Functional Design of a Voice - Switched Speakerphone
W. F. Clemency, W. D. Goodale · Bell System Technical Journal · 1961
A new hands-free telephone, known as the 3A Speakerphone System, is described. It provides, by means of switched-gain techniques, almost complete freedom from distant-end talker echo and singing. The gain-switching action is virtually free of clipping or blocking — objectionable side effects that are often introduced with voice control of gain. The switching threshold is varied automatically in accordance with room noise to avoid blocking in the receive channel. Performance characteristics are shown, with particular emphasis being given to the parameters chosen to meet rather stringent performance objectives.