1. A method for automatically detecting unusual waveforms during monitoring of a generally repetitive electrical signal by a digital oscilloscope, the method comprising the steps of: determining a number of “new” pixels affected by each waveform rasterization process; comparing the determined number of “new” pixels with a first threshold number of pixels; forming a histogram of a frequency of new pixels; using said histogram to define a second threshold number of pixels; and if the determined number of pixels is more than said second threshold number of pixels, classifying that rasterized waveform as an unusual waveform.