total harmonic distortion (THD)
The distortion factor, Total Harmonic Distortions (THD), is a measure of the freedom from distortion of signals. It is a non-linear, frequency-dependent distortion that results from the ratio of the effective value( RMS) of all harmonics to the effective value of the total signal and is specified as a percentage or decibel.
The distortion factor is calculated from the ratio of the sum of all harmonics to the RMS value of the fundamental frequency.
The distortion factor is influenced equally by the recording, storage and playback electronics. Thus by passive and active components of the audio chain, the microphone, preamplifier, tape recorder, power amplifiers and loudspeakers. In addition to the THD value, there is also the THD+N value ( noise), which also includes the background noise of the audio components.