DC-free line code and bit and frame synchronization for arbitrary data transmission
 A method of encoding data for transmission over a communication link. A cumulative polarity of previously-transmitted frames is maintained. A frame is prepared for transmission by combining a data word with a plurality of additional bits. The additional bits provide a master transition. A phantom bit is encoded in the additional bits. If the polarity of the frame is the same as the cumulative polarity, the data bits or in some instances all the bits are inverted so as to maintain balance. Control words and fill words are provided and are distinguished from data words by encoding the additional bits. Control words carry additional data or control instructions and are distinguished from fill words by the number of transitions. The phantom bit either conveys additional data or is used for such purposes as error checking. 
Hewlett-Packard Company
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Thomas Richard C. Patrick Cheryl L. Benny W. H. Jieh-Tsorng Chu-Sun
Hornak Walker Petruno Stout Lai Wu Yen