Audio system for drive-in theaters

Claim 1

In a drive-in theater having an array of pre-designed parking spaces, a central audio output device for generating a carrierless audio signal, and a network of conductors for distributing said signal to said parking spaces, wherein the improvement comprises a plurality of interface devices each located at and associated with a particular one of said parking spaces and connected in parallel to said audio output device by said network to receive said audio signal as an input, each of said interface devices comprising: oscillator means for generating a carrier signal, said oscillator means including an inductor and a capacitor arranged as a resonant circuit and a modulation transistor to which said resonant circuit is connected; a secondary winding arranged for mutual inductance with said inductor to form an output transformer; oscillator control means for activating said oscillator means in response to the presence of said audio signal and for deactivating said oscillator in response to the absence of said audio signal, said oscillator control means comprising a field-effect transistor and a control capacitor connected to the gate of said transistor and arranged to forward bias to said transistor upon charging of said control capacitor; means for rectifying said audio signal to charge said control capacitor; voltage divider means for supplying a portion of said audio signal to said oscillator transistor; a rechargeable battery connected to said oscillator means to provide power for said oscillator means; means for charging said battery with said audio signal comprising a diode arranged to rectify said audio signal, said diode being light emitting to indicate the presence of said audio signal; and transfer means connected to said output transformer for inputing said audio signal to the radio antenna of an individual automobile.