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.

Abstract: A system for measuring the attenuation on a transmission path for a random signal in which the propagation time of said signal is between a lower limit T1 and an upper limit T2, the system comprising:a first maximum value indicator circuit with immediate response and a holding time not less than T2, to which the signal at the input end of said path is applied;a second maximum value indicator circuit with immediate response and a holding time not less than T2-T1, to which the signal at the output end of said path is applied;a detector which detects that the output of said first indicator has been maintained constant during a time interval not less than T2, connected to receive the output signal from said first indicator and determining times at which the indicator output signals are recognized; anda processor connected to receive the indicator output signals, controlled by said detector and generating a measurement signal which is a function of the instantaneous value of the ratio of the indicator output signa