Circuit identifying different AF test signals in telephone exchange - measures signal and pause durations and compares them with values held in store

摘要

The circuit indentifies different audible characters transmitted to a subscriber station for test purposes. The a.f. signal is applied through a Schmitt trigger (ST) to an evaluation circuit (D), at whose outputs (T, P) signals appear corresponding to the presence of the signal proper or of its pauses. The durations of the signal and pause are measured by a counter (Z). Each change from signal to pause starts a high clock frequency interrogation of a store (Sp), whose individual contents corresponds to counter (Z) readings for the signal and pause periods of the a.f. signal. These store contents are compared with the corresponding counter (Z) readings at the instant of level change. The result is applied to a logic evaluation circuit (V) in which individual comparison results are allocated to corresponding a.f. signals. In the case of coincidence the comparator outputs are applied to a corresponding display circuit (AUs).