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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?

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    $\begingroup$ 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. $\endgroup$ – Peter K. Oct 19 '16 at 15:26
  • $\begingroup$ It's "Data Communications and Networking" 5th edition. Thanks :) $\endgroup$ – wyldecat Oct 19 '16 at 16:03
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Assuming bi-polar encoding, and assuming that the number of transmitted 1's is equal to the number of 0's, then over very long time intervals the positive pulses "cancel out" the negative pulses and the average voltage is zero.

However, over smaller time intervals, there's likely to be more 1's than 0's, or vice versa. This is what introduces a DC component to the signal.

Intuitively, you may think about it this way: if you filter the line-encoded signal with a very narrow low-pass filter, you'll see a non-zero, varying output. The DC value is the expected value of the filter's output.

In order to reduce or eliminate DC you need a line encoding that forces the number of positive and negative pulses to be the same, even over very short time spans. One example is the Alternate Mark Inversion (AMI) encoding.

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