Patent ID: 8737601

Claim:
An echo canceller comprising: an adaptive filter for receiving a signal affected by an echo, and for supplying a filtered signal that is an estimate of the echo; a subtractor for subtracting this estimate from the received signal and supplying a residual signal; means for detecting a ring back tone in said residual signal, comprising: means for comparing the difference between the energy of the received signal and the energy of a transmitted signal with a first threshold, and then concluding that a ring back tone is detected when this difference is greater than this first threshold, and means for comparing the energy of the transmitted signal to a second threshold, and comparing the cumulated time during which this energy is greater than the second threshold with a fourth threshold, then assuming that the ringing phase is elapsed when this cumulated time is greater than this fourth threshold; means for blocking the received signal and replacing it by a locally generated ring back tone when a ring back tone has been detected in the received signal, and until the ringing phase is assumed to be elapsed.