In a system and method for detecting electrical cardiac events, and a heart stimulator embodying such a system, cardiac events are detected in respective chambers of a heart by sensing electrical signals in at least two different chambers of the heart and forming a difference signal from the sensed signals, and using the difference signal to automatically distinguish between events originating from one of the chambers and events originating in another of the chambers. At least one of the sensed signals is sensed in a coronary vein on the left atrium or the left ventricle.