[PD] feedback control

[matthew jones]

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 
note...

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...

cheers,
Matt

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----- 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 
>>around
>>so that the frequencies at which the system is unstable actually move
>>around, done via slow variable delays or something like that.  Thats just 
>>a
>>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..... 
>>but
>
> 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
>>power!
>
> 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?
>>
>>Matt
>