20. The method of claim 13 wherein the separating step includes passing the detected energy through a plurality of lowpass filters, and amplifying the output of adjacent lowpass filters with difference amplifiers to separate the detected energy into a plurality of frequency bandwidths.
20. The method of claim 13 wherein the separating step includes passing the detected energy through a plurality of lowpass filters, and amplifying the output of adjacent lowpass filters with difference amplifiers to separate the detected energy into a plurality of frequency bandwidths. 23. The method of claim 20 wherein the energy spectrum deriving step includes passing the output of adjacent sets of difference amplifiers through a plurality of summing amplifiers, and summing the outputs of the summing amplifiers.