Telephone system with alternate voice-video signal transmission - feeds immune code output signals to receiver, to ensure transfer units connection together prior to operation

摘要

The special telephone system, for police, allows voice and video signals to be sent alternately via a 4-wire line. Immune code output signals are transmitted to an immune code receiver before the transfer units are connected through, in order to ensure that the units can be connected together. When a single output signal is present, the through connection is prevented. But when more than one output signal is present at the receiver the through connection can take place.