A system and method of identifying focal sources is presented. The method can comprise detecting, via sensors, electro-cardiogram (ECG) signals over time, each ECG signal detected via one of the sensors having a location in a heart and indicating electrical activity of the heart, each signal comprising at least an R wave and an S wave; creating an R-S map comprising an R-to-S ratio for each of the ECG signals, the R-to-S ratio comprising a ratio of absolute magnitude of the R wave to absolute magnitude of the S wave; identifying, for each of the ECG signals, local activation times (LATs); and correlating the R-to-S ratios for the ECG signals on the R-S map and the identified LATs and using the correlation to identify the focal sources.