Total harmonic distortion
The total harmonic distortion (THD or THDi) is a measurement of the harmonic distortion present in a signal and is defined as the ratio of the sum of the powers of all harmonic components to the power of the fundamental frequency. Distortion factor, a closely related term, is sometimes used as a synonym.
In audio systems, lower distortion means the components in a loudspeaker, amplifier or microphone or other equipment produce a more accurate reproduction of an audio recording.
In radio communications, devices with lower THD tend to produce less unintentional interference with other electronic devices. Since harmonic distortion tends to widen the frequency spectrum of the output emissions from a device by adding signals at multiples of the input frequency, devices with high THD are less suitable in applications such as spectrum sharing and spectrum sensing.
In power systems, lower THD implies lower peak currents, less heating, lower electromagnetic emissions, and less core loss in motors. IEEE std 519-2014 covers the recommended practice and requirements for harmonic control in electric power systems.
Definitions and examplesEdit
To understand a system with an input and an output, such as an audio amplifier, we start with an ideal system where the transfer function is linear and time-invariant. When a sinusoidal signal of frequency ω passes through a non-ideal, non-linear device, additional content is added at multiples nω (harmonics) of the original frequency. THD is a measure of that additional signal content not present in the input signal.
When the main performance criterion is the ″purity″ of the original sine wave (in other words, the contribution of the original frequency with respect to its harmonics), the measurement is most commonly defined as the ratio of the RMS amplitude of a set of higher harmonic frequencies to the RMS amplitude of the first harmonic, or fundamental, frequency
where Vn is the RMS voltage of the nth harmonic and n = 1 is the fundamental frequency.
In practice, the THDF is commonly used in audio distortion specifications (percentage THD); however, THD is a non-standardized specification and the results between manufacturers are not easily comparable. Since individual harmonic amplitudes are measured, it is required that the manufacturer disclose the test signal frequency range, level and gain conditions, and number of measurements taken. It is possible to measure the full 20–20 kHz range using a sweep (though distortion for a fundamental above 10 kHz is inaudible).
Measurements for calculating the THD are made at the output of a device under specified conditions. The THD is usually expressed in percent or in dB relative to the fundamental as distortion attenuation.
These can be distinguished as THDF (for "fundamental"), and THDR (for "root mean square"). THDR cannot exceed 100%. At low distortion levels, the difference between the two calculation methods is negligible. For instance, a signal with THDF of 10% has a very similar THDR of 9.95%. However, at higher distortion levels the discrepancy becomes large. For instance, a signal with THDF 266% has a THDR of 94%. A pure square wave with infinite harmonics has THDF of 48.3%, or THDR of 43.5%.
The IEC defines the term "total harmonic factor" as : 
THD+N means total harmonic distortion plus noise. This measurement is much more common and more comparable between devices. It is usually measured by inputting a sine wave, notch filtering the output, and comparing the ratio between the output signal with and without the sine wave:
Like the THD measurement, this is a ratio of RMS amplitudes, and can be measured as THDF (bandpassed or calculated fundamental as the denominator) or, more commonly, as THDR (total distorted signal as the denominator). Audio Precision measurements are THDR, for instance.
A meaningful measurement must include the bandwidth of measurement. This measurement includes effects from ground loop power line hum, high-frequency interference, intermodulation distortion between these tones and the fundamental, and so on, in addition to harmonic distortion. For psychoacoustic measurements, a weighting curve is applied such as A-weighting or ITU-R BS.468, which is intended to accentuate what is most audible to the human ear, contributing to a more accurate measurement.
For a given input frequency and amplitude, THD+N is reciprocal to SINAD, provided that both measurements are made over the same bandwidth.
The distortion of a waveform relative to a pure sinewave can be measured either by using a THD analyzer to analyse the output wave into its constituent harmonics and noting the amplitude of each relative to the fundamental; or by cancelling out the fundamental with a notch filter and measuring the remaining signal, which will be total aggregate harmonic distortion plus noise.
Given a sinewave generator of very low inherent distortion, it can be used as input to amplification equipment, whose distortion at different frequencies and signal levels can be measured by examining the output waveform.
There is electronic equipment both to generate sinewaves and to measure distortion; but a general-purpose digital computer equipped with a sound card can carry out harmonic analysis with suitable software. Different software can be used to generate sinewaves, but the inherent distortion may be too high for measurement of very low-distortion amplifiers.
