Abstract: The analog broadband output signal of a medical Doppler device is sampled at a constant rate to convert the signal to digital form. The sample values are stored consecutively in a random access memory component. Such values are recalled from such component at a readout rate different from the sampling rate so as to create a frequency-shifted intermediate broadband signal corresponding to the Doppler output signal. Such intermediate signal is filtered and the power of the filtered intermediate signal is continuously compared to the power of the unfiltered intermediate signal. The readout rate from the memory component is adjusted to maintain approximately the same proportionate relationship of the power of the filtered intermediate signal to the power of the unfiltered intermediate signal. The continuously adjusted readout rate from the memory component indicates a "band edge" frequency above or below which a preselected proportion of the total power of the Doppler signal resides.