Perception Oriented, Delay-controlled Echo Cancellation in Ip Based Telephone Networks

Echo cancellation is one of the most challenging tasks in the design of IP based telephone networks. The degradation of conversational speech quality caused by impairments introduced by echo cancellers can be considerably reduced when using an echo delay controlled approach: The non-linear processor provides a sufficient, but limited additional attenuation based on the talker echo tolerance curves of the corresponding ITU-T Recommendation. The important control parameter is the network delay. While restricting the echo level below the perception threshold, the near end signal of the echo canceller is transmitted with higher quality during double talk periods, especially when the double talk detection operates unreliably. A third party listening test has been carried out to assess different speech quality parameters during a conversation. This test focused on the comparison of an echo canceller implementation with a standard non-linear processor and a delay-controlled residual echo attenuation as suggested in this contribution. The results show substantial improvements for one-way delays up to around 100 ms.