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RE: Null Test Difference - test to measure real fidelity of a device

Hi Jon,

I am going to try giving some indications on the test. Please bare in mind that English is still not my main language and that my partner, Tom Gefrusti, is the author of the test.

I will give you few words, but the method is so complicated and complex that we can write for days about it...

The capture card cannot generate errors...it sounds obvious...it has to be very stable. precise and constant.

The same card doing a "self" NTD in loop, therefore placing a complex signal in antiphase, must generate a null signal. If one changes a component, such as a power cable, differences can be noticed (if variations with a power cable are possible) by doing a double acquisition of a piece of music with one cable first, and then with the other cable taking into account the very same capture level (dB).

The two signals are placed in a sort of sequencer, then time alignment on the first samples is needed and, finally, one of the two signals is going to be reversed (absolute phase).

Simultaneous playback through the sequencer will make the two acquisitions face each other. Like into a mirror, the two are compared in terms of both amplitude and time domain.

The produced signal (or not produced), which is the result of the two signals put in antiphase, is going to be sampled (with the same capture card). Then moving it onto an analyzer, further assessments/considerations on the level of error produced can be performed.

All this explanation is for testing a single component, that is trying to catch a difference between the two.

However, when you test the degree of fidelity between two devices, after having acquired the signal, it will be necessary to compare directly with the original track. It is possible to assess the deviations in amplitude and time domain. Obviously, an extremely precise, micrometric procedure of calibration is required, as the system is quite sensitive. At least you should maintain a range of difference not exceeding 0.01 dB.



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