Tell me about all-pass filters

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DJ Warmonger
KVRAF
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4577 posts since 7 Jun, 2012 from Warsaw

Post Sun Nov 27, 2022 7:52 am

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.
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rd2play
KVRer
11 posts since 17 Nov, 2022 from USA, SoCal

Post Sun Nov 27, 2022 8:18 am

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

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mystran
KVRAF
7331 posts since 12 Feb, 2006 from Helsinki, Finland

Post Sun Nov 27, 2022 1:11 pm

DJ Warmonger wrote: Sun Nov 27, 2022 7:52 am [*] 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.
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dmbaer
KVRAF
1592 posts since 11 Nov, 2009 from Northern CA

Post Sun Nov 27, 2022 1:17 pm

DJ Warmonger wrote: Sun Nov 27, 2022 7:52 am 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.

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Full Bucket
KVRian
875 posts since 25 May, 2010 from Hessisch Uganda, Germany

Post Wed Nov 30, 2022 8:35 am

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

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MrBeagleton
KVRist
43 posts since 5 Nov, 2015

Post Wed Nov 30, 2022 3:05 pm

dmbaer wrote: Sun Nov 27, 2022 1:17 pm 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.

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nix808
KVRAF
4843 posts since 22 Jul, 2006 from Tasmania, Australia

Post Thu Dec 01, 2022 2:54 pm

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
KVRAF
1592 posts since 11 Nov, 2009 from Northern CA

Post Fri Dec 02, 2022 1:44 pm

MrBeagleton wrote: Wed Nov 30, 2022 3:05 pm
dmbaer wrote: Sun Nov 27, 2022 1:17 pm 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?

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Music Engineer
KVRAF
4180 posts since 8 Mar, 2004 from Berlin, Germany

Post Fri Dec 02, 2022 2:00 pm

dmbaer wrote: Fri Dec 02, 2022 1:44 pm 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.
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mystran
KVRAF
7331 posts since 12 Feb, 2006 from Helsinki, Finland

Post Fri Dec 02, 2022 3:24 pm

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...
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dmbaer
KVRAF
1592 posts since 11 Nov, 2009 from Northern CA

Post Sat Dec 03, 2022 12:17 pm

Music Engineer wrote: Fri Dec 02, 2022 2:00 pm 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?

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mystran
KVRAF
7331 posts since 12 Feb, 2006 from Helsinki, Finland

Post Sat Dec 03, 2022 12:31 pm

dmbaer wrote: Sat Dec 03, 2022 12:17 pm 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
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DJ Warmonger
KVRAF
Topic Starter
4577 posts since 7 Jun, 2012 from Warsaw

Post Sat Dec 03, 2022 12:32 pm

I've got another related question: what is the difference between flanger and phaser? I've seen these terms used interchangeably?
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Tricky-Loops wrote: (...)someone like Armin van Buuren who claims to make a track in half an hour and all his songs sound somewhat boring(...)

MrBeagleton
KVRist
43 posts since 5 Nov, 2015

Post Sat Dec 03, 2022 1:19 pm

DJ Warmonger wrote: Sat Dec 03, 2022 12:32 pm 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
KVRAF
3177 posts since 4 May, 2012

Post Sat Dec 03, 2022 1:23 pm

DJ Warmonger wrote: Sat Dec 03, 2022 12:32 pm 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: []
Last edited by Unaspected on Sat Dec 03, 2022 1:38 pm, edited 1 time in total.

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