NonReturntoZero

Non-Return-to-Zero (NRZ) is a binary line encoding used in digital communications in which information is represented by two distinct voltage levels and there is no return to a baseline level between successive bits. In NRZ, a 1 and a 0 are encoded by different levels; in its common variants, NRZ-L maps bit values directly to levels, while NRZ-I uses transitions to signal a 1, with a 0 defined as no transition.

NRZ-L (NRZ-Level) encodes each bit by a constant level for the duration of the bit interval. For example, a 1 might be represented by a positive voltage and a 0 by a negative voltage. The scheme is simple and spectrally efficient but has drawbacks: long runs of identical bits produce few transitions, making clock recovery harder and, in real channels, introducing baseline wander due to DC offsets or drift.

NRZ-I (NRZ-Inverted or NRZI) encodes a 1 as a transition and a 0 as no transition. The current level depends on the previous level and whether a 1 occurs. This tends to create more transitions than NRZ-L, aiding clock recovery, but it can still suffer from baseline wander and phase ambiguity on long sequences of identical bits.

NRZ encodings are often contrasted with schemes that guarantee transitions or synchronization per bit, such as Manchester encoding. The main advantages of NRZ are simplicity and high data rates with minimal overhead, but practical use requires consideration of DC balance, clock recovery, and channel distortion to maintain reliable communication.