I don't know why you guys think this is impossible.  The dbx unit, or copies thereof,  you see in small PA systems all the time works fine.  Not as good as an experienced engineer with a 1/3rd octave EQ, but good enough to be very useful to performers running sound from the stage or for ad hoc PA systems in small venues.

I don't know of an open source implementation, but the engineering problem itself is not intractable. 

On Sat, Oct 22, 2016 at 1:49 PM, Len Ovens <len@ovenwerks.net> wrote:
On Sat, 22 Oct 2016, Johannes Kroll wrote:

I'm looking for a solution for adaptive feedback cancellation. The type
of feedback that occurs between a microphone and speaker, not echo
cancellation. Preferrably in the form of a plugin (LADSPA, LV2,
DSSI, ...) or even as a standalone JACK app. This must be a common
problem, but I haven't found a solution.

 That would be because it requires a different solution every situation. What happens when the "fat lady" sings? The plugin nulls out her vocal thinking it is feedback. As happens, my acoustic instrument amp includes an anti-feedback section. It works in two parts:
polarity reversal
notch filter

The polarity of the FOH/monitor speakers does matter in this case. Piezo pickups are bad in this respect because the polarity may be different depending on mounting position (inside vs outside of sound plate). Mics _should_ be wired corectly and a FOH mixer/amp/speakers _should_ be too. Those are things to check. I have seen FOH mixers where the main outs are purposely 2- and 3+ for this reason. I don't know if this is standard or not.

Aim for as low of stage monitor level as possible. Setting mains level first may allow talent to use lower stage levels... but some are just deaf! In ear would be better in such a case. Note that I am using my (limited) experience with small venue work. A large venue is different.

Notch filters are the thing we think of first, but should probably be last. As already noted, this will not give you much more level and it does change the sound. The frequency to use changes with the hall, mic placement, talent mic know how, etc, etc. (teach your talent if they must hold the mic to hold it away from the capsule)

The other thought I have is that maybe you mean something to remove feedback noise on an already recorded file... notch filter is the tool for that... but it is really already too late by then.

--
Len Ovens
www.ovenwerks.net


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