Wailing siren detecting circuit
First Claim
1. A wailing siren detecting circuit including, in combination:
- (a) a microphone for mounting on a car in a position to detect outside sounds;
(b) a pre-amplifier connected to said microphone including automatic gain control means to provide improved linearity at high signal input levels;
(c) a high Q voltage tunable band pass filter connected to the output of said pre-amplifier;
(d) a frequency to voltage converter connected to the output of said high Q voltage tunable band pass filter;
(e) a feedback means from the output of said frequency to voltage converter to said high Q voltage tunable band pass filter to shift the center frequency of said band pass filter as a function of the voltage output of said frequency to voltage converter whereby tracking of a dominant frequency by said band pass filter is achieved;
(f) a low frequency band pass filter receiving said voltage from said frequency to voltage converter for passing only low frequency signals;
(g) a full wave rectifier and differentiating circuit connected to the output of said low band pass filter for providing an output signal only in response to a continuously slowly varying voltage signal corresponding to the pitch variation of a wailing siren;
(h) an indicating means for mounting in said car;
(i) energizing means connected to said indicating means and to the output of said full wave rectifier and differentiating circuit for energizing said indicating means in response to the presence of said output signal from said full wave rectifier and differentiating circuit;
(j) high pass filter and amplifier means connected to the output of said frequency to voltage converter for passing only erratically rapidly varying high frequency noise signals developed as a result of tracking dominant noise frequencies;
(k) full wave rectifier and integrating circuit means connected to the output of said high pass filter and amplifier means providing an inhibit signal only when said voltage signal is erratically varying as a result of tracking said dominant noise frequencies; and
(l) inhibiting means connected between said indicating means and said full wave rectifier and integrating means for inhibiting operation of said indicating means in response to receiving said inhibit signal whereby said indicating means is only energized in response to a wailing siren.
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Abstract
The detecting circuit is used on cars to indicate the presence of emergency vehicles utilizing wailing sirens the instant invention residing in the unique circuitry itself to result in an extremely inexpensive electronics package for carrying out the desired function. Essentially, the circuit utilizes a band pass filter having a voltage tunable center frequency. Dominant frequencies in a sound spectrum received through the band pass filter are converted to voltages by means of a frequency to voltage converter and fed back to the band pass filter to shift the center frequency of the filter thereby tracking the dominant frequency signal. When the dominant frequency results from a wailing siren, the fed back voltage from the frequency to voltage converter constitutes a slowly continuously varying AC signal corresponding to the pitch variation of the siren. This signal is detected by a low pass filter and differentiating circuit to operate a flasher to indicate the presence of the siren. When the dominant frequency results from noise or steady state sounds or the like resulting in a random shifting of the center frequency of the band pass filter, further filter and integrating means responsive to such random signals generate an inhibiting signal preventing spurious operation of the flasher.
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Citations
3 Claims
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1. A wailing siren detecting circuit including, in combination:
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(a) a microphone for mounting on a car in a position to detect outside sounds; (b) a pre-amplifier connected to said microphone including automatic gain control means to provide improved linearity at high signal input levels; (c) a high Q voltage tunable band pass filter connected to the output of said pre-amplifier; (d) a frequency to voltage converter connected to the output of said high Q voltage tunable band pass filter; (e) a feedback means from the output of said frequency to voltage converter to said high Q voltage tunable band pass filter to shift the center frequency of said band pass filter as a function of the voltage output of said frequency to voltage converter whereby tracking of a dominant frequency by said band pass filter is achieved; (f) a low frequency band pass filter receiving said voltage from said frequency to voltage converter for passing only low frequency signals; (g) a full wave rectifier and differentiating circuit connected to the output of said low band pass filter for providing an output signal only in response to a continuously slowly varying voltage signal corresponding to the pitch variation of a wailing siren; (h) an indicating means for mounting in said car; (i) energizing means connected to said indicating means and to the output of said full wave rectifier and differentiating circuit for energizing said indicating means in response to the presence of said output signal from said full wave rectifier and differentiating circuit; (j) high pass filter and amplifier means connected to the output of said frequency to voltage converter for passing only erratically rapidly varying high frequency noise signals developed as a result of tracking dominant noise frequencies; (k) full wave rectifier and integrating circuit means connected to the output of said high pass filter and amplifier means providing an inhibit signal only when said voltage signal is erratically varying as a result of tracking said dominant noise frequencies; and (l) inhibiting means connected between said indicating means and said full wave rectifier and integrating means for inhibiting operation of said indicating means in response to receiving said inhibit signal whereby said indicating means is only energized in response to a wailing siren. - View Dependent Claims (2, 3)
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Specification