[PD] feedback detection

[Hans-Christoph Steiner]

On Jul 31, 2006, at 6:43 AM, Jamie Bullock wrote:

>
> There is an cross-correlation class [cc~] in the flib library,  
> which you can use to do auto-correlation. It has both a time domain  
> and a frequency domain CC function. I found the output of the time  
> domain implementation to be easier to interpret.

I've forgotten all my DSP math, could you enlighten me as to how I  
might use this?

> There are also number of other feature extraction objects in flib  
> that might help with what you need, including [peak~], which does  
> the same as [peakit~] except that with [peak~] the magnitude  
> spectrum isn't calculated by the object, so you can precalculate  
> the power spectrum, log mag spectrum, cepstrum and pass it to the  
> object. With [peak~] you can also set a threshold as a percentage  
> below the maximum peak found.

I checked out [peak~], it looks like its outputting data similar to  
what I am looking for, but in a strange format.  How can I get just  
the frequency of the highest peak?  Also, I would like to have much  
higher resolution.  Is that possible?

> You might find [bmax~] useful, as it returns the bin numbers of the  
> n most prominent partials, I think similar to ekext/[hasc~]. (ekext/ 
> [hssc~] might also be useful, but you have probably already tried).

I played a bit, and didn't really get anywhere.  More on that later...

> Also you might want to try flib/[irreg~], flib/[ss~] which measure  
> spectral irregularity and spectral smoothness respectively.

I will play with those in the future, for sure.

.hc

> I guess it depends how prominent you are expecting the feedback to  
> be, and how it is distributed across the spectrum.
>
> Let me know how you resolve this issue, as I am very interested in   
> this.
>
> HTH,
>
> Jamie
>
>
> BTW: sounds like a good installation!
>
> On Mon, 31 Jul 2006 01:01:56 -0400
> hans at eds.org wrote:
>
>>
>> This isn't a standard feedback suppression system that I am after.  I
>> want to be able to play with the feedback while having the ability to
>> prevent it from taking over.  Its part of an interactive installation
>> I am working on, the next generation of this:
>>
>> http://at.or.at/hans/swirler
>>
>> The autocorrelation stuff sounds interesting, plus I just found Ed
>> Kelly's [peakit~], which gives amplitude info of FFT channels.
>>
>> .hc
>>
>>
>> On Jul 31, 2006, at 10:23 AM, padawan12 wrote:
>>
>>>
>>> Feeback tends toward periodicity at 1/t for a delay of t
>>> so you could do an autocorrelation (expensive - does Pd
>>> have an object to help with that?) of te feedback signal
>>> to look for it. Since you know the frequency in advance
>>> a simple bp filter is more effective. Not sure what you mean by
>>> "dominant" part. If gain is greater than unity then it will
>>> eventually be, but if you think of a Karplus=Strong it is by
>>> definition once the excitory signal has vanished. Am I understanding
>>> your question(?) if I say the best way is to simply monitor the
>>> amplitude
>>> , because if G>1 the amplitude will *always* keep growing.
>>>
>>> On Sun, 30 Jul 2006 19:07:12 -0400
>>> Hans-Christoph Steiner <hans at eds.org> wrote:
>>>
>>>>
>>>> Has anyone done anything with trying to detect feedback in a  
>>>> signal?
>>>> I am not looking so much for feedback suppression techniques, but
>>>> rather detection techniques.  I am working on a system that has a
>>>> slow feedback loop.  I want to detect when feedback is the dominant
>>>> part of the signal, and make the system respond in novel ways.
>>>>
>>>> .hc
>>>>
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>>
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>>
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