[PD] phase-canceling

Nicholas Mariette nicholas.mariette at limsi.fr
Thu Feb 26 15:07:39 CET 2009


On Feb 25, 2009, at 1:00 AM, mrz wrote:

> yes,i find it also not an absurd question.
> I had build a loop machine, and i did want to phase cancel the sound  
> is coming out of the speakers to be able recording and play again  
> new stuff over that loop in "realtime" (overdubbing). But as far as  
> i understand it is a really complex thing to do so as chuck allready  
> mentioned. But someone build allready a kind of "feedback canceller"  
> or is it simply not possible in the real world?
> Like, it's the best way simply EQ'ing your Soundsystem to have the  
> best result?

Best to do as bands do for singers or any other instrument when  
feedback is not desired... use a cardioid microphone with low  
sensitivity, and make the desired input signal much closer to the  
microphone than the speakers.  If possible, don't face the speakers  
into the microphone at all.  Even to achieve *some* feedback  
cancellation is very difficult, and true cancellation is impossible  
(except maybe theoretically for a totally known electro-acoustical  
system, with same number of emitters and receivers).  Feedback  
avoidance is slightly more possible (e.g. by small frequency shifts).   
At best, feedback suppression by narrow band filtering can only get  
you a few extra db of gain before feedback pops up somewhere else.
Nick


> all the best,
> moritz
>
> On Mon, Feb 23, 2009 at 7:19 PM, Charles Henry <czhenry at gmail.com>  
> wrote:
> Hi, yohannes
>
> No, it's not absurd.  Tell us a little bit more about your
> application.  I am out of the project game for now, but have some long
> term goals for solving this sort of problem.
>
> For example, you send a sound out your speakers.  You want to recieve
> a new sound from the room on microphone that is not the sound coming
> from the speakers.
>
> This becomes a system identification problem.  You need to find the
> delay between the speakers and microphone and the transfer function (a
> filter) between speakers and mic.  Then, you digitally apply the
> filter and delay to your signals as they would be played and subtract
> that copy from the signal received by the mic.
>
> Chuck
>
>
>
> On Mon, Feb 23, 2009 at 4:02 PM, yohannes <THIS_IS_POP at web.de> wrote:
> > hello everybody,
> >
> > maybe sounds absurd:
> >
> > is it possible to cancel an whole signal of an mic input through
> > phase-canceling?
> >
> > if yes, how can i do dat in pd?
> >
> >
> >
> > thanks a lot, yohannes
> >
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