19. A circuit for recognizing a string of spoken characters according to claim 18 wherein said group correspondence signal forming means comprises means for assigning each reference character to a predetermined one of a plurality of classes; means responsive to the classes for the reference characters of each reference character set for assigning the reference character set to a predetermined group; means responsive to the character correspondence signals for each input character for selecting a class correspondence signal for said input character; and means for combining the selected class correspondence signals for the input characters to generate a correspondence signal for each group.
19. A circuit for recognizing a string of spoken characters according to claim 18 wherein said group correspondence signal forming means comprises means for assigning each reference character to a predetermined one of a plurality of classes; means responsive to the classes for the reference characters of each reference character set for assigning the reference character set to a predetermined group; means responsive to the character correspondence signals for each input character for selecting a class correspondence signal for said input character; and means for combining the selected class correspondence signals for the input characters to generate a correspondence signal for each group. 20. A circuit for recognizing a string of spoken characters according to claim 19 wherein said reference character assigning means comprises means for assigning each reference character to a predetermined one of first and second classes; and said means for selecting a class correspondence signal for each input character comprises means responsive to the input character correspondence signals for determining the minimum character correspondence signal for the reference characters assigned to said first class and the minimum character correspondence signal for the reference characters assigned to said second class.