Wavelet speech enhancement based on time–scale adaptation

We propose a new speech enhancement method based on time and scale adaptation of wavelet thresholds. The time dependency is introduced by approximating the Teager energy of the wavelet coefficients, while the scale dependency is introduced by extending the principle of level dependent threshold to wavelet packet thresholding.This technique does not require an explicit estimation of the noise level or of the a priori knowledge of the SNR, as is usually needed in most of the popular enhancement methods. Performance of the proposed method is evaluated on speech recorded in real conditions (plane, sawmill, tank, subway, babble, car, exhibition hall, restaurant, street, airport, and train station) and artificially added noise. MEL-scale decomposition based on wavelet packets is also compared to the common wavelet packet scale.Comparison in terms of signal-to-noise ratio (SNR) is reported for time adaptation and time–scale adaptation of the wavelet coefficients thresholds. Visual inspection of spectrograms and listening experiments are also used to support the results. Hidden Markov Models speech recognition experiments are conducted on the AURORA-2 database and show that the proposed method improves the speech recognition rates for low SNRs.