1. A system for comparing groups of digitized data, comprising: a. means for processing a first group to obtain a first signature representative of a first information portion of said first group of data; b. means for selecting a set of subsignatures from said signature, said subsignatures representing selected respective portions of said first signature; c. means for selecting one of the set of said subsignatures which has the maximum relative information content, by successive comparison of the members of said set of subsignatures to said group; d. means for processing a second group to obtain a second signature representative of a corresponding first information portion of said second group; and e. means for comparing said selected subsignature to respective portions of said second signature in order to determine a comparison error for each said portion of said second group, and for designating the respective portion of said second group with respect to which the lowest comparison error is determined, as being the portion of said second group to which said selected subsignature corresponds.