I am not a fan of CCIF IMD test because music spectrum does not at all have full amplitude 19 and 20 kHz spectrum. The SMPTE test I run has a low and mid-high frequency tones that are more realistic. And demonstrate low frequency distortion where music is loudest. Here is a random track from my library:
Furthermore, dual-tone tests are difficult to related to audibility.
I am not a fan of CCIF IMD test because music spectrum does not at all have full amplitude 19 and 20 kHz spectrum. The SMPTE test I run has a low and mid-high frequency tones that are more realistic. And demonstrate low frequency distortion where music is loudest. Here is a random track from my library:
[...]
Once I run the dashboard, the story is told with some rare exception.
Furthermore, dual-tone tests are difficult to related to audibility.
Amir -- I think you're doing some good work here, but I would argue that dual tone testing can be very related to audibility--far more so than THD testing. High frequency dual tone testing, in particular, can reveal the presence of nasty stuff that is completely unrelated to the musical signal, and thus arguably far more damaging than merely harmonic distortion. THD testing and dynamic range testing is somewhat akin to assessing how a car performs based on how it does on a dyno, and ending there. It's useful if all you want to do is drag race, but it possible to test a lot more things that will really tell you how the DUT "handles", so use an inapt analogy.
While the CCIF IMD test has no distortion within the audio band--that's not the point of the test. What it does do is tell you whether a lack of linearity is resulting in potentially audible garbage lower in the audio band. And this stuff is real garbage. I think Bruno Putzeys has an article out there somewhere about the nCore and why they substituted CCIF IMD results over 6kHz for the "real" THD results. Those points are valid. Stereophile has used the test for ages. In a 1980 AES paper, Richard Cabot (who I believe started Audio Precision) concluded that "[t]he distortion products generated in this test are usually very far removed from the input signal. This positions them outside the range of the auditory system's masking effects.
If a test which measures what the ear might hear is desired, the CCIF test is a good candidate." Arguably, it was the desire to be able to view the results of test like this which precipitated Audio Precision and its spectrum analyzers. You may have read the Cabot article at some point, but if not, you can get it for free on ResearchGate:
https://www.researchgate.net/public...n_of_Nonlinear_Distortion_Measurement_Methods. Also useful are Doug Self and Bob Cordell's books on audio power amplifier design. They talk a lot of circuit linearity.
For a practical example of the importance of testing at high frequencies, see
https://www.stereophile.com/content/emotiva-xpa-gen3-two-channel-power-amplifier-measurements. Look at all the stuff through Figure 8. Those are basically the tests you run regularly, with the exception of figure 6. And you know what? It appears to ace them. But Fig. 6 is enough to make you go, "Hmmmm....". Now look at the CCIF IMD test:
That's why I think a good review that is trying to link up measurements with audible performance must involve a high power frequency sweep or a CCIF IMD test on every DUT. Else, you're just kicking the tires and racing between stoplights. Useful data, but not enough to make an informed purchasing decision or draw meaningful judgments about sound quality.
Note, though, that the frequency sweep alone is not enough. The nCore doesn't do that well on the sweep, but it still does okay on the CCIF test:
https://www.stereophile.com/content/theta-digital-prometheus-monoblock-power-amplifier-measurements. Just because distortion is rising at high frequencies quite a bit, doesn't necessarily mean that the non-linear distortion is going through the roof. Often, but not always. The inverse, though, very rarely occurs.
Now, my practical examples are power amps (and unusual examples I've come across in the past, which I why I mention them here), but the same set of principles holds true on headphone amps or DACs. If you test for this stuff, you will have a far better idea of which products perform and which do not, in ways that are actually sonically meaningful. As interesting as the SINAD drag race is, for 97% of all recorded music, it's largely irrelevant since it's already vastly better than the source material. 90dB or 120dB SINAD will almost never have any audible consequence, in a vacuum. But are any of these great products botching it over 1kHz, and thereby smearing and mucking up stuff lower in the spectrum? For a significant number of products, I really don't know. That holds true for both of the products reviewed here. While this chart:
https://www.audiosciencereview.com/...ated-portable-headphone-amps.6312/post-140671 at least
suggests both of these products are fairly clean, it's certainly not a conclusion anyone can draw on the basis of the data presented. The only conclusions that can actually be reached on the basis of the data presented is that they both have adequate dynamic range to reproduce CD quality audio, and that they don't suffer from harmonic distortion problems. Whether they suffer from NON-harmonic distortion problems is anyone's best guess.