Abstract: The level of the signal present on the telephone channel is evaluated at successive time intervals. Those evaluated signal levels which lie within a given range of noise levels are selected and they are expressed in terms of r discrete values in said range. A count is maintained for each of said discrete values indicative of the frequency with which it is selected in comparison with the frequency with which the other discrete values are selected. The discrete value having the highest count, i.e. the most frequently selected discrete value, is considered to be indicative of the level of noise on the telephone channel in question.

Abstract: The presence of such signals is detected each time the following detection condition is satisfied, namely: that, for the duration of a predetermined "prior activity" period the amplitude of the signal on the telephone channel exceeds a threshold level while successive maximum values taken at short time intervals remain fairly constant in level. Apparatus for performing the method comprises a threshold level comparator (10) and a difference comparator (11) which control a detector (50) followed by a holdover circuit (60). The invention is particularly applicable to inhibiting echo suppressor circuits during telephone signalling or data transfer by telephone line.