Party intercept in selective multifrequency ringing systems

摘要

A system for directing calls to selected parties on a multiparty line in exchanges equipped with selective multifrequency ringing generators is disclosed. Ringing applied to the line is converted to a DC logic level pulse train and is compared to logic level pulse trains generated for each frequency of the ringing generator. If the party represented by the frequency on the line has been manually selected for intercept and the pulse trains match for 250 milliseconds, the call is intercepted. Since the pulse trains are at logic level and are generated by the same ringing cycle, they are immune to phase shifts, voltage variations, and electrical noise.