| 摘要 |
An information terminal incorporates a network interface and an extension interface, and in the case of outside call, an audio send signal is modulated into an audio signal frequency band, and sent to the exchange through a private telephone line and a subscriber's line by way of the network interface. An audio receive signal sent from the exchange through the subscriber's line and private telephone line is demodulated into an ordinary audio frequency band through the network interface. On the other hand, in the case of extension service, an audio send signal is converted into a packetized digital signal by an extension interface circuit, and is further modulated into a higher frequency band so as not to overlap with the audio signal frequency band, and is transmitted through the private telephone line.
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