A method of characterizing a newly acquired waveform with respect to previously acquired waveforms during monitoring of a generally repetitive signal, where the previously acquired waveforms have been rasterized into a two-dimensional array of memory locations, reads history values for those memory locations associated with an active portion of the newly acquired waveform, compares the history values with history value ranges, increments a count for one of a plurality of recent pixel counters corresponding to the history value ranges, each counter having a different history value range, and modifies the history values in the memory locations. From the counts accumulated for each of the history value ranges the variability of the newly acquired waveform from the generally repetitive signal is determined.