DC FREE ENCODING FOR DATA TRANSMISSION SYSTEM

A method and apparatus provide for the elimination of any net DC component from the transmission of binary data sequentially in successive clocked bit cells of a transmission channel wherein logical first bit states, e.g., 0's are normally transmitted as signal transitions relatively early in respective bit cells, preferably at cell edge, and logical second bit states, e.g., 1's are normally transmitted as signal transitions relatively late in respective bit cells, preferably at mid-cell, and any transition relatively early in a bit cell following a transition relatively late in the next preceding bit cell is suppressed. The onset of a sequence of second bit states following a first bit state that might introduce a DC component into the transmitted signal with normal transmission is detected by producing a first indicating signal, and in response to the first indicating signal, the state of a current bit and the state of the next succeeding bit, the transmission of signal transitions is modified to eliminate any DC component. Preferably, the end of a sequence of second bit states that would introduce a DC component is detected by producing a second indicating signal utilized to modify the signal transitions at the end of a troublesome sequence, as by suppressing the transition corresponding to the last second bit state in such a sequence.