A system includes an electrical interface and a processor configured to receive, via the electrical interface, a signal sensed by at least one electrode in contact with tissue of a subject's heart, the signal spanning successive time periods that are each of length T1 and including multiple signal points, to calculate respective thresholds for the time periods, to select a set of points that includes, for each of the time periods, a signal point of greatest magnitude in the time period, provided that the greatest magnitude is greater than the threshold for the time period, to remove, from the set, one of any pair of the selected points that are within an interval T2 of one another, T2 being less than T1, and to generate, subsequently to the removing, an output that is based on the points remaining in the set corresponding to respective electrical activations of the tissue.