| 摘要 |
A subscriber's circuit for a four-wire-system local switch is provided with a two-wire/four-wire hybrid circuit that includes a two-wire/four-wire converter circuit. Connected between the converter circuit and a time-division switch section is an incoming circuit that amplifies an incoming voice current, supplies a DC current to a subscriber's telephone set, and generates a subscriber's line signal of large amplitude, such as a bell and a howler signal, by repeatedly reversing the polarity of the DC current. An alarm signal is produced by stepwise changing the duration and the frequency of the repetitive polarity reversal in response to digital command signals supplied from the time-division switch section. An outgoing circuit device, which is connected between the converter circuit and the time-division switch section amplifies the output signal of the converter circuit, transmits a signal representative of a subscriber's call or answer to the time-division switch section in response to the state of the DC current supplied to the subscriber's telephone set, and terminates the operation thereof in response to the cut-off of the DC current to thereby prevent singing.
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