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Term: Intermodulation distortion (audio)

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Term: Intermodulation distortion (audio)


IMD results when two or more signals of different frequencies are mixed together and additional signals are formed at frequencies that are not, in general, at harmonic frequencies (integer multiples) of either source signal. Intermodulation causes spurious emissions that can create minor to severe interference to other operations on the signal. Although IMD may affect many types of signal data, this definition focuses on sound. Intermodulation should not be confused with general harmonic distortion (which does have widespread use in audio effects processing; see THD). Intermodulation specifically creates non-harmonic tones due to undesired mixing of near frequencies.

There are various ways to measure IMD. In the United States, the SMPTE (Society of Motion Picture and Television Engineers) standard is often used. It specifies a two-sinewave test signal consisting of a low-frequency, high-amplitude tone linearly combined with a high-frequency sinewave at 1/4 the amplitude of the low-frequency tone, specifically tones at 60 Hz and 7 kHz. When a non-linear device is subjected to a two-tone test signal, intermodulation products appear as sidebands around the high-frequency tone. The percentage intermodulation distortion is defined as the percentage of amplitude modulation, represented by the second and third order pair of sidebands, of the high-frequency signal. Second order sidebands around the high frequency tone are spaced at a frequency equal to the low-frequency tone (FH ± FL). Third order sidebands are spaced at twice the low-frequency tone or FH ± 2FL. (FH and FL correspond to the highfrequency and low-frequency tones, respectively. There is also a European measurement standard, specified by ITU-R.

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