| 摘要 |
PURPOSE:To decrease the required bit number of an operation processing circuit by combining a processing pre echo canceller and an adaptive type digital processing echo canceller and eliminating a part of an echo signal from an input signal of the latter in advance so as to narrow the dynamic range of an A/D converter. CONSTITUTION:A transmission signal from a transmitter 2 is given a specific frequency characteristic by a filter circuit 22 in a pre echo canceller 21 and attenuated properly by an attenuation circuit 23, then subtracted from a reception signal including an echo signal incoming through a terminal 6 and a hybrid transformer 5 by a subtraction circuit 24. When the frequency characteristic of the filter 22 and the attenuation of the attenuation circuit 23 are decided properly, the absolute amount of the echo signal included in the signal inputted to the digital processing adaptive type echo canceller 13 is decreased, the number of bits required for an A/D converter 7 is reduced and the number of bits of the operation processing circuit is reduced.
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