# 1.4 Definition of terms

#### This section introduces many of the terms and definitions relating to the communications channel. Some of these terms can have specific meanings within the context of a digital communications system, and these are defined here.

## Characteristics of Message Type

### Analogue

#### An analogue signal is defined as a physical time varying quantity and is usually smooth and continuous, e.g. acoustic pressure variation when speaking. The performance of an analogue communications system is often specified in terms of its fidelity or quality, hence the term HIFI – HIgh FIdelity.

### Digital

#### A digital signal on the other hand is made up of discrete symbols selected from a finite set, e.g. letters from the alphabet or binary data. The performance of a digital system is specified in terms of accuracy of transmission e.g. Bit Error Rate (BER) and Symbol Error Rate (SER).