A threshold control system for controlling a non-linear processor in an echo canceller, the non-linear processor being configured to remove any signal energy below a threshold that remains in a microphone signal after the echo canceller has subtracted an echo estimate from it, the threshold control system comprising a convergence unit configured to determine an indication of the stability of an adaptive filter, the adaptive filter being configured to continuously model an echo path so as to generate the echo estimate, and a threshold tuner configured to adjust the threshold of the non-linear processor in dependence on the indication.