The Reasearch On Post Processing And Frame Erasure Concealment For An Embedded Variable Bit-Rate Speech Codec

Embedded speech coding is a newly developing technique, which also is a significant orientation of the speech coding's application. The post processing algorithm and frame erasure concealment algorithm for a wideband (50-7000 Hz) embedded variable bit-rate speech codec are described in this thesis.Firstly, a post proscessing algorithm including excitation and synthesized speech is designed for embedded variable bit-rate speech codec based on conventional adaptive post filtering and wideband speech post processing techniques. In this method, the selected anti-sparseness processing,fixed codebook gain smoothing and higher frequencies enhancement of the excitation for different layers are finished in the first step. Then adpative long-term post filter is used to treat with the sum of excitation. The short-term post filter and other processes are operated on synthesized speech in the last step. The noise between pitch harmonics is reduced with this adapative long-term post filter operated in residual domain compared with traditional method operated in speech domain. The short-term post filter is designed for narrowband and wideband input speech concurrently. It reduced the nosie between formants. The experiments show that these methods improved the quality of synthesized speech to some extent.In addition, an efficient frame erasure concealment(FEC)method for wideband embedded speech codec was proposed in this thesis. The erased speech frame is classified as voiced,unvoiced, silence, unvoiced transit to voiced and voiced transit to unvoiced at decoder. The energy of excitations is carefully controlled based on the classification of the speech. To match with the configuration of the embedded speech codec, the adaptive codebook for erased frame will be recovered with the last frame's bit-stream. In order to increasing the robustness of the codec, the contribution of adaptive codebook is properly constrained at encoder. The proposed FEC method is very simple and has a good performance without extra delay and bits in codec.These methods have been applied to an embedded variable bits rate codec submitted to ITU-T as a candidate.