Run-length reduction of 3PM code via look-ahead technique

The binary 3PM code has at least two ZEROS between transitions, and thus has an increase in density ratio over MFM. While the original 3PM had at most 11 ZEROS between transitions, this number has been reduced to 7 by look-ahead techniques, whereby the present code word can be a function of input bits yet to come. The ability to "see into the future" enables look-ahead codes to achieve rates close to capacity with simple logic: so long as m/n≤capacity, it is possible to achieve an m-into-n code with an m and n relatively prime. Encoding is done one word at a time by observing the contents of the present code word, the previous code word, and the look-ahead region 4 bits wide, and substituting patterns not used in the basic encoding. Changes are made only in the present word and its preceding bit. Decoding is done likewise, with an observation field of 14 positions.