1. Feedback Cancellation in Public Address systems using notch filters 12th Nov. 2008 Shailesh Kulkarni Vaibhav Mathur Digital Signal Processing-Advanced Topics

2. Presentation Outline Feedback in audio systems Design of feedback controller  Feedback detection method  Design of notch filter - filterbank

3. Feedback in audio systems Acoustic coupling between microphone and loudspeaker Causes oscillation of audible frequencies and high pitch howling sometimes

4. Feedback Controller Feedback Detection 3- notch filter Filterbank Audio Feedback frequencies IN OUT

5. 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

6. Feedback Detection block 1024 point BUFFER 1024 point FFT normalize FFT 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? Audio IN

7. Design of Notch Filter Second order parametric equalizer filter Cascade of such filters will be used

8. FFB = NEW ? Set NEW notch filter Depth = 3 dB Increase depth of existing notch filter Depth = depth + 3 dB NO YES Width = 1/10 octave Feedb ack freq Design of Notch Filter

9. 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

10. Thank you Questions ?