A method, including receiving a bipolar signal froma pair of electrodes in proximity to a myocardium of ahuman subject and receiving a unipolar signal from aselected one of the pair of electrodes. The methodfurther includes computing a local unipolar minimumderivative of the unipolar signal, and a time ofoccurrence of the unipolar minimum derivative. The methodalso includes computing a bipolar derivative of thebipolar signal, evaluating a ratio of the bipolarderivative to the local unipolar minimum derivative, andwhen the ratio is greater than a preset threshold ratiovalue, identifying the time of occurrence as a time ofactivation of the myocardium.
