Abstract

A data communications system for transmitting and receiving 64 different audio-frequency signals representing 64 respective alpha-numeric code symbols. A synchronizing signal consists of three time-spaced bursts where each burst consists of three audio-frequency signals. A following information signal consists of a three time-spaced message bursts where each burst consists of twenty audio-frequency signals, all three of the message bursts containing the same audio-frequency signals but in different orders. A receiver includes synchronizer means to recognize the predetermined synchronizing signal, and decoder means enabled and synchronized by the synchronizer means to decode the information signal. The audio-frequency signals may be 64 different fixed-frequency tones, or may be 64 differently-changing signals such as may be produced by unidirectional phase shift keying by 64 different pseudo-random number codes.