| 摘要 |
PROBLEM TO BE SOLVED: To miniaturize a DTMF(dual tone multi frequency) signal generating circuit. SOLUTION: This DTMF signal generating circuit is constituted of a frequency-divider circuit 13 which frequency-divides a reference clock signal from a transmitting circuit 11, first and second up/down counters 14-1 and 14-2 which up/down count a clock signal from the frequency-divider circuit 13 based on a control signal from a control circuit 12, first and second sine wave circuits 15-1 and 15-2 which output first pseudo sine wave data for generating a DTMF signal with low group frequencies and which output second pseudo sine wave data for generating a DTMF signal with high group frequencies respectively based on the control signal from the control circuit 12 by using the count values of the first and second up/down counters 14-1 and 14-2 as addresses, an adder 16 which adds the outputs of the first and second sine wave circuits 15-1 and 15-2, and a D/A converter 17 which converts the output of the adder 16 into an analog signal, and outputs the DTMF signal.
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