### Data encoding and the telephone channel

#### While data encoding can effectively move the signal energy away from 0 Hz and permit ac-coupled operation, the encoding process does not provide sufficient shifting of the frequency components within a baseband data symbol stream to allow them to pass effectively through the bandpass response of the telephone link. |
| #### In order to ensure minimum distortion of the data passing over the telephone channel, it is clear that the spectral energy of the data signal must be placed in the centre of the channel passband where there is the flattest gain response and the smoothest group delay. This requirement is achieved by modulating the baseband data stream on to a carrier signal and is described in Chapter 5.