A medical device and associated method for classifying an unknown cardiac signal that includes 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, 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；determining an R-wave onset and an R-wave offset in response to the fourth order difference signal of the unknown cardiac cycle signal, determining an R-wave width as the difference between the R-wave onset and the R-wave offset, determining a morphology analysis window in response to the R-wave width, and determining a first morphology match metric across the morphology analysis window.
