| 摘要 |
A signal detecting device can make determination whether to carry out or not automatically dialling transmission by detecting an identification signal sent from an exchange system via a telephone line. The frequency variation of the identification signal is constant since the identification signal is a composite wave of a plurality of constant frequency signals. A pulse train having substantially a constant number of pulses can be obtained by comparing the level of the identification signal provided from the telephone line with a predetermined reference voltage by voltage comparators. Frequency counters count for a constant time period the number of pulses included in the pulse train provided from voltage comparators. According to the count result output of frequency counters, a determining circuit makes determination whether an identification signal is received or not.
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