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Reference.org
Bipolar encoding
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<p>In telecommunication, bipolar encoding is a type of <a href="/facts/Return-to-zero/Pq239gv7">return-to-zero</a> (RZ) <a href="/facts/Line_code/x9IuaYNL">line code</a>, where two nonzero values are used, so that the three values are +, −, and zero. Such a signal is called a duobinary signal. Standard bipolar encodings are designed to be <a href="/facts/DC-balanced/crU5EHzl">DC-balanced</a>, spending equal amounts of time in the + and − states. </p><p>The reason why bipolar encoding is classified as a <a href="/facts/Return_to_zero/Pq239gv7">return to zero</a> (RZ) is that when a bipolar encoded channel is idle the line is held at a constant "zero" level, and when it is transmitting bits the line is either in a +V or -V state corresponding to the binary bit being transmitted. Thus, the line always returns to the "zero" level to denote optionally a separation of bits or to denote idleness of the line. </p>