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An adaptive filter routine is presented in which the frequency-selective characteristics are related to the sampling frequency. The sampling frequency can be locked to the input frequency within a predetermined range, and consequently implement self-adaptive characteristics. The routine utilises one set of 2nd-order notch-filter coefficient values as its vehicle. No external reference signal is required. The phase relationship between the input and output signals is used to perform the adaptive procedure. The real-time implementation of this adaptive technique is presented, and time-estimation curves are provided. The system is extended to consider adaptive noise cancellation where the adaptive mechanism removes unwanted noise superimposed upon the input signal. The system used to implement the adaptive routine is a 16-bit microprogrammable microprocessor based on the Intel 3000 series of computing components.