OK, let's see if we can skin this cat, i.e. "you can't measure this device with FFT" this way: by using UpTone's own statements of what it does. Here is a paragraph from their product page:
"While some DACs and converters do incorporate digital isolators for galvanic isolation—they are ALWAYS after the USB input PHY chip and processor system. And unfortunately, USB input noise of all sorts still makes it through to some extent and reaches the DAC master clock (Impacts on the DAC master clock are ultimately why ANY upstream variations—in computer, USB, cables, etc.—are heard at all. If you are interested, please see our easy to digest “white paper” on the subject.)"
So the impact is on master clock for the DAC. As I explained before, any modulation/changes on master clock of the DAC creates extra energy on either side of the DAC. And the test for this? J-test that we have been using! What this means is that we do NOT need anything more complex than a single tone. The tone will be modulated and in frequency domain we either get wider skirts around it due to random noise, or spikes showing correlated noise -- both of which you all have seen in my DAC measurements.
The white paper explains the same:
"PACKET NOISE: In a DAC the data packets coming in on the USB bus are not continuous—there is significant time in-between each packet. Thus the processing of these packets produces noise on the power supply and ground plane that come in bursts, and we refer to this as "packet noise". Since the rate of USB packets is 8KHz there are strong components of this noise in the audio band. This noise can cause jitter in clock oscillators, re-clocking flops, and DAC chips. It can also go directly into noise on the output of DAC chips."
Sadly there is no measurements to show any of these assumptions to be true. They could have opened a DAC and instrumented its clock, power, etc. to see if their assumptions are correct.
Well, I tested the original regen which as the same hub architecture as ISO Regen (and the above paper refers to that). Here was the outcome with and without ISO Regen (powered with a premium power supply):
We see that there are indeed 8 Khz and its harmonic at 16 Khz distortion spikes. Problem is, the original UpTone Regen created them!!! They were not in the output of the Meridian Explorer DAC without it. This result was confirmed by two other people making their own measurements.
That embarrassing bit aside, this should make it amply clear that our instrumentation is superbly sensitive to finding the exactly the sort of problem they say exists and they try to fix it. Packet noise at 8 Khz intervals that impacts either the DAC clock, its reference voltage or power supply absolutely shows up in Jitter tests with J-test signal.
Why is our instrumentation so sensitive? Because we have picked a 12 Khz tone that is pretty high frequency and importantly, it is nearly zero db FS. The jitter components are proportional to levels of our test signal. By using such a strong, high frequency tone, we are able to amplify the jitter components and make them visible at such extreme low levels (-115 db or so).
Real music does not have such strong content at 12 Khz. Not anywhere close or you would run away from it
. So in this regard,
our measurements are far harder on hardware than real music will ever be.
Worse yet,
real music will have lots of components that will mask these distortions. Can you imagine the audibility of those 8 Khz distortion spikes in the midst of real music with content at 8 Khz that is orders of magnitude louder?
So no, real music is not your friend. That pre-echo I showed earlier with Castanet will become next to impossible if you throw some classical music at it instead. Even I can't always find it despite my training to spot and hear them.
People who insist on these things can't be measured argue with words. If they think such effects can't be measured, they should take a DAC, distort it electrically in these manners and show the output remaining the same.
Before any such gear is designed, engineers need to first instrument and find real problems and document them. And here, I am not talking about USB bus, but audio performance. Once there, they can then show before and after. Until then, it is really technical jargon thrown at consumers to make them believe there is a problem, and further believe that they have solved the problem where in reality the problem may not be there, and neither their solution per above measurement.
