A system and method for narrowband sinewave modulation. The system includes an input buffer for storing input digital data and a sub-periodic modulator for encoding the input digital data in a periodic waveform. The sub-periodic modulator encodes one or more bit values of the input digital data within each period of the periodic waveform. One or more digital-to-analog converters generate an encoded analog waveform from a digital representation of the periodic waveform wherein the encoded analog waveform is of a frequency f and a power P. The encoding is performed by the sub-periodic modulator such that any signal of frequency f′ resulting from the encoding is of a power P′ at least 50 dB less than power P, where f′ is offset from f by more than 25 Hz.