Abstract: A desired signal having a wide frequency band and including sequential symbols representing digital values is discriminated from a narrow band interfering signal having a carrier frequency in the wide band by initially applying both signals to a filter so both signals are passed to an output of the filter. In response to the filter output, estimates of the values of received symbols of the desired signal are derived and compared to the filter output to derive an error representing signal. A characteristic of the filter is controlled in response to the error representing signal so the filter is adjusted to reject the interfering signal and pass the desired signal.