| 摘要 |
PURPOSE:To realize inexpensively the performance equal to that of an echo canceller and to facilitate talking even when noise level on a transmission line is high by providing a white noise generating circuit and applying white noise to the opposite party when a switch provided to the transmission line is opened. CONSTITUTION:A listening improving circuit 25 is provided with a white noise generating circuit 27. The threshold of the comparator circuit 35 is set so that the level of a sending signal Sin is around the boundary of regions X, Y shown in figure and a fixed attenuator 11 is controlled by a control circuit 23 so as to inert the loss at the regions X, Y. When the transmission signal Sin and a reception signal Rin are in the region X, a sample-and-hold circuit 37 outputs a transmission signal from a detection circuit 21. When the signals Sin and Rin are moved to the region at the outside of the region X, the attenuation of a variable attenuator 29 is set so that the white noise level fed to an adder 32 is equal to the level of the transmission signal Sin outputted from the circuit 37. Then the white noise of the said level is added to the signal Sin at the echo suppression so as to reduce the sudden change in noise level on the transmission line.
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