[PD] feedback control

matthew jones mj at isvr.soton.ac.uk
Mon Mar 21 15:42:58 CET 2005

Hi, thanks for that feedback Sven,
I'm just curious as to how some piece of external hardware would know what 
is feedback and what is, say, just an electric guitar holding a sustained 

I suppose if you were to set it all up first when nobody was playing, then 
anything 'new' could be left untouched.
But I doubt these behringer units are designed like that?

By the way, any other musos out there both thankful and a little bemused 
that behringer can make so many units at such little cost??  I mean it's 
unbelievable what their condenser mics are going for, for example...


----- Original Message ----- 
From: "sven" <ml.sven at subscience.de>
To: "matthew jones" <mj at isvr.soton.ac.uk>; "PD-List" <pd-list at iem.kug.ac.at>
Sent: Monday, March 21, 2005 2:15 PM
Subject: Re: [PD] feedback control

> At 13:48 21.03.2005, matthew jones wrote:
>>>if so multiply that bands which an adjustable value <1
>>but wouldn't that have weird effects when the signal you are attempting to
>>remove the feedback from is the sound of you whistling??
>>Or is this method the one that many feedback supressors use?  I thought it
>>was something like mimicking the effect of constantly moving the mic 
>>so that the frequencies at which the system is unstable actually move
>>around, done via slow variable delays or something like that.  Thats just 
>>guess though.
> well, ofcourse a feedback suppressor would effect the sound. just as any 
> noise/click or
> whatever suppressor also filters some of the wanted sound.
> the fft idea was just a very simple solution. a better one would be to 
> find the
> freq at what the feedback happens and then filter w/ a very narrow (high 
> q) notchfilter (FIR)
> let's say feedback freq is 2khz, you filter 2khz -/+ 10Hz (hanningwindowed 
> or similar)
> so you filter a band which is a most 20Hz wide, which is just 1/50 (down) 
> or 1/100 (up) of an
> octave. the process is bearly hearable.
> and ofcourse you don't need to cut out the band completely.
> the suppressing factor just had to be a little lower than the speed with 
> which the feedback builds up.
> but before thinking about feedback suppression you should always try to 
> setup
> your equipment that feedbacks can't happen.
>>You could use an adaptive line enhancer, if it was feasible to regularly
>>calculate the inverse of a relatively large matrix and apply long FIR
>>filters... tho on a laptop this is maybe a bit too hopeful.
> sounds too computationally expensive. filtering w/ a simple notch should
> be enough. what's important is just to find the frequency of the feedback
> as exactly as possible.
>>The true way would be to keep the mic fixed in place and measure the
>>response from loudspeakers to the mic, then calculate the optimal response
>>to cancel the signal as it arrives at the microphone with a second source
>>(maybe the right side of a stereo pair) and use an adaptive filter..... 
> if you keep the mic fixed in place and the speakers are not moving it 
> should
> be easy to set it all up that feedback won't happen anyway.
>>as Thomas says it would require near zero latency and heck, a lot of cpu
> the latency is the biggest problem imho. just did a quick test with 
> testtones
> and guessing the right frequency with some accurary from the fft needs
> a window of at least 512 samples which is ~ 11ms @ 44.1kHz/s.
>>Any ideas/improvements anybody?

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