| 摘要 |
PURPOSE:To prevent deterioration in tone quality when sound is low, by setting the output voltage of a D/A converter controlled by a bus line as the reference voltage of the D/A converter. CONSTITUTION:The D/A converter 1 is controlled by the bus line. The reference voltage is applied on the D/A converter 2 via an RC filter 3. A sound output level that is the conversion output of the D/A converter 2 changes almost in proportion to the reference voltage. Meanwhile, a sound volume data is sent to the D/A converter 1 via the bus line, and the converter 1 generates corresponding DC voltage by the RC filter 3. The output voltage of the filter 3 can perform sound volume control by setting it as the reference voltage of the converter 2. Since the converter 1 generates only a sound volume control voltage, it is enough to prepare the one operated at low conversion speed, and the converter is constituted of the one of simple type such as a PWM type converter, etc.
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