duobinary
Duobinary is a pseudo-binary coding that works with two different hi-levels, one positive and one negative, and with a zero level. It thus belongs to the three-level or ternary coding methods.
In duiobinary representation, the logical 0 is represented by a zero level, and the logical 1 is represented by a positive or negative level. Which level state is represented depends on the number of logic zero states after the last 1 state. If there is an even number of zeros since the last 1-level, the positive 1-level is used, if there is an odd number of zeros, the logical 1 is represented by the negative 1-level.
Duobinary signals require less bandwidth than NRZ encoding. Also, some errors can be detected without additional parity bits.