Adaptive Howling Canceller (Acoustic Feedback Cancellation/Canceller) - Acoustic path is not feedback path!

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Howling canceller LSI is under development now.

Our research & development of howling canceller is not the research of the simulation of howling canceller.

Block Diagram canceller_blockdiagram.pdf (65KB)
Block Diagram (Expranded) canceller_blockdiagram_expand.pdf (743KB)

FIR filter section of adaptive howling canceller
FIR filter section


LMS algorithm section of adaptive howling canceller
LMS algorithm section



Improvement of Howling Canceller using Adaptive Filter with Delay Circuit  [in Japanese, with English abstract and figure captions]
IEICE technical report EA 108(491), 13-18, 2009-03-20
https://ci.nii.ac.jp/naid/110007328695



IEICE = The Institute of Electronics, Information and Communication Engineers
EA means Electro-Acoustics

Basic adaptive howling canceller block diagram (without limiter)
Basic adaptive howling canceller block diagram (without limiter)

Acoustic path is not feedback path.



Cut feedback path → There is not closed signal path. It means the cutted signal path is really feedback path.
Cut acoustic path → System is not stable because still there is closed signal loop via adaptive filter. Acoustic path is not feedback path!
Acoustic path is not feedback path !
H(ω) : Acoustic system
W(ω) : Adaptive filter

A limiter cicuit is added to ensure the stability of adaptive howling canceller. (see the figure of experiment below)
Limiter prevents saturation of D/A, power amp, speaker, microphone, mic amp and A/D.

Wrong model of adaptive howling canceller

Don't use wrong model (left).
Many research of the simulation of adaptive howling canceller use this wrong model.
(e.g. Toon van Waterschoot and Marc Moonen, "Fifty years of acoustic feedback control: state of the art and future challenges," Proc. IEEE, vol. 99, no. 2, Feb. 2011, pp. 288-327)

Wrong model and true model of adaptive howling canceller

Acoustic path including power amp and mic amp has gain grater than 0dB. It means acoustic path is not feedback path. (Correct model, right )
Feedback path don't have gain grater than 0dB.

Experiment at Tokyo Metropolitan Industrial Technology Research Instuitute (TIRI)




Limiter characteristics
K is a threshold value.


Anechoic room




Experiment result

Please see and listen following video.


video.avi (61MB),
video.wmv (4MB)




Adaptive filter coefficient




Adaptive filter gain (grater than 10dB) = PA system gain

This PA system gain includes A/D gain, D/A gain, scaling gain and the gain of fixed gain block (15dB)



How to measure the gain of acoustic path

y[n]rms/x[n]rms is the gain of the acoustic path of adaptive howling canceller system.





Howling canceller simulation program for Scilab

Scilab simulation program (Acoustic feedback cancellation code) scilab_canceller.zip (710KB)

Simulation output signal (howling is suppressed) sim_out.wav (400KB)

Scilab simulation result of howling canceller


Howling canceller demo system for echoic room using DSP (Digital Signal Processor)

Howling canceller demo system
This system is used for demonstration at the meeting of AES (Audio Engineering Society) Japan Section.

AES Section Meeting Reports, Japan - December 15, 2016



Inside of DSP unit (Howling canceller demo system)
Inside of DSP unit

We use C6713 DSK DSP board (TI/Spectrum Digital).



Example of coeffcients of the adaptive filter of howling canceller demo system
and its amplitude response (= acoustic sysytem gain grater than 0dB)

This is experiment result in rather small room (Reverberation time = 300ms)







Example of the acoustic path impulse response of the room we use to test our adaptive howling canceller (fs=8kHz). room_impres.txt
Please consider there is conversion delay of A/D, D/A and omitted leading zero data etc.
The distance between speaker and microphone is 2.3m.
It looks slight strange response shape because it includes phase rotation of measurement system etc.



Experiment at Tokyo University of the Arts, Senjyu Campus





The microphone suited for adaptive howling canceller is omnidirectional ! (or binomial array)
This is one of incredible characteristics of adaptive howling canceller.



Line array speaker for howling canceller experiment

Japanese page is here

Line array speaker for howling canceller experimentLine array speaker for howling canceller experiment

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