[PD] feedback detection

[noisternig at iem.at]

hi hans,

there is a simple method for feedback stabilization coming from 
adaptive line enhancement theory using FIR N-LMS algorithms. i've 
checked in my adaptive filter library into CVS today 
(externals/iem/iem_adaptive); so it should be easy to implement ... 
back to your problem:

the basic scheme of an adaptive line enhancer with FIR structure is 
depicted in the figures attached.  due to short term periodic 
components within the target signal, the filter tries to suppress also 
parts of the target signal. with a small stepsize parameter mu={0.01, 
0.00001} this behaviour can be avoided and only long term periodic 
components of the signal will be suppressed; this also guaranties 
filter stability. because of the small step size the filter converges 
slowly and feedback distortions will be attenuated after a 
nonnegligible period of time - so it will be possible to "play" with 
feedbacked sounds. the main parameter is the step size. the delay ND 
considers the short term stationarity of the target signal (eg speech 
~20ms). you may also use a feedbacked FIR structures (cp IIR) that 
provide sharper notches, i.e. less distortion of your target signal. 
using IIR may become unstable!!

i used this algorithm for residual feedback suppression in 
communication setups and it worked quite well.

cheers

markus

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