Duobinary signal
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In telecommunication, a duobinary signal is a pseudobinary-coded signal in which a "0" ("zero") bit is represented by a zero-level electric current or voltage; a "1" ("one") bit is represented by a positive-level current or voltage if the quantity of "0" bits since the last "1" bit is even, and by a negative-level current or voltage if the quantity of "0" bits since the last "1" bit is odd.
Note 1: Duobinary signals require less bandwidth than NRZ.
Note 2: Duobinary signaling also permits the detection of some errors without the addition of error-checking bits.
Source: from Federal Standard 1037C and from MIL-STD-188