| 摘要 |
<p>A siren detector for detecting siren sounds which precess at known yelp, wail and high-low warble rates within a selected frequency band. A transducer (8) detects siren sounds and produces a corresponding electrical output signal. The output signal is filtered (10) to reject signal frequencies outside the selected frequency band. The signal amplitude is monitored and varied as required to produce a constant amplitude output signal. The constant amplitude output signal is low pass filtered (15) for detection of siren high-low sounds and is also low pass filtered (16) for detection and output of precession signals which pass through a selected centre frequency. Subsequent filters (19,20,21) are provided for detecting signals which vary at the wail and yelp warble rates. Wail, high-low and yelp warble rate signals output by the various filters respectively trigger don't walk and yelp clock generators (23,24) which in turn drive don't walk and red light output flip-flops (26,27) respectively. Operation of the red light output flip-flop is controlled by a delay mechanism (25) which provides a further filtration level to present false triggering.</p> |
