Abstract: It is important that a data transmission system should be fast, utilize a minimum of band width and be accurate in the presence of high noise levels. The invention provides a frequency shift keying data transmission system for transmission of binary coded information in which the mark of the coded pulse waveform is represented by a half cycle of one keying frequency and the space is represented by a half cycle of another keying frequency. An improvement in speed, band width utilization and accuracy at high noise levels is obtained by providing that the transmission signal is an approximately sinusoidal waveform of which the phase is continuous at the transitions, the time lengths of the mark and space symbols thereof are matched to the half time periods of the respective keying frequencies and the slope is constant at the cross-over points.