| 摘要 |
PURPOSE:To prevent a noise level from being increased in a silent state by avoiding the passage of a level signal included in a range for increasing the amplification of a level between 1st and 2nd levels when a call-enable state in which a level signal in a sound signal exceeds the 1st level is shifted to a silent state in which the level signal is lower than the 2nd level. CONSTITUTION:A sound signal to be an AC signal is inputted to comparators 21, 22 in a level detection part 13 and a comparator part 41 in a level detection part 31 as a DC signal through a rectifying part 12. The comparator part 41 compares set voltage Ref1 as the DC level signal to detect the level of the sound signal. When the DC level signal exceeds the voltage Ref1, the comparator part 41 turns a detection signal to be applied to a switch part 32 to 'H' to turn on the switch 32. Thereby both ends of a resistor R are short-circuited, voltage Ref2, Ref2 are set to the same potential and capacitors C11, C12 are charged with the same electrostatic charge. When this state is suddenly changed to a silent state having no calling, the DC level signal is dropped lower than the voltage Ref2 in accordance with the discharge of the capacitor C1 and reduced down to a ground level, so that the increment of noise can be prevented. |
