# Why NRZ-L and NRZ-I line coding have DC component?

I can find the effect of having a DC component in the Internet which is "When the AC waveform has a DC component, the average voltage would be equal to the DC voltage instead".

But in my text book, it says NRZ-L and NRZ-I both have a DC component although NRZ-L and NRZ-I can have the average voltage $0$.

Why is it?

• Can you let us know what your textbook is? One reason simple encodings have a DC offset is because the data is not equally distributed between ones and zeros. Another reason might be that the NRZ uses a physical 0V for the zero level and a non-zero voltage for the one level. That will also cause a DC component. – Peter K. Oct 19 '16 at 15:26
• It's "Data Communications and Networking" 5th edition. Thanks :) – wyldecat Oct 19 '16 at 16:03