Emergency signal warning system

Claim 1

Apparatus for giving a warning indication of a siren which produces an audio frequency signal which changes in frequency between F1 hz. and F2 hz. at the rate of about F3 times per minute comprising, in combination: a. a transducer for picking up the siren signal and changing it to electrical signals; b. amplification means tuned to amplify signals from said transducer which are within the F1 to F2 frequency range; c. a phase-locked-loop circuit consisting of electrical circuit means for producing electrical signal oscillations within the frequency range of F1 to F2 and additional electrical circuit means for comparing the signal from said amplification means to the signal produced by said oscillation means and for producing an error signal to change the frequency of the latter to substantial agreement with the former; d. band-pass amplification means coupled to said additional electrical circuit means for producing an electrical signal output of a predetermined level only when the frequency of the said error signal is about F3; and e. means coupled to the output of said band-pass amplification means for producing a warning signal when the output is at least said predetermined level.