A method, including receiving a bipolar signal from a pair of electrodes in proximity to a myocardium of a human subject and receiving a unipolar signal from a selected one of the pair of electrodes. The method further includes computing a local unipolar minimum derivative of the unipolar signal, and a time of occurrence of the unipolar minimum derivative. The method also includes computing a bipolar derivative of the bipolar signal, evaluating a ratio of the bipolar derivative to the local unipolar minimum derivative, and when the ratio is greater than a preset threshold ratio value, identifying the time of occurrence as a time of activation of the myocardium.
