Circuit for switching between ring signal and message waiting signal without bell tap

Claim 1

A circuit for switching between a ring signal and a message waiting signal without bell tap in a telephone system, comprising: (a) means for generating a composite RAC/MW signal which comprises an A-C signal and a D-C signal alternately spaced in time, the generating means comprising an RAC signal generator, an MW signal generator, and switching means for combining the RAC and MW signals into a composite RAC/MW signal; (b) a switch enable-signal generator comprising a comparator for comparing the instantaneous value of the RAC signal with the value of the MW signal, the switch enable generator providing a signal for changing the composite signal from an A-C signal to a D-C signal only when the instantaneous value of the RAC signal equals the value of the MW signal and the slope is negative; (c) means for producing either a RAC or MW signal to a particular subscriber station of a telephone system in response to an external command synchronous signal; (d) a RAC voltage zero crossing detector and synchronous signal generator for detecting when the RAC signal crosses zero point, and controlling when the appropriate RAC or MW signal is provided to a subscriber station in the telephone system; and (e) a synchronous edge suppression circuit for supressing the first ring synchronous pulse from the ring synchronous signal when the means for providing either an RAC or MW signal to a particular subscriber station signals that an MW signal should be provided during the time interval between the A-C signal crossing the D-C signal with a negative slope and before the subsequent positive going ring synchronous signal.