| 摘要 |
PROBLEM TO BE SOLVED: To accurately detect whether or not a signal on a line is a prescribed tone signal by deciding that a signal to be detected is the prescribed tone signal when the counted frequency is in a frequency range proper as the prescribed tone signal. SOLUTION: A tone frequency detecting part 7 detects whether the level and frequency of the signal to be detected on a line inputted via a network control part is proper or not as a tone signal desired to detect. In this case, an analog switch for controlling PWM is provided on a prestage of a comparator 74 and a low-pass filter 73. Then, a read tone detecting frequency is compared with upper/lower limit frequencies corresponding to an installing area set as installing area setting information in a tone detection parameter table by each installing area. As the result of the comparing, whether the tone detecting frequency is within the range of the upper/lower limit frequencies. In the case of being within the range, the tone signal is decided to be present, and in the case of being outside of the range, the tone signal is decided to be absent.
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