Correlation in real time

Abstract

The process of correlation can be effected in real time only by methods of time compression, or by replacement of the fundamental integration over time by an integration over distance. The Deltic correlator is an example of the first, while optical correlators and tapped delay line matched filters illustrate the second; the paper summarizes these techniques. Two new forms of real-time correlator are described. The first provides correlation of an incoming waveform against a reference waveform which is known in advance. The reference signal is stored in the device as a thin copper waveform made, for ease and economy, by a standard printed circuit process. A magnetic tape passes over and along this waveform, and the input signal is recorded on this tape in the conventional manner. The output from the copper waveform, after equalization, is the cross correlation function of the two waveforms. The second form of correlator provides real-time operation where neither signal is known in advance. Both in put signals are recorded in duplicate on magnetic tapes. From these tapes magnetic fields representing the sum and difference of the two waveforms are derived, and these fields are used to energize two extended magnetoresistive elements. These operate in the manner of a quarter-squares multiplier to produce a change of resistance which follows the cross correlation function of the two waveforms. Both correlators can be realized in several configurations adapted to particular needs within and outside of seismology.