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Limitations of a synthetic tests (THD, IMD etc.)
IgorC
post Jan 13 2013, 21:50
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THD is measured on one single frequency (tone) at once, IMD with two tones.
So these measurements are made with synthetic signals while music and speech aren’t just pure tones but rather complex sounds.

For example IMD is calculated on two tones 19 kHz and 20 kHz like here. http://nwavguy.blogspot.com.ar/2012/04/odac-released.html
Just one example of possible many. IMD between 20 kHz and 19 kHz tones ends with 1 kHz tone (20-19 kHz) in audible band. But at the same time there can be 20.1, 20.11 … and 19.1, 19.11… kHz tones those will also cause the same 1 kHz tone in audible band. A classic IMD measurement is calculated only for one pair of tones and not for all other. Wouldn’t be it a huge oversimplification? I’m afraid a testing of audio equipment will be very hard on a real life signals (music, speech).

This post has been edited by IgorC: Jan 13 2013, 21:55
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saratoga
post Jan 13 2013, 22:01
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QUOTE (IgorC @ Jan 13 2013, 15:50) *
THD is measured on one single frequency (tone) at once, IMD with two tones.


Thats one way you can do it, but not the only way. RMAA for instance will do an IMD sweep. The link you posted measures THD as a function of frequency. So you can look at just a few frequencies, or you can look at lots.

QUOTE (IgorC @ Jan 13 2013, 15:50) *
A classic IMD measurement is calculated only for one pair of tones and not for all other. Wouldn’t be it a huge oversimplification? I’m afraid a testing of audio equipment will be very hard on a real life signals (music, speech).


Its only an oversimplification if you expect non-linearity to be strongly frequency dependent. Generally if a device is linear at a couple frequencies, its linear at all the ones in between as well, so just testing a subset of combinations gives you a very good idea of the overall performance.
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