Post noise smoother to improve low bit rate speech-coding performance

A new post-process called a post noise smoother (PNS) for the CELP decoder is proposed in order to improve low bit rate speech-coding performance under various background noise conditions. In the PNS, spectral amplitude smoothing and phase randomizing are performed on the decoded speech in order to obtain smoothed background noise. The decoded speech, the smoothed signal, and an automatically generated imitative noise signal are multiplied by adaptive gains and are summed up in the final output speech. These gains are computed from each frame´s estimated ratio of background noise to signal. Evaluation test results show that the PNS significantly improves the subjective quality of a 4-kbps speech coder under various conditions of background noise