1. An echo path monitoring system for controlling an adaptive filter configured to estimate an echo of a far-end signal comprised in a microphone signal, the system comprising: a comparison generator configured to compare the microphone signal with the estimated echo to obtain a first comparison result and compare an error signal, which represents a difference between the microphone signal and the estimated echo, with the estimated echo to obtain a second comparison result; and a controller configured to combine the results of the first and second comparisons to form a parameter indicative of a state of the microphone signal and, in dependence on said parameter, control an operating mode of the adaptive filter, wherein the parameter is indicative of the presence of an echo path change in the microphone signal, and the controller is configured to form the parameter by dividing the result of the second comparison by the sum of the results of the first and second comparisons.