Abstract: An electronic recognition unit indicates that a flow of received information is starting when a desired bi-phased pulsed ratio signal starts to be received. The recognition unit discriminates against both noise and spurious radio signals. Desired signals are made recognizable by reversing, upon transmission, the phase of the carrier for a prescribed pattern of pulses. This technique prevents responding to unqualified continuous-wave signals. The relative phases of adjacent radio-frequency pulses of the desired pulse train convey the recognition information. When the phases of two adjacent pulses are the same a positive audio-pulse output is obtained, and if their phases differ by 180 degrees the output is negative. For a predetermined number of pulses, for example fourteen, the resulting positive and negative audio pulses are added in an audio delay circuit with appropriate inversion of the sign of the intentionally negative pulses.