1. A method for encoding a digital information signal consisting of a plurality of N-bit data words, comprising the steps of: generating M-bit data words in which M is an even integer greater than N and each of said N-bit and M-bit data words is comprised of a plurality of logic level "1" bits and logic level "0" bits; selecting those M-bit data words having a sequence of no more than two bits all at logic level "1" or "0" extending inwardly from both boundaries of those M-bit data words and in which all sequences of bits all at logic level "1" or "0" are no longer than four bits within those M-bit data words; classifying the selected M-bit data words into a first group consisting of those M-bit data words having an equal number of said "1" and "0" bits, a second group consisting of those M-bit data words having a greater number of said "1" bits than of said "0" bits, and a third group consisting of those M-bit data words which have a greater number of said "0" bits than of said "1" bits; assigning each of said N-bit data words to at least a respectively one of said M-bit data words, some of said N-bit data words being assigned alternately to M-bit data words of said second and third groups and the remainder of said N-bit data words being assigned to M-bit data words of said first group; and emitting an output stream of immediately contiguous assigned M-bit data words in response to an input stream of N-bit data words, said output stream being adapted for improved self-clocking with reduced low frequency components.