| 摘要 |
In a telephone system where frequency multiplexed analog telephone signals and digital data signals are transmitted over a line, incoming calls are announced to the customer installation by a ringing indication signal having an amplitude between 1 and 30 V RMS, which is much less than the regular ringing and therefore easier to be generated, and moreover having a smoother shape than a rectangular shape. Upon receiving the ringing indication signal, the customer installation locally generates and provides a full ringing signal to a telephone. The amplitude of this local ringing signal is higher than the amplitude of the ringing indication signal, but the line with the digital data signal is isolated from the customer telephone equipment at the time the ringing signal is generated, so as to avoid disturbance of the digital data transmission by the ringing current, which is a source of impulsive noise.
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