A method includes receiving a plurality of mappingelectro-cardiogram (ECG) signals from respective mappingelectrodes coupled to a surface of a heart of a patient.Multiple reference ECG signals are received fromrespective reference electrodes coupled to the patient.The multiple reference ECG signals are jointly processed,so as to produce a single timing reference indicative ofcardiac cycle timing of the heart. The single timingreference is applied to the mapping ECG signals.