Abstract: A security identification system for telephone, teleprinter, computer, and similar communication applications comprising a plurality of transmitter station identification units, one for each transmitter station and each including an identification signal generator for generating a pulse code modulated identification signal, unique to the transmitter station, that comprises a repeating sequence of uncoded signal segments each including N pulse intervals of given duration alternating with coded signal segments each including M pulse intervals of like duration, coding being effected by the presence or absence of pulses in the coded signal segment intervals. Each pulse is transmitted as a burst of cycles of a carrier signal preferably having a carrier frequency above 3 Khz.