A bipolar electrogram and a unipolar electrogram are recorded from electrodes of a probe, and differentiated with respect to time. Peaks are identified in the differentiated bipolar electrogram. An activity window is defined that includes bipolar activity about the peaks. An extreme negative value in the differentiated unipolar electrogram within the activity window is reported as a unipolar activation onset. In one aspect, an annotation is selected from candidate minima in the differentiated unipolar electrogram within the activity window by excluding candidates that fail to correlate with activity in the bipolar electrogram.
