Abstract: The invention comes within the field of half echo-suppressor for four-wire telephone lines and relates more particularly to the transmitted speech detector, said suppressor comprising an accumulator for accumulating characteristic indices of the samples, to control of the blocking element in the send channel depending on the accumulated value.In accordance with the invention, there is provided at least one digital comparator with a threshold, which receives the content of the accumulator and whose threshold is fixed between the limits of the accumulator. The blocking element can therefore be controlled with several operation times and via different hold circuits.

Abstract: The invention relates to a device for digitally detecting a frequency in a PCM signal.In accordance with the invention, an up/down counter is provided which receives a up-count pulses when there is a change of sign in the signal to be checked and which receives r down-count pulses when there is no change of sign in this signal. The numbers a and r are determined as a function of the ratio between the sampling frequency of the PCM signal and the frequency to be detected, so that it is possible to deduce the presence of this frequency from a balance between the number of up-count pulses and the number of down-count pulses.Application: detecting the neutralization tone for an echo suppressor in a PCM telephone line.

Abstract: The invention relates to the frame synchronization of a first binary pulse train with a second binary pulse train. According to the invention, three looped shift registers are provided which each have a capacity of one complete frame and two identical control circuits are provided which cyclically respectively connect the input and output of the device to the inputs and outputs of the registers. In the case of divergence of frequencies between the two pulse trains, one complete frame is either repeated twice or completely omitted.