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Let's go back to Beyonce

No new measurements, just new analysis.

If you look closely at the FFT I posted, you'll see a small double peak near 25kHz. I went back to Audition so that I could get a closer look and found that the highest of those two peaks is at 24.75kHz, at a level of -89.76. So do the math: That's Nyquist +2.7kHz.

So what's happening at Nyquist - 1.7 kHz? Notice anything interesting? Frequency resolution is limited, even in Audition, but there's obviously a very similar-looking double peak at 19.35kHz, or Nyquist - 2.7 kHz. See it? Returning to Audition so that I can get better resolution, I find that as close as I can get to 19.35 (Nyquist - 2.7kHz), the level is -73.85. What I'm getting at is that, if you start with the assumption of a symmetric spectrum--aliasing--then the peak at Nyquist + 2.7kHz is down by about 16dB relative to Nyquist - 2.7kHz. Note also that with any of the filters in question, 19.35kHz--Nyquist - 2.7--should be essentially flat.

Now let's go to JA's measurements. Look at the MQA filter. Resolution is poor (because I'm reading a small graph and the line has significant spread), but to me it looks like "flat" on the filter is at about -43dB. At about 25kHz, the filter is down to about -59dB by my estimate; it really is hard to tell. Take a look and see if you agree. That would mean that the filter is down about 16dB at Nyquist + 2.7kHz, which is exactly what we observed above.

This spectrum matches up well with the MQA filter.

Now--finally--take a look at the image below. In the Beyonce spectrum (that's CD and MQA versions; you can barely see the red shining through the blue), I've zeroed in on the region near the Nyquist frequency. Look at it closely. Start in the middle and work your way out toward the sides. Except for the fact that it's increasingly attenuated at higher frequencies, the symmetry is, I think, apparent.

That is aliasing.





Edits: 06/06/17
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