Analog signal

An analogue signal is a continuously varying representation of physical quantities used for the transmission, processing, and measurement of information. Unlike digital signals, which operate through discrete binary values, analogue signals fluctuate smoothly over time and can assume an infinite range of values within a given interval. They are fundamental to classical communication systems, audio and video technologies, instrumentation, and many natural processes, making them essential to understanding both traditional electronics and modern hybrid systems.

Characteristics and Fundamental Properties

Analogue signals are defined by their capacity to vary continuously in both amplitude and time. This variability allows them to represent physical phenomena such as sound waves, light intensity, temperature, pressure, and motion. Key properties include amplitude, frequency, phase, and wavelength, each describing a particular aspect of the signal’s behaviour.
Analogue signals commonly take two forms:

• Continuous-time signals: Defined at every moment in time.
• Continuous-amplitude signals: Capable of taking any value within a certain range.

A typical analogue signal may appear as a sine wave, although in practice many signals consist of complex waveforms formed from multiple frequency components. The smooth nature of analogue signals enables precise representation of gradual changes but also makes them susceptible to noise, distortion, and attenuation during transmission.

Sources and Representation

Analogue signals originate from both natural and engineered sources. Physical sensors convert real-world phenomena into electrical analogue signals; for example, microphones convert sound pressure into voltage, thermocouples convert heat into electrical potential, and photodiodes convert light intensity into current.
Representation methods commonly include:

• Time-domain graphs, showing amplitude variations over time.
• Frequency-domain plots, obtained using Fourier analysis, illustrating the spectral components of the signal.
• Phasor diagrams, particularly useful in alternating-current systems.

These forms help engineers analyse behaviour, detect distortion, and design circuits that preserve signal integrity.

Analogue Signal Processing

Analogue signal processing involves manipulating signals using continuous-time electronic circuits. Traditional methods rely on components such as resistors, capacitors, inductors, op-amps, and transistors. Common operations include filtering, amplification, modulation, mixing, and differentiation or integration.
Typical analogue devices and systems include:

• Amplifiers for boosting weak signals.
• Analogue filters (low-pass, high-pass, band-pass) for frequency selection.
• Oscillators for generating periodic waveforms.
• Modulators and demodulators in radio communications.
• Phase-locked loops for frequency synchronisation.

Analogue systems often possess high bandwidth and real-time responsiveness, making them suitable for applications where continuous monitoring or low-latency processing is essential.

Analogue Communication Systems

Communication technologies historically relied on analogue signalling to transmit information over wires, airwaves, or optical channels. Examples include traditional radio broadcasting, landline telephony, and early television. Analogue communication typically uses modulation schemes such as:

• Amplitude Modulation (AM)
• Frequency Modulation (FM)
• Phase Modulation (PM)

In these systems, the properties of a carrier wave are varied according to the information signal. Although digital communication has become predominant due to improved noise immunity and data efficiency, analogue techniques remain relevant in broadcasting, amateur radio, audio transmission, and specialised industrial contexts.

Noise, Distortion, and Limitations

Analogue signals are inherently vulnerable to degradation. Noise—random, unwanted variations—can enter a signal during generation, transmission, or measurement. Sources of noise include thermal fluctuations, electromagnetic interference, and imperfections in electronic components.
Major forms of signal impairment include:

• Attenuation: Reduction in amplitude over transmission distance.
• Distortion: Alteration of waveform shape due to nonlinearities.
• Interference: External signals overlapping the desired waveform.

Because analogue systems lack discrete encoding, noise accumulates continuously and is difficult to remove without affecting the information content. This limitation is a principal reason for the extensive adoption of digital technology.

Analogue-to-Digital Conversion and Hybrid Systems

Modern electronic systems often integrate analogue and digital components. When physical data must be processed by digital computers, it undergoes analogue-to-digital conversion (ADC), a process involving sampling, quantisation, and encoding. Conversely, digital-to-analogue conversion (DAC) is used to reproduce continuous waveforms from digital data, essential in audio playback, video output, and control systems.
Key steps in ADC include:

• Sampling: Measuring the signal at regular time intervals.
• Quantisation: Assigning values to discrete levels.
• Encoding: Converting quantised values into binary form.

Hybrid analogue–digital systems combine the responsiveness and authenticity of analogue components with the precision and stability of digital processing, offering advantages in fields such as telecommunications, medical imaging, radar, and industrial automation.

Applications and Technological Relevance

Analogue signals continue to play an important role across numerous domains. In engineering, they provide real-time measurements in control systems, instrumentation, and robotics. In audio technology, analogue sound is valued for its continuous representation, often perceived as warmer or more natural than digital sound.
Further applications include:

• Sensors and measurement devices used in scientific research.
• Analogue control loops in mechanical and electrical systems.
• Biomedical equipment measuring heart rate, brain activity, and muscle signals.
• Radio-frequency systems, from aviation communication to maritime navigation.