Satoru Ibaraki

Pre-howling howlback detection method

This report relates to a howling detection unit used in teleconferencing systems, one which is capable of detecting howling before it occurs. An analytical study was made of the characteristics of pre-howling conditions, in which it was found that howling components are present immediately following spoken syllables in conversational speech. Based on these results, the differential value of signal amplitude was employed to extract these howling components, thus creating a new howling detection method for frequency analysis. Computer simulation confirmed that howling components can be detected accurately using this detection method, even under pre-howling conditions of howlback margin 3dB. This system was implemented on a trial basis, using a Digital Signal Processor, and favorable results were obtained.

Loudspeaking telephone using echo cancellers and a voice‐switching circuit

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 talkers 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.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 talkers voice, the inserted loss decreases to 3 dB. Duplex communication can b...