Method of and means for detecting digitized multi frequency-coded signals

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. If the threshold used as a reference is exceeded for a predetermined length of time, the output signal is recognized as being a true signaling pulse. A significant discrepancy between the absolute magnitudes of the two pseudo-correlator outputs, detected by the microcomputer, gives rise to an alarm signal indicative of a malfunction. The several frequency correlators and pseudo-correlators can all be constituted by a single circuit component to which the various signaling frequencies are successively applied in any time slot assigned to an incoming signaling channel.