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In Reply to: RE: OK, now we are on the same page - it is aliasing *and* it is a bug. posted by Charles Hansen on June 06, 2017 at 13:47:03
Charles, I lack your experience-based perspective, so I guess to me it doesn't seem that odd. I go back to (one of) the patent application(s) and see this language:
>>Preferrably, the downsampler comprises decimation filter specified at the first sample rate, wherein the asymmetric component of the response of the decimation filter is characterized by an attenuation of at least 32dB at frequencies that would alias to the 0-7kHz range.
The range 0-7kHz is where the ear is most sensitive. The amount of attenuation required varies greatly according to the spectrum to be encoded in the vicinity of its Nyquist frequency, and may [sic] signals will require more than 32dB of attenuation.<<
For CD-res, 7kHz is Nyquist - 15kHz. So what's the attenuation of that MQA filter JA measured at Nyquist + 15kHz = 37.1kHz? It looks to be down about 55dB, give or take, which even with this slow roll-off easily meets that spec. Looking at the Beyonce track--comparing 7kHz with 37kHz--just eyeballing it this time, not going back to the Audition file--the levels seem to be about -70dB at 7kHz and about -120dB at 37kHz--so, again, aliasing spec easily met--even with this very gentle filter.
MQA has been quite explicit about the desire to trade aliasing for time resolution. They appear to be doing just what they said they would do--so I don't see why you'd call it a bug. Surely they'd call it a feature.
One thing I don't understand is why you say they're using a double-rate filter. The filter shown in JA's Fig. 6 accurately describes the data, is flat not quite to the 44.1 Nyquist frequency (that is, about 22kHz). It describes what's happening but it would never work for 96kHz data.
Otherwise, we seem to have converged on an interpretation.
OK, now I get it!
I was wrong and you were right, and now I understand where I was led astray with regards to the aliasing. I only read JA's Brooklyn review close enough to notice the single-rate MQA filter response just recently (even though it is ~9 months old) and it hadn't fully sunk in. Instead I've been operating under the (presumably still accurate) premise that the limiting factor in the MQA "end-to-end system response" was the digital reconstruction filter (in the D/A converter), which at the quad-rate sampling frequency yields this frequency response curve:
and mistakenly assuming that was the filter also being used with single-rate audio data. I now completely agree with your analysis using your measured spectra combined with (the other) JA's measurements of the Brooklyn single-rate filter.
And now we also an see the likely reason why the notch appears in the spectrum of single-rate MQA files originally recorded at 44.1kHz. Virtually all digital equipment (recording or playback) uses "halfband" digital filters, as these require only half as many taps and half as much storage for the memory coefficients. One key characteristic of a halfband filter is that it will always have a response of -6dB at the Nyquist frequency (Fs/2).
If that output is later fed to a "leaky" reconstruction filter that mirror-images the audio data as out-of band aliases, there will be a continuous graph with no gap between the original music and the aliased version. But that is not what we see in your Beyonce track. Instead it appears that at some point the audio data was fed through an "apodizing" filter, specifically designed to filter out the "ringing" created by the anti-aliasing filter of the A/D converter at the Nyquist frequency (Fs/2). I would guess that this was part of the MQA process, as I am unaware of any commercially available A/D converters with built-in "apodizing" filters.
When that filtered audio data is then sent through yet another digital filter - this time the "leaky" MQA filter in the D/A converter, the sharp rolloff between 21kHz and 22kHz seen in your original spectrum of the Beyonce track is mirror-imaged at 22.05kHz due to the aliasing. The combination of the "apodizing" filter and the "leaky" MQA filter creates the "notch" seen in the spectrum. Thank you for your input in helping to solve this mystery!
As far as MQA's claim that 32dB of anti-aliasing is sufficient for signals in the 0 (DC) to 7kHz range, I think that is open for debate. It would seem that high-performance audio generally revolves around the idea of continuous improvement, and not dictating "sufficient" levels of performance.
As always, solely my own opinions and not necessarily those of my employer or butcher.
Charles, thanks for this. I may have been right (approximately) from early on, but exchanges like this help solidify my tenuous knowledge and provisional thinking. And it should be noted that any insight I have derives also from conversations with--and in some cases specific ideas expressed by--the other JA.
Your explanation here is consistent with one thing Bob Stuart has said repeatedly--that what MQA does depends on the recording. The other 44.1 MQA I posted results from didn't have this wide, obvious gap. Presumably the Beyonce needed a lot of apodizing. Curious, then, that the CD version and the MQA version should be nearly identical--what do you make of that? (The possibility that this near-identity is an implementation error--a mistake or compromise made by this particular converter and not necessarily a specifically MQA phenomenon--of course remains.)
