Thats... not quite what I meant, perhaps I worded that a little bit weird. I meant frequency invariant, not amplitude normalised. As in divide the frequencies b

Thats... not quite what I meant, perhaps I worded that a little bit weird. I meant frequency invariant, not amplitude normalised. As in divide the frequencies by the fundamental frequency (i.e. subtract their logs on the 'scope), not divide the amplitudes by the amplitude on the fundamental. Any deviation from the profile of the FT irrespective of its base frequency, from the normal sinus rhythm, would easily show. There might be a few problems dealing with cardiac load transitions, and I suppose if you are trying to differentiate the exact class of arrhythmia (as in the paper) then perhaps not, but differentiating normal (sinus), noise, abnormal it should work fine.