| 摘要 |
PURPOSE:To remarkably improve simultaneous transmission/reception performance by providing a frequency shift circuit to at least one channel of a transmission path or a reception path. CONSTITUTION:Frequency shift circuits 3, 12 are provided to both of the transmission and reception paths and the output signal of the channel is shifted to a higher or a lower frequency in the range of several tens several hundreds Hz with respect to the input signal. Since the frequency is shifted by the frequency shift circuits 3, 12 in case of a route of microphone 1 2 wire/4 wire converter 8 speaker 9 microphone 1, even if the gain of the closed loop at a frequency is the unity or over, no oscillation is caused. Thus, it is possible to reduce the attenuation of variable gain attenuators 4, 11 thereby improving the simultaneous transmission/reception performance. That is, the insertion loss is enough to be a quantity to suppress the echo.
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