Signal transmission and surveillance system using a subscriber{3 s telephone line without interfering with normal telephone line operation

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, said first and second units being of sufficiently high and balanced impedance to telephone system AC voltage on said line as to substantially reduce attenuation of said telephone system AC voltage and unbalancing of said line by said units, said transmitter comprising low-frequency means for modulating said telephone line in a way which is discernible to said receiver, said first unit being adapted to receive an event signal, said transmitter delivering a modulating signal current to said line in accordance with said event signal, said receiver delivering a counter-modulating signal current to said line in response to said modulating signal current, said counter-modulating signal current at least partially compensating for said modulating signal current and hence reducing voltage on said line due to said modulating signal current so as to reduce system modulation of the telephone system voltage on said line, said second unit providing an output signal related to said counter-modulating signal current, said output signal being electrically connectable to a suitable means for utilization.