For many purposes different types of harmonics are not equivalent. For instance, crossover distortion at a given THD is much more audible than clipping distortion at the same THD, since the harmonics produced are at higher frequencies, which are not as easily masked by the fundamental. A single THD number is inadequate to specify audibility, and must be interpreted with care. Taking THD measurements at different output levels would expose whether the distortion is clipping (which increases with level) or crossover (which decreases with level).
THD is an average of a number of harmonics equally weighted, even though research performed decades ago identifies that lower order harmonics are harder to hear at the same level, compared with higher order ones. In addition, even order harmonics are said to be generally harder to hear than odd order. A number of formulas that attempt to correlate THD with actual audibility have been published, however none have gained mainstream use.
The sawtooth signal possesses
The pure symmetrical triangle wave has THDF of
and logically, reaches the minimum (≈0.483) when the signal becomes symmetrical μ=0.5, i.e. the pure square wave. Appropriate filtering of these signals may drastically reduce the resulting THD. For instance, the pure square wave filtered by the Butterworth low-pass filter of the second-order (with the cutoff frequency set equal to the fundamental frequency) has THDF of 5.3%, while the same signal filtered by the fourth-order filter has THDF of 0.6%. However, analytic computation of the THDF for complicated waveforms and filters often represents a difficult task, and the resulting expressions may be quite laborious to obtain. For example, the closed-form expression for the THDF of the sawtooth wave filtered by the first-order Butterworth low-pass filter is simply
where μ is the duty cycle, 0<μ<1, and
see for more details.
- Iaroslav Blagouchine and Eric Moreau. Analytic Method for the Computation of the Total Harmonic Distortion by the Cauchy Method of Residues. IEEE Transactions on Communications, vol. 59, no. 9, pp. 2478–2491, September 2011.
- Total Harmonic Distortion and Effects in Electrical Power Systems – Associated Power Technologies
- Recommended Practice and Requirements for Harmonic Control in Electric Power Systems
- On the Definition of Total Harmonic Distortion and Its Effect on Measurement Interpretation, Doron Shmilovitz
- Slone, G. Randy (2001). The audiophile's project sourcebook. McGraw-Hill/TAB Electronics. p. 10. ISBN 0-07-137929-0.
This is the ratio, usually expressed in percent, of the summation of the root mean square (RMS) voltage values for all harmonics present in the output of an audio system, as compared to the RMS voltage at the output for a pure sinewave test signal that is applied to the input of the audio system.
- THD Measurement and Conversion "This number indicates the RMS voltage equivalent of total harmonic distortion power, as a percentage of the total output RMS voltage."
- Kester, Walt. "Tutorial MT-003: Understand SINAD, ENOB, SNR, THD, THD + N, and SFDR so You Don't Get Lost in the Noise Floor" (PDF). Analog Devices. Retrieved 1 April 2010.
- IEEE 519 and other standards (draft): "distortion factor: The ratio of the root-mean-square of the harmonic content to the root-mean-square value of the fundamental quantity, often expressed as a percent of the fundamental. Also referred to as total harmonic distortion."
- Section 11: Power Quality Considerations Bill Brown, P.E., Square D Engineering Services
- VOLTAGE WAVE QUALITY IN LOW VOLTAGE POWER SYSTEMS José M. R. Baptista, Manuel R. Cordeiro, and A. Machado e Moura
- The Power Electronics Handbook edited by Timothy L. Skvarenina "This definition is used by the Canadian Standards Association and the IEC"
- AEMC 605 User Manual "THDf: Total harmonic distortion with respect to the fundamental. THDr: Total harmonic distortion with respect to the true RMS value of the signal."
- 39/41B Power Meter Glossary
- Total Harmonic Distortion Calculation by Filtering for Power Quality Monitoring
- Electric Machines By Charles A. Gross
- Calculation of harmonic amplitude sum
- Total Harmonic Distortion of a square wave
- Distortion factor
- IEEE 519
- Harmonics and IEEE 519
- Rane audio's definition of both THD and THD+N
- Op Amp Distortion: HD, THD, THD + N, IMD, SFDR, MTPR
- Introduction to the Basic Six Audio Tests "Since the sum of the distortion products will always be less than the total signal, the THD+N Ratio will always be a negative decibel value, or a percent value less than 100%."
- Distortion – Valves vs. Transistors