1. A method for classifying cardiac beats for use in detecting cardiac rhythms, comprising: sensing a cardiac signal over a plurality of cardiac cycles; 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 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, resulting in an aligned fourth order difference signal of the cardiac signal having an unknown morphology; determining an R-wave onset and an R-wave offset from the aligned fourth order difference signal of the cardiac signal having an unknown morphology; determining an R-wave width as a difference between the R-wave onset and the R-wave offset; determining a morphology analysis window in response to the R-wave width; determining a first morphology match metric between the aligned template and the cardiac signal having an unknown morphology across the morphology analysis window; and classifying the cardiac signal having an unknown morphology based on the first morphology match metric.