| 摘要 |
PROBLEM TO BE SOLVED: To separately control each speaker by providing a preamplifier for public addressing for every vehicle, incorporating a power amplifier for every speaker box, operating a speaker as a microphone and performing a sound volume setting by relating the setting to car inside noise. SOLUTION: A public address starting is set by a public address starting SW, public address sound produced from a microphone for public addressing is amplified by a public address controller, public address sound is amplified by a preamplifier via a circuit for car inside public address and the public address sound is delivered to a speaker box 6 at each vehicle. A speaker box 6 is constituted of a power amplifier 7 and a speaker 8. After sound is amplified in the power amplifier 7, sound is delivered from the speaker 8. Car inside noise (b) enters from the speaker 8, a subtraction is performed by public address sound (a) and a subtraction amplifier 22 and only car inside noise (b) is amplified. The level of a public address signal to be inputted in a transistor 21 by amplification is controlled in responce to the car inside noise. Thus, sound volume can be automatically adjusted for every installation location of the speaker 8 within the vehicle. |
