- Thread Starter
- #21
@BE718
Well I get the same results with Audacity and Wave Spectra. Here are some measures of an actual device. The cyan line is an 18i20 which is running 77 ppm fast. The gold line is the same signal once the speed has been corrected within about 10 ppb more or less. The second image has a pink line which is a synthetically created file 77 ppm fast. Hann window, 32 K fft. Again it only matters close in. In the second image you see how they all come together 200 hz either side of the 12 khz tone. 77 ppm fast btw is offset by .924 hz. You see the point at which -120 db crosses the graph is very close to where your .8 hz offset occurred at roughly 75 or 80 hz left and right.
Now trying to get back on topic. I originally brought all of this up because you showed the SMSL as having less close in phase noise than the miniDSP. Yet the amount of the difference or most of it could be nothing more than speed differences in the clocks of the two DACs vs the clock in the QA. Or it could be a real difference in close in timing.
If you still have both devices an easy quick check to see if that is some of it would be to look at the 12 khz waveform in Audacity to see if one is smooth and one lumpy which would indicate such a speed difference.
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Nonetheless it still doesnt make sense to have zero leakage The variations you show above seem more in tune with what I would expect, and yes you are correct in saying that some of the difference between the SMSL and minidsp could be due to this frequency difference. Unfortunately I dont have the SMSL anymore to confirm.
What this does make clear is that we should be using much higher FFT resolutionsto minimise the issue.
262k point FFT min max