Special purpose digital apparatus is described for off-line correlation of time varying functions in analogue form. The necessary storage, repetition and relative scanning are afforded by an instrumentation magnetic tape recorder through the extension of its ‘closed loop’. A new value of lag coincides with the complete replacement of that length of tape contained within the ‘closed loop’, and a fresh value of correlation coefficient is printed out for each lag. The method takes up only two tracks, so hose remaining can be used in the normal acquisition role at any convenient speed, later transferring their signals at high speed to the analysis circuits.