| 摘要 |
PURPOSE:To enable to efficiently supervise the encoding and decoding circuits of an optional channel, by utilizing the time when the reverse channel is not used since a subscriber has become a listener, and providing a supervisory signal to said reverse channel. CONSTITUTION:The channel selective circuit 10' selects channel to which a supervisory signal is sent, and detects an applied supervisory signal. This circuit 10' checks whether a decoded signal level is in the tolerance or not against the signal level which has been prescribed in advance, and supervises whether the operation of the decoding circuit 8' or the encoding circuit 1' is normal or not. In order to prevent a supervisory signal provided to the channel which has been selected in said case, from being sent out to the line, the output signal is suppressed by the sound suppressing circuit 9' by selecting the channel to which the supervisory signal has been applied, by the channel selective circuit 11'. Subsequently, to the reverse direction, too, both the encoding circuit and the decoding circuit are supervised in the same procedures. In this way, the encoding and decoding circuits of an optional channel can be supervised efficiently. |
