## Data encoding for AC coupled channels

### NRZ vs Manchester Encoding

#### In order to allow unmodulated data symbols to pass uncorrupted through ac-coupled channels, it is necessary to ensure that the symbol stream has very little or no energy at or near 0 Hz. This means that the data must be either scrambled or encoded to ensure that no long strings of 1s or 0s are transmitted. |

#### Scrambling of the data involves reordering of the data symbols in such a way that the chances of long strings of 1s or 0s occurring in the scrambled waveform is minimized or eliminated. This process introduces a processing overhead and also a latency in the encoding and decoding process which may introduce unacceptable time delay in the communication process – particularly if digital voice is being carried. In most cases, a data encoding process is used in preference to scrambling, albeit at the expense of occupied bandwidth in most cases. There are many data encoding schemes for reducing the dc content of a data signal, the most common of which is Manchester encoding. It can be seen that compared with the original non-return-to-zero (NRZ) data stream, the Manchester encoded signal never contains long strings of 1s or 0s. For a more detailed discussion of line coding techniques, see Haykin (1989).