Pulse signalling systems

Claim 1

A MULTIPLE-FREQUENCY SIGNAL RECEIVER INCLUDING A FREQUENCY DETECTOR FOR EACH NORMAL COMBINATION OF SIGNAL FREQUENCIES TO WHICH THE RECEIVER IS REQUIRED TO RESPOND, EACH SUCH FREQUENCY DETECTOR COMPRISING AN ACCEPTOR CIRCUIT TUNED TO ONE OF THE FREQUENCIES OF THE RELEVANT COMBINATION AND A REJECTOR CIRCUIT CAPABLE OF RESPONDING TO ALL THE SIGNAL FREQUENCIES AND EFFECTIVE TO RENDER THE ACCEPTOR CIRCUIT INEFFECTUAL TO RESPOND TO ANY FREQUENCY OTHER THAN THAT TO WHICH IT IS TUNED, SAID FREQUENCY DETECTORS BEING DIVIDED INTO GROUPS IN EACH OF WHICH THERE IS AT LEAST ONE COMMON FREQUENCY IN THE DIFFERENT FREQUENCY COMBINATIONS TO WHICH THE DETECTORS CONSTITUTING THE GROUP RELATE, BAND-STOP FILTER MEANS PRECEDING THE REJECTOR CIRCUITS OF THE DETECTORS IN EACH GROUP, SAID FILTER MEANS COMPRISING A COMMON BAND-STOP FILTER TUNED TO EFFECTIVELY STOP SAID COMMON FREQUENCY FOR THE GROUP, A BOARD BAND DETECTOR RESPONSIVE TO THE RECEIPT OF ANY FREQUENCY SIGNAL IRRESPECTIVE OF THE NUMBER OF FREQUENCIES SAID SIGNAL CONTAINS, AND ERROR-DETECTING MEANS FOR DETECTING THE RECEIPT OF A FALSE SIGNAL CONTAINING ONLY A SINGLE FREQUENCY OR MORE FREQUENCIES THAN IN A NORMAL COMBINATION, SAID ERROR DETECTING MEANS COMPRISING A RELAY HAVING AN OPERATING WINDING THEREOF CONNECTED IN AN ENERGIZING CIRCUIT WHICH INCLUDES CONTACTS CONTROLLED BY ALL THE DETECTORS, WHICH CONTACTS ARE INTERCONNECTED TO ESTABLISH AN OPERATING CONDITION FOR THE RELAY WHEN THE BOARD BAND DETECTOR RESPONDS TO A RECEIVED SIGNAL ACCOMPANIED BY RESPONSE OF MORE OR LESS THAN ONE OF THE FREQUENCY DETECTORS.