Editing Total harmonic distortion (section)

==THD+N==
'''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 filter|notch-filtering]] the output, and comparing the ratio between the output signal with and without the sine wave:<ref>{{Cite web|url=http://www.rane.com/note145.html|title=Audio Specifications|website=www.rane.com}}</ref>
:<math>
 \text{THD+N} = \frac{\displaystyle\sum_{n=2}^\infty \text{harmonics} + \text{noise}}{\text{fundamental}}.
</math>

Like the THD measurement, this is a ratio of RMS amplitudes<ref name="MT-003"/><ref>[http://www.analog.com/static/imported-files/tutorials/MT-053.pdf Op Amp Distortion: HD, THD, THD + N, IMD, SFDR, MTPR].</ref> and can be measured as THD<sub>F</sub> (bandpassed or calculated fundamental as the denominator) or, more commonly, as THD<sub>R</sub> (total distorted signal as the denominator).<ref>[http://www.ap.com/solutions/introtoaudiotest/thd+n 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%."</ref>

A meaningful measurement must include the [[Bandwidth (signal processing)|bandwidth]] of measurement. This measurement includes effects from [[Ground loop (electricity)|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 does not take into account the [[Equal-loudness contour|non-linear behavior of the ear]].<ref>{{cite book |doi=10.1533/9781782421955.208 |chapter=Low power transmission plastic gear trains: Which parameters affect the subjective acoustic quality? |title=International Gear Conference 2014: 26th–28th August 2014, Lyon |date=2014 |last1=Faventi |first1=R. |last2=Hopper |first2=H. |last3=Torrente Rodriguez |first3=M. |pages=208–218 |isbn=978-1-78242-194-8 }}</ref> The loudness model proposed by Zwicker includes these complexities. The model is described in the German standard DIN45631<ref>The loudness model proposed by Zwicker includes these complexities. The model is described in the German standard DIN45631.</ref>

For a given input frequency and amplitude, THD+N is reciprocal to [[SINAD]], provided that both measurements are made over the same bandwidth.