Abstract: Digitized samples of switching or supervisory signals in the form of specific frequency pairs, selected from n predetermined signaling frequencies, are received at a PCM terminal of a telephone exchange and fed to n frequency correlators, one for each signaling frequency used in the system, where the samples are multiplied by a sine function and a cosine function of the respective signaling frequency and the two products are integrated and vectorially added to form a resulting binary signal. The absolute value of the incoming-signal sample in each PCM time slot is multiplied in a pair of pseudo-correlators with two fixed values of approximately equal magnitude but opposite sign. A microcomputer compares the output signals of the several frequency correlators with a fixed threshold, or with an adaptive threshold derived from the output of one of the pseudo-correlaors if that threshold surpasses the fixed one.