| 摘要 |
PURPOSE:To prevent MODEM data from being shifted at once to a sound signal when it comes below a threshold level, and also to prevent a mismatch between a coding side and a decoding side at the time of the momentary disconnection of a transmission line by means of switching only the decoding side by switching the MODEM data into a sound mode when the power of the MODEM data continues to be lower than the threshold level for a prescribed period. CONSTITUTION:A sequence circuit 4 is provided for the output of a signal power decision circuit 3. When a MODEM training signal is detected by a MODEM training signal detector 2, it is switched to a MODEM coding mode, but hereafter the sequence circuit 4 does not output a switching signal to the sound coding mode, unless a state that the input signal power P of the MODEM data is below the threshold level T continues for the prescribed period. Since the MODEM data is scarecely below the threshold level for the prescribed period during a transmission, the switching to the sound coding mode during the inputting of the MODEM data is gone. Since the same procedure as this is given to the decoding side, the switching to the sound coding mode of only the decoding side is gone as far as the momentary disconnection period is within the above mentioned prescribed period.
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