A spectrometry method and apparatus for filtering step-like pulses output by a preamplifier to measure the energy or other desired characteristics of events occurring in a detector attached to the preamplifier. The spectrometer provides a set of filters that are capable of measuring the desired characteristic with different accuracy or resolution, detects the pulses in the preamplifier signal, and measures the time between consecutive pairs of detected pulses. For each detected pulse, the spectrometer selects a filter from the set of available filters, based on the measured time intervals between the pulse and its immediately preceding and succeeding detected pulses, applies it to the pulse, and indexes the output of the filtering operation with one or more indices identifying the selected filter.