Speech Communication - Special issue on speech processing in adverse conditions
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Based on the bit-stream of ITU-T G.722.2 speech coding standard, through the modification of codebook gains in the codec, a compressed domain speech enhancement method that is compatible with the discontinuous transmission (DTX) mode and frame erasure condition is proposed in this paper. In non-DTX mode, the Voice Activity Detection (VAD) is carried out in the compressed domain, and the background noise is classified into full-band distributed noise and low-frequency distributed noise. Then, the noise intensity is estimated based on the algebraic codebook power, and the a priori SNR is estimated according to the noise type. Next, the codebook gains are jointly modified under the rule of energy compensation. Especially, the adaptive comb filter is adopted to remove the residual noise in the excitation signal in low-frequency distributed noise. Finally, the modified codebook gains are re-quantized in speech or excitation domain. For non-speech frames in DTX mode, the logarithmic frame energy is attenuated to remove the noise, while the spectral envelope is kept unchanged. When frame erasure occurs, the recovered algebraic codebook gain is exponentially attenuated, and based on the reconstructed algebraic codebook vector, all the codec parameters are re-quantized to form the error concealed bit-stream. The result of performance evaluation under ITU-T G.160 shows that, with much lower computational complexity, better noise reduction, SNR improvement, and objective speech quality performances are achieved by the proposed method comparing with the state-of-art compressed domain methods. The subjective speech quality test shows that, the speech quality of the proposed method is better than the method that only modifies the algebraic codebook gain, and similar to the one with the assistance of linear domain speech enhancement method.