2-wire-4-wire telephone converter for use on unstable 4-wire circuits

Claim 1

IN A COMMUNICATION SYSTEM COMPRISING FIRST AND SECOND STATIONS A TWO-WIRE CIRCUIT COUPLED TO EACH OF SAID FIRST AND SECOND STATIONS, A FOUR-WIRE CIRCUIT BETWEEN SAID STATIONS PROVIDING TWO SEPARATE CHANNELS FOR COMMUNICATION, ONE FOR EACH DIRECTION BETWEEN THE STATIONS, AND MEANS FOR SWITCHING THE TWO-WIRE CIRCUITS AT EACH STATION BETWEEN TWO POSITIONS, ONE COUPLING SAID TWO-WIRE CIRCUIT TO ONE PAIR OF SAID FOUR-WIRE CIRCUIT OR TO THE OTHER OF SAID PAIR OF SAID FOUR-WIRE CIRCUIT, SAID MEANS COMPRISING MEANS AT EACH STATION FOR GENERATING A CONTINUOUS COMMAND SIGNAL WITH A FREQUENCY SHIFT IN RESPONSE TO THE TRANSMISSION OF A SIGNAL FROM THE STATION, MEANS AT THE OTHER STATION RESPONSIVE TO THE FREQUENCY SHIFT SIGNAL TRANSMITTED THERETO FROM SAID FIRST STATION FOR CAUSING SAID TWO-WIRE CIRCUIT TO BE CONNECTED TO THE OTHER OF THE PAIR OF SAID FOUR-WIRE CIRCUIT AND RESPONSIVE TO THE ABSENCE OF FREQUENCY SHIFT IN SAID CONTINUOUS COMMAND SIGNAL FOR RETURNING THE CONNECTION TO SAID FIRST PAIR OF SAID FOURWIRE CIRCUIT.