A device for assigning to the unknown onset of a signal in a noisy time series a probability of that onset having a signed excursion away from the trace mean including one or more means arranged to: define a polarity at a given time sample by reference to the sign of the amplitude difference between the extrema immediately before and after the given time sample in the time series; define a positive and a negative polarity probability density function; take the respective products of the positive and negative polarity probability density functions with an onset probability density function defining the probability that the onset of the signal occurs at a given time sample; and marginalize the respective products of the positive and negative polarity probability density functions with the onset probability density function over time to estimate final probabilities that the onset has a positive or a negative polarity.