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OK, now we are on the same page - it is aliasing *and* it is a bug.

Posted by Charles Hansen (M) on June 6, 2017 at 13:47:03

In Reply to: Let's go back to Beyonce posted by Jim Austin on June 6, 2017 at 07:47:16:

>> That is aliasing. <<

Yep, agree 100%. Thanks for your patience and diligence with this. And I also see now why I said "something is very wrong here". Your explanation detailed in your post is right on the money. What is wrong about the graph is that it seems quite apparent that MQA sent a single-rate signal through a dual-rate filter.

This is essentially the equivalent of using a so-called "NOS" (non-oversampling) D/A converter. The only "reconstruction filter" in those cases is simply due to the bandwidth limitations of the analog circuitry. (In some cases the analog bandwidth for NOS DACs is deliberately restricted - say by using transformers in the audio signal path.)

Now all of these graphs make sense, and I would assert that there *is* a bug in this implementation of the MQA system. Specifically when a dual- or quad-rate source file is used, the digital filter used is a slow rolloff type that may allow for some *slight* "leakage" which contributes to very low levels of aliasing. But when a single-rate source file is used, the dual-rate MQA filter provides so little filtering as to act more like a filterless ("NOS") D/A converter. All of the energy in the top octave is simply non-harmonic (ie, unpleasant sounding) distortion. It doesn't make sense to me that one would sharply reduce aliasing at clearly inaudible frequencies above 48kHz, yet allow high levels of aliasing in potentially more troublesome frequencies an octave lower.

Furthermore we can now understand why some single-rate MQA files exhibit the notch centered at 22.05kHz. If the original source file was sampled at 44.1kHz, subsequent MQA processing apparently occurs at 48kHz and all content above 22.05kHz (=Fs/2, the Nyquist frequency) is simply aliasing that is only mildly attenuated. The same is true if the original source file was sampled at 48kHz, except there would be no gap between the original cutoff due to the anti-aliasing filter in the A/D converter and the mirror-imaged aliasing (purely artifacts comprising non-harmonic distortion) would connect seamlessly with the original audio.

In this case if a listener prefers the sound of an MQA-processed single-rate signal to that of the original, I assert they would likely prefer the sound of a filterless ("NOS") D/A converter even more. Apparently the main difference created by the MQA processing of a single-rate source file is the addition of the non-harmonic distortion created by the aliasing.

When I spent many months auditioning digital filters roughly a decade ago, one of the first things I tested was a filterless ("NOS") solution, as at that time they were fairly popular and making a "buzz" in the market. While there were many attractive aspects about the sound quality, in my opinion the filter we settled on retained the good qualities of the filterless approach while improving significantly on what I felt were its weaknesses.

As always, strictly my own opinions and not necessarily those of my employer or favorite athlete.

This post is made possible by the generous support of people like you and our sponsors:

Atma-Sphere Music Systems, Inc.

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