Why bipolar encoding such as AMI is called bipolar?
Bipolar signalling simply means that the signal goes both positive and negative. Both RZ and NRZ signals can be bipolar; RZ always is, to my knowledge.
Unipolar signalling, on the other hand, only ever goes positive with respect to the return/ground. (It could also only go negative, but that's less common.)
I haven't heard it called just &quot;polar&quot;, but I would assume that refers to unipolar, by the way you contrast it with bipolar. It's not the number of valid signal states, but rather the number of signs used--either just positive, or both positive and negative.