Signal transmission and surveillance system using an operational telephone line

Claim 1

An electronic signal transmission system comprising a first unit, said first unit comprising a transmitter, a second unit, said second unit comprising a receiver matched to said transmitter, a telephone transmission line having two wires, conductive means connecting said first unit to said telephone line, conductive means cooperatively connecting said second unit to said telephone line, a telephone, a telephone exchange, said telephone line connecting said telephone and said telephone exchange, said telephone and said telephone line and said telephone exchange together defining a telephone system, said telephone exchange causing a DC bias current on said telephone line when said telephone is electrically connected to said line sufficient for message transmission between said telephone and said exchange, normal current on said telephone line which is not caused by said signal transmission system being telephone system current, said first and second units being of sufficiently high resistance so as to substantially reduce DC loading of said line, said first and second units of sufficiently high and balanced impedance to telephone system AC voltage on said line as to substantially reduce attenuation of said voltage and unbalancing of said line by said telephone system AC units, said first and second units comprising means to detect the state of said telephone system, said transmitter comprising audio-signal means for modulating said telephone line voltage, said first unit being adapted to receive an event signal, said transmitter modulating the voltage of said line in accordance with said telephone system state and said event signal in a way which is discernible to said receiver, said receiver sensing said modulated telephone line voltage, said second unit transforming said modulation into an output signal, said output signal being electrically connectable to a suitable means for utilization.