| 摘要 |
PURPOSE:To prevent howling due to a large change in an echo path by inserting a temporary loss to a howling group when the echo path changes largely and the estimating operation of an echo canceller is not in time. CONSTITUTION:An echo path fluctuation detecting circuit 310 transmits always a supervisory signal without a listening range indoors at a prescribed time interval via an amplifier 309 and a speaker 307. This signal is reflected by, e.g. a caller and inputted to a detection circuit 310. The detection circuit 310 detects only the supervisory signal by using a filter, detects the delay from a transmission timing and the change in the attenuation, and discriminates it as the presence of a large change in the echo path when the amount of change exceeds a prescribed value and transmits a control signal to a transmission/reception decision circuit 303 so as to command the operation of voice switches 301, 305. When it is discriminated that the cancellation capability of th echo canceller is recovered afterward, the detection circuit 310 outputs a voice switch release signal. |
