- Thread Starter
- #21
Yes the total distortion from a complex input such as music will always be higher than that for sine tones, but what I was referring to with those difference signal results I linked to is that they do not show a monotonic correlation between the median difference signal level (Df value as Serge calls it) when playing sine tones and the difference signal when playing music. For example, from those results the FiiO M11 has a slightly lower Df value (less signal degradation) for a 1kHz sine wave than the Questyle QP1R, yet the former has a significantly higher Df value (more signal degradation) when playing real music or the Program Simulation Noise. This suggests sine tones tests are not adequate for measuring the true signal degradation and distortion of a device when actually playing music, and so music (real or simulated) would be the required test signal for a true measure of this.
As for audibility, Dr. Sean Olive and Steve Temme for example have shown in this AES paper that a non-coherent distortion metric using real music has a much better correlation to sound quality than standard multitone distortion metrics, which along with IMD are poor in this regard, as well as beating THD (results are at 31:24 in the video):
More details on the non-coherent distortion metric can be found in this paper.
Okay, I skimmed through it and I have to say, my initial thought (if the measurements are accurate) is that most of the reported "Df" for complex signals is coming from error in the nulling algorithm that compares the two signals, which would naturally get larger as the signal got more complex and it became more difficult to phase align them. A tiny error in phase can cause two audibly identical signals to catastrophically fail a null test.
As another poster noted above, Amir typically does a 32-tone test that never shows distortion products of significantly larger magnitude than those found in the single-tone tests. But Serge's square wave tests (which only contain about 12 tones) are showing a 50 dB difference from their single-tone tests.
Furthermore, it seems this person is claiming that all of the tested DAPs are subjectively distinguishable, yet we have no evidence of this from blind tests.
I have to admit I'm skeptical up front, but I haven't looked very deep into it.
Last edited: