3GPP AMR Codec中丢帧隐藏机制的改进

he AMR (Adaptive Multi Rate) Speech Codec is the best solution of next generation network. It consists of the multi-rate speech coder, a source controlled rate scheme including a voice activity detector and a comfort noise generation system, and an error concealment mechanism to combat the effects of transmission errors and lost packets. The purpose of the error concealment procedure is to conceal the effect of lost AMR speech frames. The purpose of muting the output in the case of several lost frames is to indicate the breakdown of the channel to the user and to avoid generating possible annoying sounds as a result from the error concealment procedure. This paper has compared the existing five error concealment mechanisms. We analyze the curves in three different cases, where the frame error rate is 1%, 3% or there appear continuous errors. The simulation results have shown that the first speech frame after the SID frame is more important than other frames. However, there exists a mechanism shortage in the 3GPP TS 26.091 standard for the loss of this kind of frames, so that the speech is hardly comprehensive at the beginning of talk. Therefore, a particular backup mechanism is proposed to reduce the error probability of this kind of frames. Not only is the theoretic analysis elaborated, but the effectiveness of the mechanism has been proved. The simulation results have shown the error probability of the first speech frame is reduced significantly. When calculating using typical values, the loss probability of the important speech frame is decreased by 97.8%, with only 0.3% overhead imposed on the system. Through simulations, the performance improvement has been verified using the proposed backup mechanism. The perception quality has been improved by enhancing the similarity to the original wave by 60%.