Noise Reduction in Mobile Phones

This project has been about reducing acoustical noise transferred in a communicationsystem intended for speech. The communication system is inthe form of a mobile phone and the intended use is in environments withhigh noise intensity. Spectral subtraction, adaptive filtering, fixed beamformingand adaptive beamforming are noise reduction methods that havebeen explored. For complexity reduction a polyphase subband structure hasbeen proposed. Implementations and evaluation has been made in Matlab.An analog beamformer provided by National Semiconducter (LMV1089) hasbeen calibrated according to the manufactures instruction and tested in simulatednoise environments.For evaluation purposes four microphones were integrated in a mobile phone.Recordings were made in an isolated chamber with the phone strapped to thehead of a dummy torso. With one speaker in every corner and in the mouth ofthe dummy head different noise and speech situations were recorded. Theserecordings were used as guidelines to simulate the noise in Matlab. Usingthe recordings directly for evaluation failed because of what might be a verysmall time delay between the two channels.The spectral subtraction method is based on a noise estimation made duringspeech free segments. Using a voice- activity-detector (VAD) the meanvalue of the noise is continuously updated. Since the VAD and noise estimationare both based on a mean value non-stationary noise will diminish theperformance. There is also a chance that the altered noise gets even moredistracting. Using an adaptive filter as a noise canceler a second microphoneis needed as a reference signal. In a mobile handset application difficulties inisolating the reference source from the wanted signal limits the noise reductionconsiderably. A Beamformer utilizes the interference pattern that occurswhen adding multiple inputs with different time delay to isolate sounds comingfrom specific angles. It is therefor dependent on the predefined spatialrelationship between speaker and microphone placement. A beamformer canbe made both stationary and adaptive. The effectiveness of the stationarybeamformer depends on the spatial relationship while and adaptive beamformercan adjust to changes made in the spatial domain. Considering complexity,required hardware and convenience for the user digital beamformingis recommended for further study and real-time implementation.Validerat; 20120504 (anonymous