Adaptive Echo Cancellation And Voice Codec Design And Implementation

This work is supported by the pre-research project being conducted in 140 lab: " Researches on Key Signal Processing Technologies in IP Telephone Systems". And the main focus is the design, implementation and testing of an adaptive line echo canceller and a vocoder based on Multi-band Excitation (MBE) model.Because IP telephone systems employ the packet switching technology, the echo signals generated by the Hybrid due to the impedance mismatch usually experience longer transmitting delay, which significantly decrease the speech communication quality. Therefore, echo cancellers are indispensable devices for any IP telephone system. In this paper, an echo canceller based on NLMS algorithm and Geigle algorithm is implemented on TMS320C5402 Digital Signal Processor (DSP). In order to improve its performance, extended precision techniques and a high-efficiency implementation scheme of Geigle algorithm are employed. The testing results show that the performance of the echo canceller exceeds the requirements specified by ITU recommendation G.165.Speech compression technologies are of great importance in increasing the capacities of IP telephone systems. Vocoders based on the MBE model are popular due to their good synthesis speech quality. In this paper, a vocoder based on the MBE model is designed, its algorithms are detailed and it is implemented using C programming language. And a new pitch extraction algorithm, an active/inactive frame decision algorithm and a voiced/unvoiced frame decision algorithm are developed with the aims to improve the quality of the vocoder and reduce its overall computation load. The testing results show that the vocoder can synthesize high-quality speech when transmission rate is set to be 2.4kbps.