Abstract: An electronic measurement system for extracting a small AC signal from a dominant DC background signal, which can be changing at a rate similar to that at which the desired signal changes. The invention is particularly useful for pulse rate measurement of a subject even while undergoing vigorous motion such as running, by means of pulse oximetry. The measurement technique utilizes a moving window for selecting a part of the input signal, and processing in an A/D converter, an offset part of the signal which falls within a range which covers the window. The method is also more generally employable to any measurement task, where the signal to be extracted is a small AC signal buried within a dominant DC or quasi-DC background, which itself can be changing, and even at a rate similar to that expected of the sought-after AC signal.