Aspects of the present disclosure are directed to detecting Atrial Fibrillation (AF). As may be implemented in accordance with one or more embodiments, a time series of inter-beat intervals is computed from a recording of activity of a beating heart. The time series is decomposed into subcomponents, and an envelope of at least one of the subcomponents is computed. The presence of atrial fibrillation (AF) is detected based upon characteristics of the envelope that are indicative of AF.