Security identification system using pulse code modulation

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.