A Kalman filtering noise canceler for PDC speech enhancement

Speech enhancement techniques suitable for digital mobile communication systems are proposed. The proposed methods improve the speech quality and enlarge the continuous conversation time of any mobile radio system. The enhancement techniques, whose complexity is about 1/10th that of the speech codec, are easily implemented in the same chip as the speech codec. The core of the proposed techniques is a noise canceler based on a Kalman filter and signal-state estimation. We employ the Kalman filtering algorithm for its optimum filtering performance for time varying signals. In order to achieve effective noise cancelation with the filter, the filtering parameters are controlled according to the estimated signal-state. The speech enhancement performance of the proposed noise canceler is evaluated in terms of mean opinion score (MOS). The proposed techniques improve noisy speech quality by half a point in MOS at the noise level of 50 dBA. Improvements in transmission time ratio by employing the proposed noise canceler in the Japanese PDC (personal digital cellular) system are evaluated. The combination of voice operated transmission (VOX) in PDC and the noise canceling techniques save the battery life. The transmission period ratio is improved by 20% at the noise level of 60 dBA. We describe a speech detection scheme and processing method for reducing the speech degradation due to VOX operation.