- Total harmonic distortion
The total harmonic distortion, or THD, of a signal is a measurement of the
harmonic distortionpresent and is defined as the ratio of the sum of the powers of all harmonic components to the power of the Fundamental frequency.Lesser THD, for example, allows the components in a loudspeaker, amplifier or microphone or other equipment to make a violinsound like a violinwhen played back, and not a celloor simply a distortednoisedubious.
In most cases, the transfer function of a system is linear and time-invariant. When a signal passes through a non-linear device, additional content is added at the harmonics of the original frequencies. THD is a measurement of the extent of that distortion.
Other calculations for amplitudes, voltages, currents, and so forth are equivalent. For a voltage signal, for instance, the ratio of the squares of the
RMSvoltages is equivalent to the power ratio::
In this calculation, "Vn" means the RMS voltage of harmonic "n", where n=1 is the fundamental harmonic. One can also calculate THD using all harmonics (n=∝)::
Other definitions may be used. Many authors define THD as an amplitude ratio rather than a power ratio. This results in a definition of THD which is the square root of that given above. For example in terms of voltages the definition would be::
This latter definition is commonly used in audio distortion (percentage THD) specifications. It is unfortunate that these two conflicting definitions of THD (one as a power ratio and the other as an amplitude ratio) are both in common usage. Fortunately if the THD is expressed in dB then both definitions are equivalent. This is not the case if the THD is expressed as a percentage.The power THD can be higher than 100% and is known as IEEE, but for audio measurements 100% is preferred as maximum, thus the IEC version is used (Rohde & Schwartz, Bruel and Kjær use it).:
A measurement must also specify how it was measured. Measurements for calculating the THD are made at the
outputof a device under specified conditions. The THD is usually expressed in percentas distortion factor or in dB as distortion attenuation. A meaningful measurement must include the number of harmonics included.
THD+N means total harmonic distortion plus noise. This measurement is much more common and more comparable between devices. This is usually measured by inputting a
sine wave, notch filtering the output in question, and measuring the ratio between the output signal with and without the sine wave::
A meaningful measurement must include the bandwidth of measurement. This measurement includes effects from
intermodulation distortion, interference, and so on, instead of just harmonic distortion.
For a given input frequency and amplitude, THD+N is equal to
SINAD, provided the bandwidth for the noise measurement is the same for both (the Nyquist bandwidth). [http://www.analog.com/en/training-and-tutorials/tutorials/design-center/tutorials/CU_tutorials_MT-003/resources/fca.html MT-003: Understand SINAD, ENOB, SNR, THD, THD + N, and SFDR so You Don't Get Lost in the Noise Floor] — Analog DevicesTutorial MT-003 by Walt Kester]
Audio system measurements
* [http://www.dogstar.dantimax.dk/tubestuf/thdconv.htm Explanation of THD measurements]
* [http://www.rane.com/note145.html Rane audio's definition of both THD and THD+N]
* [http://www.sengpielaudio.com/calculator-thd.htm Conversion: Distortion attenuation in dB to distortion factor THD in %]
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