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 ?