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; sensing an unknown cardiac signal over an unknown cardiac cycle; determining a fourth order difference signal; determining a template alignment point and an unknown cardiac signal alignment point in response to the fourth order difference signal; aligning the template and the unknown cardiac signal across an alignment window by aligning the template alignment point and the unknown cardiac signal alignment point; determining a first morphology match metric between the aligned template and the unknown cardiac signal.