Acoustic Echo Cancellation
In recent years, the development of new communication systems has been directed more and more by the customer desired for more comfort. An important factor for commercial success is the high quality of the hands-free interface.
Due to the lack of an acoustical barrier between loudspeaker and microphone, the signal of the communicating party at the far end is picked up by the microphone and transmitted back to its origin. The far speaker perceives this as an echo. The subjective disturbance of this echo increases with with a longer transmission delay, e.g. in mobile radio systems. Thus, a cancellation of the acoustical echo is necessary.
An adaptive filter is used to identify and reconstruct the acoustic echo path and thus the room impulse response. The result of the filtering is an estimated echo signal, which can be subtracted from the microphone signal to reduce the disturbing echo. Adaption is performed mainly in single talk phases where the near speaker does not talk. To reduce the impact of double talk, usually adaptive stepsize control is employed.
A statistical postfilter is used to reduce non-deterministic influences of the acoustic echo path. These influences are introduced by the time-variant room impulse response, as well as a limited adaptive filter length, which in real systems is smaller than the reverberation time.
Traditional algorithms for the adaption of these components usually depend on a combination of sophisticated control mechanism to ensure the adaption and the robustness in real time-variable and disturbed circumstances. These mechanism generally use a large number of different control parameters, which need a lot of experience to adjust to the respective situation. A new, model-based approach is introduced on the next page.