1. Nico De Clercq Pieter Gijsenbergh

2.  Problem  Solutions  Single-channel approach  Multichannel approach  Our assignment Overview

3.  Speech is a highly redundant signal:  Normal people: noise not a big problem  Hearing impaired: noise reduces intelligibility  Noise = any unwanted signal that interferes with the desired signal  Assumption: additive, locally stationary noise Problem

4.  Problem  Solutions  Single-channel approach  Multichannel approach  Our assignment Overview

5.  Noise-cancelling microphones  Voice processor modifications  Preprocessor noise reduction  Single-channel  Multichannel Solutions

6.  Only one device captures the signal:  Only spectral and temporal characteristics  Techniques:  Wiener-filtering  Spectral-subtracting  Sine-wave modelling  Directional microphones

7.  Optimal adaptive filter to maximize SNR  Problem: noise and signal have to be known  Solution: use short-term spectra  speech more or less constant  Difficult approach & internal noise issues Single-channel : Wiener-filter

8.  Principle  Measure noise spectrum in non-speech activity  Take mean of measured amplitudes  Subtract mean from input signal  Spectral error Single-channel : spectral subtraction (1)

9.  Modifications: magnitude averaging, half- wave rectification, residual noise reduction, …  Expected results: noise reduced, equal intelligibility  Explanation: non-stationary noise!

10.  Problem  Solutions  Single-channel approach  Multichannel approach  Our assignment Overview

11.  Multiple sensors capture signal:  Exploits spatial diversity of the noise  Noise and signal almost always differ in location  In hearing aids  Noise microphone  Speech + noise microphone  Adaptive filtering Multi-channel noise reduction

12.  Constructive and deconstructive interference  Controls phase (delay) & relative amplitude (constraint)  Fixed or adaptive Multi-channel: Beamforming

13.  Delay-sum beamformers  Inputs are weighed (phase shift)  Filter-sum beamformers  Amplitude & phase weights frequency dependant Multi-channel: Beamforming (1)

14.  Superdirective beamformers  Maximize array gain, suppress noise from other directions  Near field superdirectivity for good low frequency performance  Amplitude + phase Multi-channel: Beamforming (2)

15.  Fixed beam former:  Points to desired signal  Mostly filter-sum beam formers used  Blocking Matrix (B):  Separates desired signal from noise: rows add up to 0  Maximum N-1 rows  Adaptive part:  Minimizes the noise power in the output  LMS, with frequency domain processing: Multi-channel: Beamforming (3) Generalized Sidelobe Canceller

16. Multi-channel: Beamforming (4) Generalized Sidelobe Canceller x´´

17.  Problem  Solutions  Single-channel approach  Multichannel approach  Our assignment Overview

18.  Implement & test algorithm  Our choice:  Generalized Sidelobe Canceller with LMS update  Frequency domain implementation of LMS  DSP II: overlap-add, adaptive filtering, time and frequency domain, multirate, … Our assignment

19.  Suppression of acoustic noise in speech using spectral subtraction, S. Boll, IEEE ASSP, vol 27, no 2, 1979  H. Levitt, "Noise reduction in hearing aids: An overview", Journal of Rehabilitation Research and Development, vol. 38, no. 1, Jan./Feb. 2001, pp. 111-121.  J.J Shynk, "Frequency-domain and multirate adaptive filtering " Signal Processing Magazine, IEEE, Volume 9, Issue 1, Jan 1992 Page(s):14 - 37.  I. A. McCowan, “Robust Speech Recognition using Microphone Arrays”, PhD Thesis, Queensland University of Technology, Australia, 2001.  G. O. Glentis, “Implementation of Adaptive Generalized Sidelobe Cancellers using efficient complex valuedarithmetic”, International Journal of Applied Mathemethics and Computer Science, vol. 13, no. 4, 2003, p. 549-566  https://gilbert.med.kuleuven.be/~koen/demo_beam/demo_beam.html https://gilbert.med.kuleuven.be/~koen/demo_beam/demo_beam.html  http://www.rp-photonics.com/interference.html http://www.rp-photonics.com/interference.html Reference

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