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Total Harmonic Distortion

Real world audio devices introduce unwanted side effects called distortion in addition to the desired response.  Total Harmonic Distortion (THD) and (THD+Noise) are two widely used measurements of this quantity.  Both measurements are expressed as a percentage.  The lower the THD value, the "cleaner" the response.

Total Harmonic Distortion (THD) - the ratio of the harmonic power to the fundamental power.  It is computed by searching the entire spectrum to find the peak frequency (fundamental) and then calculating the total power in the harmonic frequencies. The THD level is then computed as the ratio of the total harmonic power to the fundamental power.  Residual noise is not included in this calculation.

Total Harmonic Distortion + Noise (THD+N) - the ratio of the harmonic power plus noise to the fundamental power.  It is computed by searching the entire spectrum to find the peak frequency (fundamental) and then calculating the total power in the remaining spectrum (harmonics plus noise). The THD level is then computed as the ratio of the total harmonic+noise power to the fundamental power.   

THD+N values will almost always be greater than the THD values for the same signal.

A typical THD measurement will use a 1kHz reference test tone.  This reference signal is passed through the audio device under test and its spectrum computed.  It is often useful to employ long term averaging to cancel the random noise components and enhance the measurement accuracy.  

It should be noted that the purity of both the sound card and the reference signal must exceed the device being measured if meaningful results are to be expected.  The purity of the signal generator is directly dependent on the THD of the sound card used.

This utility window can be resized as required by the user and its contents will update anytime a new spectrum is computed.  If the analyzer is configured to display the spectrum of both the left and right channels, this utility window will also show the value for both channels.  The left channel value will appear above the right channel value

Notes:
  • Hanning or Blackman smoothing window is recommended because of its low noise quality.  
  • An FFT size of 2048 or greater is recommended in order to to provide adequate spectral resolution.
  • The contents of any open utility window will be printed on the right margin of the spectrum view.

    See Also: Measurement Accuracy , Application Notes - Total Harmonic Distortion,, THD+N vs Frequency