Abstract: Medical apparatus and methods for diagnostic and site determination of cardiac arrhythmias within a heart of a subject are provided. A computing device receives, records and processes electrocardiogram (ECG) signals in the form of bipolar and unipolar ECGs associated with respective cardiac tissue locations corresponding to catheter distal end sensors on locations. Unipolar ECGs that include signals from a plurality of successive heartbeats corresponding to locations within an area of study are analyzed to identify Fractionated Unipolar ECG Signal Complexes (FUESCs) of unipolar ECGs by defining complexes of the unipolar ECGs that correspond to respective bipolar activity windows. Identified arrhythmia sites for treatment include a predetermined number of unipolar ECGs that have a predetermined number of FUESCs. Atrial arrhythmia sites for treatment by ablation can be identified with respect to FUESCs of unipolar ECGs that include signals from at least ten successive heartbeats of an atrial tissue study area.