Just one more comment. You wrote:
As far as MQA's claim that 32dB of anti-aliasing is sufficient for signals in the 0 (DC) to 7kHz range, I think that is open for debate. It would seem that high-performance audio generally revolves around the idea of continuous improvement, and not dictating "sufficient" levels of performance. < <
But the MQA folks maintain--this is my interpretation; I hope it's more or less correct--that this is not "settling", but, rather, striking a different compromise, one that prioritizes the time domain over the frequency domain to a greater extent than any previous technology. It could therefore be seen as a natural extension of the last couple of decades of thinking about digital audio, by you and others. I remember reading, a few years ago, Ayre's White Paper on minimum phase filters. Viewed in that context, MQA could be seen as a natural next step--although to say it's natural is not necessarily to say it's wise. Plus, I've seen no direct evidence that they're achieving the time-domain performance they've specified. It's on this basis--that the technology is plausible and a reasonable (if radical) next step, that I've argued that it deserves an audition.
> > exchanges like this help solidify my tenuous knowledge and provisional thinking < <
Same here. One would think that MQA would simply explain the technical details clearly for all to understand, but I am also trying to "solidify my tenuous knowledge and provisional thinking".
> > Curious, then, that the CD version and the MQA version should be nearly identical--what do you make of that? < <
Again I would like more data to avoid jumping to the incorrect conclusion (as I did previously). Is the "CD version" actually a physical disc? Whatever its source, it has clearly been run through an apodizing filter, directly in contradiction to Peter Craven's (originator of the apodizing filter) recommendations (see below).
It seems unlikely that there is some obscure "pro" manufacturer making a "state-of-the-art" A/D converter with an apodizing anti-aliasing filter mistakenly attempting to wring the very best sound out of 44.1kHz sample rates. To the best of my knowledge the first proposed use of an apodizing filter was in Peter Craven's AES paper of 2004. I believe the first commercial use was likely by Meridian a few years later and know that Ayre experimented with them during 2008 and released them as the new "Listen" filter in the MP upgrade of 2009 (and also the QB-9 and subsequent digital products). But all of these were reconstruction filters for use on the D/A side.
I would think the most likely possibility is that MQA had something to do with the application of the apodizing filter to this Beyonce release. Which raises at least three questions in my mind:
1) If MQA used an apodizing filter to remove any pre-ringing from the A/D converters used to create the Beyonce CD, why not simply sell, license, use, or give away that technology to improve the sound of all recordings? Or is there some other company or mastering engineer that is already doing the same thing but without using MQA's tools?
2) It would seem that the main change upon playback between the Beyonce CD and the MQA-encoded version of the same file is that upon playback, the "leaky" MQA filter is used. In your original trace of the CD version, I saw no evidence of aliasing - presumably because the reconstruction filter was not "leaky". In contrast the "leaky" MQA filter passed higher levels of aliasing artifacts than apparently even the "Slow" filter on the Mytek Brooklyn. (Again, I am unclear on all of the details of the test conditions.) But if that is the case and the Beyonce CD already has the "time blur" filtered out in the non-MQA version, all we have learned is that different digital reconstruction filters sound different - which wouldn't seem to be a revolutionary breakthrough.
3) If an apodizing filter has been applied to the Beyonce CD, this is in direct contradiction to Peter Craven's recommendation in his AES paper. Specifically, any filter sharp enough to filter out the "pre-ringing" introduced by the A/D converter will also introduce "ringing" of its own. The only advantage of the apodizing filter is that being minimum-phase, all of its "ringing" will be more natural sounding "post-ringing" rather than the nowhere-to-be-found-in-nature "pre-ringing" created by linear-phase filters.
Again, apparently More Questions than Answers...
As always, strictly my own opinions and not necessarily those of my employer or pet wombat.
Busy today, unfortunately. I'd love to pursue this further. I won't get to that until tomorrow probably.
I can answer one question, since it takes no time: All the measurements I presented are from Tidal streams. There are two versions of "Lemonade" on Tidal (Hi-Fi/Master): the MQA version and a regular CD-res version. That's what I've presented, delivered by Roon, apparently bit-perfect (since the Blue MQA lights up on the MQA file).
I appreciate all of your work in putting this information together. Definitely an interesting puzzle. The thing that has me scratching my head is the application of an apodizing digital filter to the "CD-res version" of the Beyonce album. This is a no-no, according to Peter Craven's original paper describing apodizing filters:
"We suggest that the final digital to analogue conversion may be
the appropriate place for the apodising filter."
That is from the summary conclusion. Earlier in the paper is an entire section explaining the reasons for the end of the chain as the optimal location for the apodizing filter.
As always, strictly my own opinions and not necessarily those of my employer or minister.
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