# 3 Baseband data transmission

# 3.1 Introduction to Chapter 3

#### Many people's concept of digital communications is one of nice square voltage pulses representing 0s and 1s being passed over a piece of cable or perhaps a radio system. In actual fact, this is rarely how digital information is sent.

#### We have already seen the justification for using multi-level symbols to reduce the bandwidth content of a signal, and in this chapter we will see that square pulses become rounded pulses when passed through a channel with finite bandwidth. |

#### In section 3.2, the filtering effects of a channel are analysed for their impact on streams of data pulses – bits and symbols. What at first sight appears to be a 'show stopper' for data transmission – the intersymbol interference caused by channel filtering – is shown to be unfounded if steps are taken to achieve a Nyquist filter response for the whole system. Section 3.3 introduces the eye diagram as a powerful visual tool for observing and diagnosing problems within the modem portion of a digital communications link. This is followed in Section 3.4 by a look at a very popular type of channel filter, the raised cosine filter, which is found in almost all modern modem implementations. Section 3.5 discusses the concept of a matched filter – this in fact describes an overall channel filter response that should result in the optimum performance of the modem in the presence of noise. Finally, Section 3.6 goes against the stance taken in Section 3.3 by advocating the introduction of a controlled amount of intersymbol interference into the system by careful choice of filter (termed partial response signalling). Surprisingly, perhaps, this can achieve improved performance under certain conditions.