Bipolar line codes have two polarities, are generally implemented as RZ, and have a radix of three since there are three distinct output levels (negative, positive and zero). One of the principle advantages of this type of code is that it can eliminate any DC component. This is important if the signal must pass through a transformer or a long transmission line.
Unfortunately, several long-distance communication channels have polarity ambiguity. Polarity-insensitive line codes compensate in these channels.[6][7][8][9] There are three ways of providing unambiguous reception of 0 and 1 bits over such channels:
- Pair each code word with the polarity-inverse of that code word. The receiver is designed so that either code word of the pair decodes to the same data bits. Examples include alternate mark inversion, Differential Manchester encoding, coded mark inversion and Miller encoding.
- differential coding each symbol relative to the previous symbol. Examples include MLT-3 encoding and NRZI.
- Invert the whole stream when inverted syncwords are detected, perhaps using polarity switching
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