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You are here: ARCHI > VocabulaireElec	r1.3 - 13 Apr 2007 - 23:41 - LilianBUZER topic end
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THD: total harmonic distortion
- Plotting output versus input to determine the transfer function is not a useful method for determining distortion in audio systems though, for two reasons. Firstly, they usually respond only to changing signals (not DC), in the audio frequency range. Secondly they may suffer varying time delays (phase shift) at different frequencies. Both of these effects would produce a non-linear response to, say, a voltage ramp, even in a system free from non-linear distortion. Testing with a sine-wave input conveniently avoids these problems, while allowing the distortion to be quantified in terms of ‘harmonics’ (new components appearing at the output with frequencies that are multiples of the input sine wave frequency). The term Total harmonic distortion or THD refers to the sum of all these components, measured rms (root mean square) as a percentage of the fundamental tone amplitude.
- THD is commonly measured by using a notch filter to remove the input frequency from the output, allowing what is left to be measured, and in this case the measurement should strictly be referred to as THD+noise, since it includes any random noise on the output.

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