| 摘要 |
PURPOSE:To attain accurate gain control by inserting a high-pass filter whose cut-off frequency is 50Hz to the pre-stage of control signal generating circuit in a feedback loop controlling a variable gain circuit, detecting a digital level and generating a control signal. CONSTITUTION:The variable gain circuit 2 whose gain is varied by a digital control signal, a high-pass filter 6 to which a digital output signal of the vari able gain circuit 2 is applied, and a control signal generating circuit 7 to which an output of the high-pass filter 6 is applied and generating a digital control signal are provided. The digital high-pass filter 6 whose cut-off frequency is nearly 50Hz is inserted to the pre-stage of the control signal generating circuit 7 in the feedback loop controlling the variable gain circuit 2. The DC offset after the A/D conversion and a low frequency noise included in the input signal are eliminated by the high-pass filter 6, the digital output signal is fed to the control signal generating circuit 7, where an accurate digital level is detected to generate a digital control signal and to apply the gain control.
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