A speaker telephone comprising a microphone outputting an electrical signal, and a loudspeaker is disclosed. An amplifier has its output coupled to the loudspeaker. This amplifier has non-inverting and inverting inputs. A switch having two positions couples the microphone to the amplifier or couples...http://www.google.com/patents/US5172410?utm_source=gb-gplus-sharePatent US5172410 - Conference telephone system and method
