How to use the all-pass filter?

[bbaggins]

MDynamicEQ and MAutodynamicEQ feature an all-pass filter type, but I've no idea what to do with it. Any suggestions?

[Moonwrist]

Allpass filters are used to manipulate the phase of the signal.
http://www.uaudio.com/blog/allpass-filters/

[MeldaProduction]

Exactly . In most cases you won't need them, but if you have a problem with phase (when mixing 2 multimiked signals for example) it may be theoretically handy.

[CableChannel]

Allpass filters are used to manipulate the phase of the signal.
http://www.uaudio.com/blog/allpass-filters/

Interesting read.

But I don't here a difference

What's the secret? I want to have pro ears, I want to hear the difference. Can you give me a hint on what to listen for?

[MeldaProduction]

Well, you probably won't hear a difference!!

The thing is, brain pretty much ignores phase, unless the differences are huge. Open MOscillator, switch to harmonics mode, put there a few harmonics and play with phase. You WILL hear a difference. When you start messing with allpass filter, you probably won't.

But try this:
Take 2 exactly same tracks, and just "mix" them. They will just add up to each other. Then put MAutoDynamicEq on one of them and use one allpass filter. As you move it, you will hear the difference - the changed phase will cause the 2 tracks to sum incorrectly, because of the phase alteration. This is pretty much useless (though it is a base for phaser effects), but you may need it to fix that issue (so kind of backwards). Imagine you duplicate the tracks and use an eq on one of them, but then the phases start to be a problem, so you can use allpass filter to compensate. Well, not that I would ever do that , but some people apparently do .

[Moonwrist]

Or listen to a single drum track. Kick, snare or tom.
Put eq and then after some level meter. Your daws meters could be enough if it has peak hold.
Now move all-pass band on and off of the drums key frequency. Kick 50-80, snare 180-220, tom 80-160.
You probably won't hear a big difference but you should see a clear change in the level meter.

And like Vojtech wrote it can be used to mix two signals. Phase problems are easier to detect and hear in low frequencies.
For example if you have DI signal and mic signal of a bass and they don't mix well together you can try all-pass filter.

But brain do notice phase or phase difference between your ears.
Brain detect the direction of the sound with phase in low frequencies and with time in higher frequencies.

[MeldaProduction]

Right! I forgot about the phase difference between ears!

But rather don't exploit his to create artificial stereo, it's really not mono compatible.

[bbaggins]

I was reading this thread http://www.kvraudio.com/forum/viewtopic.php?p=5346081

It's talking about all-pass filters, particularly a plugin called Disperser from kilohearts, which is an all-pass filter. In that thread, its developer offered suggestions for how to use the plugin:

* Use the Disperser on a staccato bass line with a high frequency content for a more interesting bass sound.
* Use it on a drum loop, either as a transient shaper (medium Amount setting), or as a way to make the drums sound louder and bigger (high Amount setting). Use it before a compressor for more even more loudness.
* Put it with a low amount setting on a problematic track with a high crest factor to reduce the peaks.
* Chain it before a distortion. Keep the Amount quite low and play around with the Frequency and Pinch to change the timbre of the distorted sound. Automate to taste for dubstep.
* Experiment. It probably has lots of other uses that we haven't discovered yet.

In my experiments, I did observe a crest factor reduction of 3-4 dB, but I don't know why I wouldn't just use a limiter to achieve the same end, but with greater precision.

I also tried beefing up the low end of a drum bus, using multiple APFs in MDynamicEQ but all I could achieve was a bit of hollowness, not any desirable enhancement.

I just couldn't come up with any scenario where messing with phase was helpful.

You normally only care about phase when blending identical signals, which led to another question: why isn't there comb filtering when you use MDynamicEQ's Wet/Dry control?

[CableChannel]

I was reading this thread http://www.kvraudio.com/forum/viewtopic.php?p=5346081
You normally only care about phase when blending identical signals, which led to another question: why isn't there comb filtering when you use MDynamicEQ's Wet/Dry control?

That same question occured to me.

Interesting also: just tested this in MFilter and MEqualizer: for Highpass or Lowpass, the Dry/Wet control does not work continuously but like an on/off switch. Only for a peak filter type it works continously. Same with AllPass: it's only on/off.

[bbaggins]

I think I've answered my own question. The wet/dry parameter is not a conventional mix control, but rather modulates the filters. You aren't really mixing in the dry signal at all. I think the "wet/dry" terminology was chosen just because it's a familiar term and users would understand the concept.

[MeldaProduction]

Yes! The dry/wet is not a dry/wet. It's impossible for a minimum phase eq to do that, so we "alter" the filters to approximate the dry/wet .

[DDDPG]

Yes! The dry/wet is not a dry/wet. It's impossible for a minimum phase eq to do that, so we "alter" the filters to approximate the dry/wet .

Hi I am fresh with the audio engineering, would you like to tell that why it's impossible for minimum phase process to control the drt/wet control? Is this caused by the Phase alignment step when mixing dry and wet signal?

[dmbaer]

Yes! The dry/wet is not a dry/wet. It's impossible for a minimum phase eq to do that, so we "alter" the filters to approximate the dry/wet .

Hi I am fresh with the audio engineering, would you like to tell that why it's impossible for minimum phase process to control the drt/wet control? Is this caused by the Phase alignment step when mixing dry and wet signal?

More or less, that's right. How does a phaser effect work? It applies one or more all-pass filters to a signal and mixes the dry and the wet. So, unless you want a phaser effect, conventional dry/wet mixing would not produce desirable results.

[DDDPG]

Yes! The dry/wet is not a dry/wet. It's impossible for a minimum phase eq to do that, so we "alter" the filters to approximate the dry/wet .

Hi I am fresh with the audio engineering, would you like to tell that why it's impossible for minimum phase process to control the drt/wet control? Is this caused by the Phase alignment step when mixing dry and wet signal?

More or less, that's right. How does a phaser effect work? It applies one or more all-pass filters to a signal and mixes the dry and the wet. So, unless you want a phaser effect, conventional dry/wet mixing would not produce desirable results.

Thanks for reply. But I still interested in the implementation of the dry/wet mix function in eq, comp, saturation.... When applying the all-pass filter, what's the next step? Do you have some recommendation for articles/codes/examples for it?

[dmbaer]

Thanks for reply. But I still interested in the implementation of the dry/wet mix function in eq, comp, saturation.... When applying the all-pass filter, what's the next step? Do you have some recommendation for articles/codes/examples for it?

Well, as far as I'm concerned all-pass filters are unneeded in all but unusual scenarios. The most interesting application I've encountered (apart from within phasers) is one demonstrated in a tutorial about FabFilter's EQ plugin (or maybe it was their filter plug-in). Using an all-pass filter with some tube saturation, the track is not audibly different due to the filtering/phase-shifting, but the filter's tube quality turns the filter into a lovely source of tasteful saturation. This is found in a Dan Worrell (sp?) video tutorial - anything that fellow does is worth your time, so check the FabFilter site if interested.