The NRZ-M bivalent coding method eliminates the disadvantages of NRZ coding, namely that DC voltage fluctuations occur with long 0 and 1 sequences on the transmission link and, in addition, synchronization can no longer be established.
With NRZ-M, if the digital signal is 1, a change of state occurs. If the digital signal is 1, a transition from the previous state to the other occurs in NRZ-M. If the NRZ-M level was high before, it changes to low, if it was low before, it changes to high. If the digital signal has a 0, then no level change takes place with NRZ-M coding.
With this coding, additional level changes are generated for synchronization with long 1 sequences.