Analogue and Digital Signals

Two broad categories of electrical signal. Every electrical signal used to carry information -- whether a voltage, a current, or some other measurable quantity that varies with time -- falls into one of two broad categories: ANALOGUE or DIGITAL, and the distinction between them decides which kind of circuit is used to generate, process, and interpret the signal.

Analogue signals. An analogue signal is one that can take ANY value, continuously, across some defined range -- there is no smallest meaningful step size, and the signal can, in principle, vary smoothly between any two of its possible values. A microphone's electrical output (a direct, continuously varying representation of the sound pressure wave striking it), an ordinary AC mains supply voltage, and the raw voltage from a temperature sensor are all analogue signals. Analogue signals are naturally processed by circuits such as the amplifier of Section 9.22, which is itself designed to respond smoothly and proportionally to a continuously varying input.

Digital signals. A digital signal, by contrast, is restricted to only a small, FIXED number of discrete levels -- in almost every practical digital circuit, exactly TWO, conventionally labelled HIGH (or logic $1$) and LOW (or logic $0$), corresponding physically to two clearly separated voltage ranges (e.g. close to the supply voltage $V_{CC}$ for HIGH, close to $0$ V for LOW). A computer's internal data, a digital clock's display, and the data stored on a compact disc are all digital in nature. Digital signals are naturally processed by circuits such as the transistor switch of Section 9.23 and the logic gates built from it (Sections 9.26-9.31), both of which are themselves fundamentally two-state devices. …