Non Return to Zero Level(NRZ-L) is an extremely simple coding method for digital signals that corresponds to Non Return to Zero (NRZ). With NRZ-L, a 1-bit signal is represented by a high level, and a 0-bit signal by a low or negative level.
The NRZ-L method is not self- clocking, i.e. it has no synchronization properties. In the case of long 0 or 1 sequences without level changes, no level changes are transmitted. This causes fluctuations in the DC level and the clock recovery can get out of step.
A functional enhancement is available in the form of Enhanced Non Return to Zero Level (E-NRZ-L). It is essentially equivalent to Non Return to Zero Level (NRZ-L) with the difference that a parity bit is appended to the data strings. Depending on the user polarity, this bit is High or Low.