| 摘要 |
<p>A public address system includes a microphone (2), a variable gain amplifier (4), at least one loudspeaker (6). The differential volume adjuster includes a second microphone (8) placed in the vicinity of at least one loudspeaker (6), a substractor (10) and an amplitude equalizer (12). The gain of the amplifier (4) is controlled by means of a signal (Z) output from the substractor (10) representing only the level of the ambient noise. The substractor (10) subtracts the magnitude of a signal (Y) output from the equalizer (12) from a signal (X) output from microphone (8) being the combined audio output (A1) from the loudspeaker (6) and the ambient audio noise (A2). The equalizer (12) receives a first input signal (W) from the microphone (2) and a second input signal (V) from the output of the amplifier (4). Thus the gain of the amplifier (4) is varied so as to compensate for varying changes in the level of the background noise to improve the intelligibility of the audio output signal from the loudspeaker (6). The system could be used in a fire alarm unit where microphone (2) is replaced by an electric signal generator energised on detecting a fire and loudspeaker (6) would be replaced by a siren. <IMAGE></p> |
