A zero-crossing-based approximate realtime demodulator is described. Output digital samples, generated through linear interpolation of a suitably approximated piecewise linear model of the modulating signal, are passed through an usual reconstruction filter to recover the original modulating signal. Details of a simple microprocessor-based implementation of the demodulator are presented along with an estimate of the sample error bound for a sinusoidal modulation. A good compromise between constructional simplicity and overall output distortion is an important feature of this digital demodulator which is well suited for application in the areas of industrial telemetry and underwater communication.