| 摘要 |
PROBLEM TO BE SOLVED: To easily design frequency characteristics of a telephone line interface by easily adjusting the low group frequency component and the high group frequency component of a DTMF signal and to eliminate the need for D/A converters to reduce the cost. SOLUTION: First and second sine wave generation circuits 201 and 202, which generates sine waves having prescribed frequencies, respectively, a synthesis circuit 210 which synthesizes the outputs of first and second sine wave generation circuits in a prescribed ratio and outputs the synthesis result, a PWM modulation circuit 214 which generate a pulse which has a width corresponding to the output of the synthesis circuit, and a filter 211 which removes unwanted frequency components included in the output of the PWM modulation circuit are provided. The synthesis ratio in the synthesis circuit is variable.
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