Сигналдағы гармоникалық бұрмалаудың өлшенуі

Measurement of the harmonic distortion present in a signal
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 can potentially 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.

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). 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. A weighting is a rough way to estimate the frequency sensitivity of every persons' ears, as it doesn't take into account the non linear behavior of the ear. The loudness model proposed by Zwicker includes these complexities. The model is described in the German standard DIN45631

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 by crossover distortion are nearly as strong at higher frequency harmonics, such as 10x to 20x the fundamental, as they are at lower frequency harmonics like 3x or 5x the fundamental. Those harmonics appearing far away in frequency from a fundamental (desired signal) are not as easily masked by that fundamental. In contrast, at the onset of clipping, harmonics first appear at low order frequencies and gradually start to occupy higher frequency harmonics. A single THD number is therefore 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 decreases with an decreasing level) or crossover (which stays constant with varying output level, and thus is a greater percentage of the sound produced at low volumes). THD is a summation 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, but none have gained mainstream use.