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Total Harmonic DistortionThe total harmonic distortion, or THD, of a signal is a measurement of the harmonic distortion present, that is, any departure of the output signal waveform from that which should result from the input signal waveforms being operated on by the system's specified or ideal transfer function. In most cases, this ideal transfer function 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. This is a measurement of the extent of that distortion. The measurement is most commonly the ratio of the sum of the powers (or rms voltages/amplitudes, since the ratio is equivalent) of all harmonic frequencies above the fundamental frequency to the power of the fundamental: -
\mbox{THD} = {\sum{\mbox{harmonic powers}} \over \mbox{fundamental frequency power}} = {\sqrt{V_2^2 + V_3^2 + V_4^2 + \cdots + V_n^2} \over V_1} In this calculation, Vn means the RMS voltage of harmonic n. Other definitions may be used. A measurement must specify how it was measured. Measurements for calculating the THD are made at the output of a device under specified conditions. The THD is usually expressed in percent as distortion factor or in dB as distortion attenuation. A meaningful measurement must include the number of harmonics included (and should include other information about the test conditions). 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 it, and measuring the ratio between the signal with and without the sine wave: -
\mbox{THD+N} = {\sum{\mbox{harmonic powers}} + \mbox{noise power} \over \mbox{total output power}} A meaningful measurement must include the bandwidth. This measurement includes effects from intermodulation distortion, interference, etc. See also External links
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