IMD Measurements: "Generator" vs "Input" Level Discrepancy

[reallystate]

With identical FS sine Vrms values in the generator and RTA, the stepped measurement of the source does not differ significantly if you change "Generator" to "Input" in the lower right corner of the Distortion tab.

However, this is not the case with some IMD tests. The SMPTE, DIN, and AES-17 MD plots (all of which have a 4:1 signal ratio) show that the test reached the clipping limit (-1.7 dBFS) when "Generator" is selected. But if you switch to "Input," REW displays the test as if it started at a lower signal level and ended at -14 dBFS. The AES17 DFD shifts to the left by 3.2 dB when switched. The TDFD (Bass, akl, Phonto) shifts to the right by 3 dB. Meanwhile, switching has no effect at all on the CCIF.

At the same time, "Input" shows a noise floor value that is closer to the results of other tests.

Any idea what the reason could be?

 

[reallystate]

These are measurements of the headphone DAC/Amp

 

[John Mulcahy]

Which REW version? The V5.40 beta builds got this fix in May:

Fixed: When stepped IMD measurements are plotted against the input level (rather than the generator level) the IMD reference tone level was used for the X axis instead of the total input rms level

 

[reallystate]

I'm using version 5.31.3 on Linux.

When I measure IMD on Windows (using 5.40 beta 100), the problem disappears. Thank you. In the new version, both modes show a result identical to the Generator mode in the old version. This means that when measuring SMPTE, the TD+N and even the Noise Floor level are noticeably higher than when performing other tests (THD, CCIF, Multitone). Is this how it's supposed to be?

When I open an old measurement in the beta version, the problem persists. It seems I'll have to redo the old measurements.

I don't use the beta version on Linux because it's buggy. No matter which specific impulse response exported from REW I open in the program, the phase looks identical:

Also, in this Linux version, if you open any impulse response, its frequency response should be at a level just below 0 dBFS, or 117 dB SPL, exactly as the old version shows. But beta 101 on Linux shows it at around -98 dBFS or 19 dB SPL.

These problems do not exist in the beta version for Windows.

 

[John Mulcahy]

I don't use the beta version on Linux because it's buggy.

The code is the same on all platforms, so differences are likely due to local settings such as whether you have chosen to display phase wrapped or unwrapped, whether the impulse response is plotted normalised and the input device SPL calibration.

 

[reallystate]

Is it normal that at a high signal level the level of IMD is higher than the TD+N level?

 

[John Mulcahy]

It's possible, yes, since the measures have different reference levels.