1. A method for classifying cardiac beats for use in detecting cardiac rhythms, comprising: sensing a cardiac signal over a plurality of cardiac cycles using a plurality of electrodes coupled to a sensing module; determining a template of a known cardiac signal in response to the cardiac signal sensed over the plurality of cardiac cycles; determining a template alignment point from the template of the known cardiac signal; sensing a cardiac signal having an unknown morphology over a plurality of cardiac cycles; determining a fourth order difference signal by applying a fourth order difference function to the cardiac signal having an unknown morphology; determining an alignment point for the cardiac signal having an unknown morphology from the fourth order difference signal; aligning the template and the cardiac signal having an unknown morphology across an alignment window by aligning the template alignment point and the alignment point of the cardiac signal having an unknown morphology; determining a first morphology match metric between the aligned template and the cardiac signal having an unknown morphology; and classifying the cardiac signal having an unknown morphology based on the first morphology match metric.