| 摘要 |
PURPOSE:To enable automatic gain control depending on the input level of an audio signal, by changing the time constant of a time constant circuit depending whether or not the gain control by a variable gain amplifier is made. CONSTITUTION:A variable gain amplifier 3 is inserted and connected between an input terminal 1 and an output terminal 2. A control circuit 5 controls the gain of the amplifier 3 based on the input/output of the amplifier 3 or a signal of sum and difference of the input and output. When the output level from a time constant circuit 23 is a reference voltage or more, a transistor Tr17 is off by the output from a comparison circuit 27 and the time constant of a time constant circuit 13 is longer. Further, when the output level from the circuit 23 is less than the reference voltage, the gain control for level compression or the like is not made, and the time constant of the circuit 13 can be short only by a capacitor 16 and a resistor 14 with a Tr17 on. Thus, the gain can be changed automatically depending on the input level of an audio signal, allowing to compress and expand the signal level. |
