The present invention provides a noninvasive localization, characterization and classification apparatus and method for cardiac arrhythmias. The invention enables discrete isolation of the intricate spatial and temporal detail in morphology of the atrial activity of interest from superimposed ventricular activity of a preceding heartbeat in a particular arrhythmia. An adaptive QRST subtraction template is used that is modulated for discrepancies in voltage and rate between the QRST template and arrhythmia signal. The QRST template modulation is accomplished by using one or more fiducial points and windows that are annotated in both the QRST template and the arrhythmia signal. The invention includes, but is not limited to, the isolation of atrial activity that are commonly known as: (1) P waves in case of focal atrial fibrillation, atrial tachycardia, and orthodromic AV reentrant tachycardia; (2) fibrillation waves in case of chronic atrial fibrillation; and (3) flutter waves in case of atrial flutter.