| 摘要 |
PURPOSE:To generate no receiving blocking and to detect a transmitting signal with high accuracy by constantly measuring a scale in the acoustic coupling of an echo path from a speaker to a microphone and automatically setting a threshold corresponding to this measured value. CONSTITUTION:A threshold setting means is constituted of a peak hold part 13 for obtaining the envelope of the peak connecting the upper end of the peak of an acoustic coupling measuring amplitude ratio, a dip holding part 13 for obtaining the minimum peak value of the minimum of the envelope of this peak and a threshold calculating part 14 for setting the threshold from this minimum peak value, the acoustic coupling strength of the echo path is correctly obtained from the acoustic coupling measuring amplitude ratio in which the peak and the dip are repeatedly and violently fluctuated and the threshold corresponding thereto is automatically set. When the acoustic coupling strength of the echo path is high, the threshold is set high and when the acoustic coupling strength is low, the threshold is automatically set low. Thereby, a sound switch circuit of high sensitivity and having no erroneous detection is constantly obtained.
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