# Contents

## Idea

A signal is, roughly speaking, the use of a particular physical entity, typically a waveform of a certain kind, to convey information about another. Signals fall into two broad categories: analogue and digital.

In both the analogue and digital cases, the physical conversion from information $I$ about an entity $E$ to a signal $S_E(I)$ sent through a certain medium conveying $I$ is carried out in the final instance by a transducer?. The waveform $S_E(I)$ itself which the transducer sends out is typically constructed by modulating a waveform of a standard form, or more than one such waveform, according to an algorithm to which $I$ is an input.