# 4.4 The telephone channel

## AC coupled channels

#### In the discussions so far, we have assumed that the channel is able to pass all frequency components of a data signal from 0 Hz up to a bandwidth of B Hz. There are, however, many communications channels that are not able to pass low frequency components, either because they need to be ac-coupled for practical reasons (such as eliminating unwanted dc-offsets), or because the bandwidth allocated to a given user is in a part of the spectrum well removed from 0 Hz, for example, wireless communications. #### The domestic telephone channel is a classic example of a channel which is bandpass in nature, that is, it has a low frequency and high frequency cut-off in its gain response. |

#### The low frequency cut-off (ac-coupling) results from capacitive and/or inductive coupling of the telephone line at both the exchange and subscriber ends allowing dc power for the telephone to be passed over the same cable as the speech or data signal. The high frequency cut-off is a combination of deliberate filtering at the exchange to minimize noise on the channel and also the transmission line filtering effect of long lengths of cable. A typical telephone channel response is shown here. Notice that even within the channel, the gain response may not be flat and this in itself will introduce further symbol degradation.