Speech Enhancement in Hands-free Systems for Automobile Environments

A new microphone position in the automobile where microphones are placed on the seat belt is available. In order to be able to use this microphone a set of issues are addressed through signal processing methods. The central work of the thesis is a set of speech enhancement algorithms which are applied to the belt microphones. The speech enhancement chapters presented in the thesis form the basic units of a hands-free system. Belt microphones when integrated into hands-free systems are used to pick up the speech of the passenger/s in the automobile. This thesis presents an acoustic echo canceller to remove these echoes. In order to achieve this an improved way to control the adaptive filters of the echo canceller is presented. The chapter also presents a way to deal with the moving of the microphones when the body of the passenger moves. The speech enhancement of the microphone also deals with the estimation of the background noise in the automobile environment. The noise estimation chapter of the thesis proposes a new noise estimation scheme applicable to the belt microphones. The scheme considers noise scenarios involving nonstationary signals like the sudden change in the noise properties. By tracking long term average, the long term level, and taking into account the short term dynamics of the background noise, a multiplicative constant based scheme is proposed. The traditional Wiener filter approach to noise suppression has been re-looked in the final speech enhancement chapter of the thesis. A hands-free system is presented where the above proposed speech enhancement algorithms has been implemented in a real-time system. The software and hardware implementation details are described in the real-time implementation chapter of the thesis.