| 摘要 |
PURPOSE:To simplify the constitution of an FM detection circuit discriminating whether interference exists while a line is being connected or in talking by using an amplifier switching a gain based on the judgment whether the circuit is being connected or in talking so as to constitute an interference detection circuit at line connection. CONSTITUTION:A control section 10 sends a gain variable signal of 'H' or 'L' level to a gate switch 75 based on the judgement whether the circuit is being connected or in talking. In receiving the gain variable signal of 'H' level, the gate switch 75 is controlled to be in a short-circuited state, the gain of an operational amplifier 70 is set, and the output of a rectifier circuit 8 to be processed reaches an FM detection level discriminating whether or not interference exists while the circuit is being connected and the level is inputted to a discrimination circuit 9, where the signal is processed. When the gain variable signal of 'L' level is supplied, the gate switch 75 is controlled to the open state, the gain of the operational amplifier 70 is decreased more than that while the circuit is being connected, and the output of the rectifier circuit 8 to be processed reaches the FM detection level enough for the discriminating whether or not there is interference during talking and the result is inputted to the comparator 19 and processed. |
