Security identification system using pulse code modulation

Claim 1

A security identification system for a telephone transmission system including a plurality of individual transmitter stations at varying locations connectable through the transmission system to a receiver station, the security identification system comprising: a plurality of transmitter identification units, each connected to one of the transmitter stations, each transmitter identification unit including: identification signal generator means for generating and applying to the transmitter station telephone line a pulse code modulated identification signal unique to the transmitter station to which the identification unit is connected, the identification signal including a repeating sequence of uncoded and coded signal segments, each uncoded signal segment comprising a series of N pulse intervals of fixed duration D, each coded signal segment comprising a series of M pulse intervals of fixed duration D with coding effected by the presence or absence of pulses in those intervals, each code pulse comprising a burst of cycles of a carrier signal of frequency F, with F>>1/D, the identification signal generator means comprising a carrier signal generator, including a stabilized oscillator, for developing a continuous carrier signal at the carrier frequency F, an encoding circuit, utilizing the carrier signal as a synchronizing signal, for developing a pulse code signal corresponding to the uncoded and coded segments of the identification signal, and a modulator for modulating the carrier signal with the pulse code signal to generate the identification signal, and filter means, interposed in series in the transmitter station telephone line, to preclude transmission of the identification signal to the transmitter station; a visual display associated with the receiver station; and a receiver identification unit, associated with the receiver station, comprising: demodulator means, coupled to the receiver station telephone line, for demodulating the identification signal to develop a pulse code signal corresponding to the identification signal but with the carrier effectively eliminated, control means for utilizing the pulse code signal to actuate the display to afford a visual representation identifying the particular transmitter station from which that identification signal originates, the control means including means for identifying the beginning of a coded signal segment in the pulse code signal by counting successive pulse intervals in an uncoded signal segment to assure identification of the beginning of a coded signal segment, and filter means, interposed in series in the receiver telephone line, to preclude transmission of the identification signal to the receiver station.