Loudspeaking telephone using echo cancellers and a voice-switching circuit

Abstract

A new loudspeaking telephone is proposed that makes duplex communication possible using an adaptive voice-switching circuit and two adaptive echo cancellers. An acoustic echo canceller with a delay capacity of 20 ms cancels the acoustic echo between a loudspeaker and a microphone. An electric echo canceller with a delay capacity of 8 ms cancels the electric echo caused by the impedance mismatch between the line and the balancing network. The characteristics of the acoustic echo path are subject to change by movement of the talker. To cope with such changes, two adaptive filters were installed in an acoustic echo canceller. At the start of talk, the voice-switching circuit inserts a large loss to suppress howling, because the echo cancellers do not replicate the impulse responses of the acoustic and electric echo paths at the beginning of the process stage. When the echo cancellers are replicating the impulse responses with the talker's voice, the inserted loss decreases to 3 dB. Duplex communication can be done in this way. An experimental circuit with three digital signal processors showed that duplex communication becomes possible after repeating voice transmission and reception a couple of times.