Intercommunication system

Claim 1

SIGNAL TRANSMISSION APPARATUS INCLUDING IN COMBINATION A FIRST SIGNAL TRANSMISSION CHANNEL HAVING AN INPUT THERETO, A SECOND SIGNAL TRANSMISSION CHANNEL, A FIRST SWITCHING CIRCUIT INCLUDING A FIRST TRANSISTOR OF ONE TYPE CONNECTED IN SERIES IN SAID FIRST TRANSMISSION CHANNEL, A SECOND SWITCHING CIRCUIT INCLUDING A SECOND TRANSISTOR OF THE TYPE COMPLEMENTARY TO SAID FIRST TRANSISTOR CONNECTED IN SERIES IN SAID SECOND TRANSMISSION CHANNEL, A SWITCH CONTROL CIRCUIT INCLUDING A SWITCH CONTROL TRANSISTOR, AN INPUT CONNECTION FROM THE INPUT TO SAID FIRST SIGNAL TRANSMISSION CHANNEL TO A FIRST ELECTRODE OF SAID SWITCH CONTROL TRANSISTOR FOR CAUSING SAID SWITCH CONTROL TRANSISTOR TO OPERATE IN THE HIGH CONDUCTION STATE ONLY WHILE A SIGNAL IS APPLIED AT THE INPUT TO SAID FIRST SIGNAL TRANSMISSION CHANNEL, A FIRST OUTPUT CONNECTION FROM A SECOND ELECTRODE OF SAID SWITCH CONTROL TRANSISTOR TO SAID FIRST TRANSISTOR FOR BIASING SAID FIRST TRANSISTOR TO ITS CONDUCTING STATE WHILE SAID SWITCH CONTROL TRANSISTOR IS IN THE HIGH CONDUCTION STATE AND FOR BIASING SAID FIRST TRANSISTOR TO ITS CUT-OFF STATE WHILE SAID SWITCH CONTROL TRANSISTOR IS NOT IN THE HIGH CONDUCTION STATE, AND A SECOND OUTPUT CONNECTION FROM A THIRD ELECTRODE OF SAID SWITCH CONTROL TRANSISTOR TO SAID SECOND TRANSISTOR FOR BIASING SAID SECOND TRANSISTOR TO ITS CUT-OFF STATE WHILE SAID SWITCH CONTROL TRANSISTOR IS IN THE HIGH CONDUCTION STATE AND FOR BIASING SAID SECOND TRANSISTOR TO ITS CONDUCTING STATE WHILE SAID SWITCH CONTROL TRANSISTOR IS NOT IN THE HIGH CONDUCTION STATE.