Echo Cancellers

The echo problem exists in many fields, such as large loop delay telecommunication channels, mobile phone system and teleconference system etc. It's difficult to get smooth conversation without proper echo control. So to support high-speech-quality voice communication, the technology of echo cancellation has been a very active research field in the recent years.This paper is focused on the adaptive echo canceller, which includes the electric echo canceller and the acoustic echo canceller. The basic principle of the adaptive echo canceller can be summarized as follows:Adaptively synthesize a replica of the echo and subtract it from the echo-corrupted signal.This paper is organized as follows:The background knowledge about the echo canceller such as echo mechanism, the basic principle and basic components is introduced in the first chapter.In the second chapter, many kinds of adaptive filter algorithms are discussed. To meet the requirement of the real-time processing, we put more emphasis on the fast algorithms and the algorithms that can get a tradeoff between the convergence performance and the compute complexity. The correlation and the difference of these algorithms are also discussed.The subband technology and the subband adaptive filters are introduced in the third chapter. We especially focus on the merit of the subband adaptive filter technology and the influence to the performance of the echo canceller because of the subband alias.Then according to the implementation of the echo canceller, we discuss the applied methods to improve the convergence speed and reduce the computation complexity. A new kind of echo canceller based on the structure of dual-assistant filters is proposed to solve the double talk detection problem. Other implementation issues such as finite precision effects and the realization based on TI TMS320C549 are also discussed.Finally, the test results are given to demonstrate the performance of the design electric echo canceller and acoustic echo canceller, where the steady residual echo and the rate of convergence are the main indexes.