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In Reply to: RE: But what about...? posted by Charles Hansen on May 25, 2017 at 04:43:53
This comment from you is interesting: "My understanding is that the MQA encoding process "corrects" ('de-blurs') errors in the existing digital file created by an imperfect A/D converter."
As a so-called "end-to-end" process, my understanding that the "correction" wasn't done in the actual encoded file but, rather, at the end, in the DAC itself, along with adjusting for imperfections in the DAC.
With that in mind, if, in fact, they are correcting the original file and that MQA file has been modified for A/D imperfections then the the entire thing is even more lossy than I first thought. First off, as you mention, the lower bits are gone and some compression goes in to getting it all into the 24/48 constraint they are using. Then, if a process that corrects for timing of the A/D was applied, well, that plain and simply alters the original file. Just one of those MANY questions I had (and have).
Someone just brought a post to my attention (linked below). This is from Gordon Rankin, who designed the award-winning AudioQuest DragonFly series of USB DACs. The latest two models have been designed with MQA in mind and last week a firmware update was released that allows for some sort of MQA compatibility.
I must say that I don't yet understand this process. Remember that software decoding of the first "fold" was a surprise only announced when Tidal began streaming MQA in late January. This is the first I've heard of "custom filters for each song" and leaves me with more questions than answers:
1) If there is specific filtering done for each song, why not do it during the encode process?
2) The scheme is very reminiscent of similar schemes patented for the A/D process by Pacific Microsonics for HDCD and for the D/A process by Ed Meitner and assigned to Museatex for the BiDAT DAC. Both processes had two separate filters (anti-aliasing for the A/D side and reconstruction for the D/A side). The units would automatically switch coefficients depending on the signal level in the top octave. Is there more to this in MQA's implementation? How many sets of filter coefficients are available?
3) The DragonFly DACs will not normally decode signals higher than a 96kHz sample rate. My understanding is that the software decoding performed in the Tidal software only outputs a maximum sample rate of 96kHz. Does the DragonFly do anything more than fine-tune the filter coefficients?
It would seem that MQA is still under development. Interesting times. As always these posts are my opinions only and not necessarily those of my employer or previous employer.
> > if a process that corrects for timing of the A/D was applied, well, that plain and simply alters the original file < <
Yes, the original high-res file is altered in at least three ways:
1) The quad-rate audio data is compressed using lossy techniques.
2) The lowest 6 to 8 bits in all frequency bands are discarded to allow the dual- and quad-rate information to be "folded" underneath the audio data in the baseband. This is claimed to be "inaudible" as it is below the noise floor of the electronics. However it is well known that the ear/brain can distinguish correlated music 10dB or even 20dB below the noise floor of an LP, for example.
3) The so-called "blurring" in the original file is created by the digital anti-aliasing filter used in the A/D converter. This "ringing" is at a specific frequency - the corner frequency of the anti-aliasing filter - typically Fs/2. One concern about this filtering process is to ensure that new artifacts are not introduced.
In this sense MQA is like MP3 - both are lossy processes and the full original data can never be recovered.
All postings are strictly my own opinion and not necessarily those of my employer or my kids.
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