phaser effects are the purest example of all-pass filter application. Multiple all-pass filters with spaced center frequencies are applied to a signal and the result is mixed with the original signal result in comb filtering.

Technically not comb filtering, but a notch or series of notches.

[nix808]

I vaguely conceive,
as I am terrible with filter dsp
-that the all pass is used to affect phase, not frequency(or lesser affect frequency than phase)
to chime in
Phase can be displayed as a filter curve, less time shift downward or upward afai have scoped a bi-quad build

thinking delays then

[dmbaer]

Phaser effects are the purest example of all-pass filter application. Multiple all-pass filters with spaced center frequencies are applied to a signal and the result is mixed with the original signal result in comb filtering.

Technically not comb filtering, but a notch or series of notches.

So what's a formal definition of comb filtering? Does it imply the entire signal is altered in phase due to a delay before being recombined with the original? Or does it allow for selective phase variations depending upon frequency? Or is this just a loose term we use that really has no formal definition?

[Music Engineer]

So what's a formal definition of comb filtering? Does it imply the entire signal is altered in phase due to a delay before being recombined with the original? Or does it allow for selective phase variations depending upon frequency? Or is this just a loose term we use that really has no formal definition?

The difference between phasers and flangers is the distribution of the peaks and notches along the frequency axis. In a flanger, these are equally spaced forming a harmonic series such that the spectrum looks like a comb. That's why a (static) flanger is also called a comb filter. In a phaser, the spacing is unequal - if a comb for hair would feature unequal spacing of the teeth, it would be a very strange comb indeed and you wouldn't probably call it a comb or even recognize it as one. Technically, one could perhaps say that a flanger is a very special kind of phaser in which the general allpass is replaced by a pure delay. A delay actually *is* also an allpass, albeit a very special one - namely, one with a linear phase response.

[mystran]

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...

[dmbaer]

The difference between phasers and flangers is the distribution of the peaks and notches along the frequency axis. In a flanger, these are equally spaced forming a harmonic series such that the spectrum looks like a comb. That's why a (static) flanger is also called a comb filter. In a phaser, the spacing is unequal - if a comb for hair would feature unequal spacing of the teeth, it would be a very strange comb indeed and you wouldn't probably call it a comb or even recognize it as one. Technically, one could perhaps say that a flanger is a very special kind of phaser in which the general allpass is replaced by a pure delay. A delay actually *is* also an allpass, albeit a very special one - namely, one with a linear phase response.

Thanks for the detailed response, but maybe I didn't ask the right question. I understand the difference between flangers and phasers. Flangers do their thing with delay, and that gets you evenly spaced (on a non-log x-axis) peaks and valleys. The spacing on a phaser is up to the designer, is it not? If we have, say, a phaser with a dozen evenly spaced all-pass filters, does that qualify as comb-filtering? How many teeth have to be present for something to be called a comb? Thus my query as to whether there is a formal definition of comb filtering. Is the delay a necessary part of the deal?

[mystran]

Thus my query as to whether there is a formal definition of comb filtering. Is the delay a necessary part of the deal?

Delay is part of the deal, see https://en.wikipedia.org/wiki/Comb_filter

[DJ Warmonger]

I've got another related question: what is the difference between flanger and phaser? I've seen these terms used interchangeably?

[MrBeagleton]

I've got another related question: what is the difference between flanger and phaser? I've seen these terms used interchangeably?

I feel like the difference was already explained well enough above, but maybe a demonstration?

https://drive.google.com/file/d/1Vp8bvX ... LFvSb/view

I showed both with and without feedback.

If you compare the spectrums discerningly you'll see the notches are spaced differently. And with a phaser you can control the number of notches by varying the filter order.

[Unaspected]

I've got another related question: what is the difference between flanger and phaser? I've seen these terms used interchangeably?

Already answered in this thread: Flanger uses a delay line and sums this with the original signal; Phaser uses a bank of APFs. Both employ modulation to complete the effect. In the case of the Flanger, the delay time is modulated; whereas the cutoff frequencies of the APFs are modulated to create a phaser.

They sound quite different and distinct to my ears so I'm not sure why they get muddled up - though I know they do.

Flanging effect was originally created by physically manipulating the flange on a tape machine whilst simultaneously playing a copy in parallel with another. They would first be synced and started at the same time and then one was nudged out of time; nudge the other to bring them back to equal time and repeat as/if needed. Usually credited to George Martin.

I'm not sure about the origins of the phaser. I always preferred the sound of slow moving or static flangers (basically a comb filter when static - as mentioned above) with some feedback - though phasers are useful for watery, liquid type effects.

All pass filters can be interesting when summed and inverted in parallel. [By which I mean, play around with straight summing and inverting and summing...] You can make various types of filters this way and then morphing between settings can produce interesting results.

APFs can also be useful in feedback loops - and, of course, for creating reverbs. When used sparingly, I quite like the smearing that APFs create to add subtle depth.

And, of course, they can be employed as fractional delays to solve all kinds of issues. Some more technical but the most common is likely phase rotation.

EDIT: []