Fundamental Considerations in the Design of a Voice-Switched Speakerphone

A speakerphone 1 is a telephone whose familiar handset, which places the transmitter close to the talker's lips and couples the receiver tightly to his ear, is replaced by a separate microphone and loudspeaker that can be set on a table a few feet from the user. This arrangement gives the customer definite advantages. In addition to leaving his hands free during a telephone conversation, it is a great help for physically handicapped persons and offers other convenient features, such as the possibility of having a small group join in the conversation and the avoidance of fatigue during lengthy calls. However, these advantages are obtained at the price of some limitations. 2 To make up for the loss introduced by moving the instruments away from the head, gain is required in both the transmitting and the 265 27-1 T H E B E L L SYSTEM T E C H N I C A L J O U R N A L , M A R C H 1 9 0 0 receiving paths. This gain is limited by a "singing" problem. A signal from the microphone reaches the loudspeaker via the sidetone path and comes back to the microphone through the acoustic coupling in the room. Too much gain in this loop causes singing. Even before reaching this condition, the loudspeaker-to-microphone acoustic coupling is the source of other undesirable effects. Incoming speech at the speakerphone end is fed back from the loudspeaker through the acoustic path to the microphone, whence it is returned to the distant talker with a certain delay. This is a form of talker echo. Similar reasoning would show that the distant party is also subjected to listener echo.