| 摘要 |
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 counting first bit states and producing a first indicating signal when the count is of parity indicating such sequence, and in response to the first indicating signal, a current bit and an adjacent bit the transmission of signal transitions is modified from the onset of such sequence to eliminate any DC component. Preferably, the possibly troublesome sequences are encoded by encoding pairs of second bit states by a single transition early in the first bit cell of the pair. Further, a limited look-ahead means is provided which is responsive to the first indicating signal, a current bit and a limited number of next succeeding bits for producing a disabling signal at the onset of any such sequence that terminates within the limited number of next succeeding bits and would not introduce such DC component with normal transmission. In addition, means responsive to the disabling signal is provided for disabling the transition modifying means for the transmission of signal transitions corresponding to all of the second bit states of such sequence that would not introduce such DC omponent with normal transmission.
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