Apparatus in accordance with the subject invention maintains an exponentially decayed histogram of the counts of new pixels for each active channel, together with a running count of time and number of acquisitions. At regular intervals determined by a maximum time or by a minimum number of acquisitions it computes a new threshold for each active channel. This threshold will theoretically produce “N” nominally unusual waveforms per second where N is defined by a user “sensitivity” control. The raw histogram is smoothed to provide a stable “tail” of small probabilities. Once a threshold has been determined, subsequent waveforms with more than this number of new pixels are re-examined to determine the number of “really new” pixels. “Really new” pixels are defined as those that are not adjacent to pixels remaining from earlier acquisitions. Only those waveforms that have more “really new” pixels than a specified fraction of the basic threshold are reported as anomalies.
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