What Is Distortion?

Distortion is any systematic alteration of a signal's waveform, spectral content, or timing that was not present in the original signal. It arises when a system fails to reproduce its input faithfully, either because of nonlinear device characteristics, bandwidth limitations, or multipath propagation. Distortion degrades audio fidelity, reduces communication link margins, and introduces measurement errors in instrumentation. The field draws on circuit theory, signal processing, and electromagnetic analysis to characterize, model, and mitigate unwanted signal modification across applications ranging from audio amplification to wireless communications.

Harmonic Distortion

Harmonic distortion occurs when a nonlinear system generates output components at integer multiples (harmonics) of the input frequency. A pure sine wave at frequency f₀ entering a nonlinear amplifier will produce energy at 2f₀, 3f₀, and higher harmonics that were not present at the input. Total harmonic distortion (THD) quantifies the ratio of the root-mean-square amplitude of all harmonic components to that of the fundamental, typically expressed as a percentage or in decibels. High-fidelity audio amplifiers are specified with THD figures below 0.01%, while power conversion circuits may tolerate THD levels of several percent. IEEE Standard 519 sets limits on harmonic distortion in power systems to protect grid equipment from overheating and interference.

Intermodulation Distortion

Intermodulation distortion (IMD) arises when two or more signals at different frequencies pass through a nonlinear device and produce sum-and-difference products at new frequencies not present in the input. For input tones at f₁ and f₂, a third-order nonlinearity produces intermodulation products at 2f₁ − f₂ and 2f₂ − f₁, which fall close to the original signals and cannot be removed by simple filtering. The third-order intercept point (IP3) is the standard figure of merit used to characterize a component's IMD performance: a higher IP3 indicates greater dynamic range before intermodulation products become objectionable. IMD is a primary design concern in RF amplifiers, mixers, and analog-to-digital converters used in multi-carrier wireless systems. The IEEE Transactions on Microwave Theory and Techniques documents techniques including feedforward and digital predistortion for reducing IMD in power amplifiers.

Nonlinear Distortion

Nonlinear distortion is the general class that encompasses both harmonic and intermodulation products, produced whenever the input-output relationship of a system deviates from proportionality. It is distinguished from linear distortion, which alters amplitude or phase in a frequency-dependent but proportional manner. The Volterra series provides a mathematical framework for analyzing weakly nonlinear systems by expanding the output as a sum of convolution integrals of increasing order; truncating the series at the third order captures the dominant nonlinear effects in most practical amplifiers and mixers. Saturation and clipping are extreme forms of nonlinear distortion that arise when a signal exceeds a device's linear operating range, producing abrupt waveform truncation and a corresponding broadening of the signal's spectrum.

Signal and Acoustic Distortion

Signal distortion in communication channels includes linear mechanisms such as intersymbol interference (ISI), frequency-selective fading, and group delay variation, as well as nonlinear impairments from the transmitter chain. Digital equalization techniques, including decision-feedback equalization and maximum-likelihood sequence estimation (MLSE), are used to reverse linear channel distortion at the receiver. Acoustic distortion specifically describes the deviation of a reproduced sound pressure waveform from the original. Loudspeaker distortion arises from voice-coil nonlinearity, suspension compliance variation, and thermal effects; IEC 60268 defines the measurement standards for audio equipment. ANSI/ASA S1.1 provides the terminology framework for describing acoustic quantities including distortion in sound systems.

Applications

Distortion has applications in a wide range of disciplines, including:

• Wireless communications: digital predistortion of power amplifiers in 4G/5G base stations to improve spectral efficiency
• Audio engineering: harmonic and intermodulation measurement in microphones, amplifiers, and loudspeakers for professional and consumer audio
• Power electronics: harmonic analysis and mitigation in inverters, variable-speed drives, and power factor correction circuits
• Medical ultrasound: tissue harmonic imaging, which exploits nonlinear distortion of ultrasound pulses to improve image contrast
• Test and measurement: linearity characterization of analog-to-digital converters and RF front ends in spectrum analyzers