A paging system is entered through appropriate telephone equipment at a master station to supply an audio paging output at a remote station. A paging amplifier is connected to drive a speaker at the remote station to provide the audio output. A direction sensing circuit is disposed between the master...http://www.google.fr/patents/US4107473?utm_source=gb-gplus-shareBrevet US4107473 - Two-way paging system transmission direction sensing and control apparatus and method
Two-way paging system transmission direction sensing and control apparatus ...
