1. A method, comprising the steps of: inserting a probe into a heart of a living subject, the probe having electrodes; recording a bipolar electrogram and a unipolar electrogram from a location in the heart with the electrodes; defining a time interval comprising a window-of-interest; differentiating the bipolar electrogram and the unipolar electrogram with respect to time to define a differentiated bipolar electrogram and a differentiated unipolar electrogram, respectively; identifying peaks within the window-of-interest in the differentiated bipolar electrogram; establishing a bipolar activity window about respective peaks, the bipolar activity window having activity bounds that include activity about the peaks in the bipolar electrogram; and identifying an extreme negative value (−dV/dt) in the differentiated unipolar electrogram within the activity bounds, and reporting a time corresponding to the value as a unipolar activation onset.