Feedback Cancellation in Public Address systems using

notch filters

12th Nov. 2008 Shailesh Kulkarni

Vaibhav Mathur

Digital Signal Processing-Advanced Topics

Presentation Outline

Feedback in audio systems Design of feedback controller

Feedback detection method Design of notch filter - filterbank

Feedback in audio systems

Acoustic coupling between microphone and loudspeaker

Causes oscillation of audible frequencies and high pitch howling sometimes

Feedback Controller

Feedback Detection

3- notch filter Filterbank

Audio Audio Audio

Feedback frequencies

IN OUT

Feedback Controller

Feedback Detection Analyze to check if any feedback freq present in audio

Three frequencies of max magnitude are selected

Compared to the magnitude of their harmonics

Decision on feedback

Notch Filter filterbanks Transfer function for the 3 frequency

Check for stability

Feedback Detection block

1024 pointBUFFER

1024 pointFFT

normalizeFFT

Compare f2max to

harmonics

Find 3 largest

Mag. freq.

F2max = feedback frequency

Candidate in

3 out of 5 buffers?

Compare fmax to

harmonics

Fmax = feedback frequency

Candidate in

3 out of 5 buffers?

Compare f3max to harmonics

F3max = feedback frequency

Candidate in

3 out of 5 buffers?

Aud

io

IN

Design of Notch Filter

Second order parametric equalizer filter

Cascade of such filters will be used

H z =K⋅1−2r z cosw c z

−1r z2 z−2

1−2r pcoswc z−1r p

2 z−2

FFB = NEW ?

Set NEW notch filter

Depth = 3 dB

Increase depth of existing notch filter

Depth = depth + 3 dB

NO

YE

S

wc=2 f FBf s

Width = 1/10 octave

Feedback freq

Design of Notch Filter

References

[1] A. F. Rocha and A. J. S. Ferreira, "An accurate method of detection and cancellation of multiple acoustic feedbacks," Preprints 118th AES Conv., Barcelona, Spain, May 2005, Preprint no. 6335

[2] T. van Waterschoot and M. Moonen, "A pole-zero placement technique for designing second-order IIR parametric equalizer filters," IEEE Trans. Audio, Speech, Lang. Process., vol. 15, no. 8, Nov. 2007, pp. 2561-2565

[3] P. A. Regalia and S. K. Mitra, "Tunable digital frequency response equalization filters," IEEE Trans. Acoust., Speech, Signal Process., vol. ASSP-35, no. 1, Jan. 1987, pp. 118-120

[4] U.S. Patent Number 5245665; www.sabine.com

Thank you

Questions ?