Introduction to echo cancellation:
To improve the voice quality of the telephone calls echo cancellation method is employed to completely eliminate the echoes occurred during transmission and reception. By using the silence suppression method the echo get cancelled from the signal. The two types of echoes which occur in telephony signals are acoustic echo and hybrid echo. In this paper we are going to look into the causes for occurring and the cancellation methods proposed by the AT&T bell labs for suppression of the echoes. DSP processors are used for the echo signal from the reappearing original signal. The echo cancellation techniques were first proposed in 1950s for cancellation of echoes occurred due to long delays in satellite communication.
The acoustic echo occurs due to sound coning from loud speakers. The acoustic echo along with the original voice signal enters into direct path. The echo signal will have change in frequencies due to absorption and varied frequencies at different strengths. To suppress the signal the far end signal is delivered into the room by using a speaker and delayed to reassembled a near end signal. By subtracting both the signals we will obtain the echo free signal.
Due to reflection of the electrical energy by public switched telephone networks the hybrid echoes are generated. Because PSTN allows the same frequencies to be generated so to suppress these echoes we employ full bandwidth cancellation method by which the original speech signal is reassembled.
Due to echo suppression techniques double talk gets generated by which it echo suppressor cannot detect the signal of far end speaker. Due to alternative inserting and removing loss the clipping of the signals occurs.
Download Echo Cancellation MatLab Project Report.