MQA technical analysis

[bcwang]

Only after first playing the MQA file.

 

 

 

Because it seems to be part of the MQA rendering stage as shown by mansr's plot of the software render output in post #19. So both the hardware and software seem to be doing same stuff. Why it does that is another question for which I don't have answer...

 

Does the dac revert back to normal if you stop and play back the non-MQA file again? Does it require a full signal stop or sample rate change to get out of this buggy mode?

 

This is quite disturbing to learn about.

[bcwang]

I double checked: bizarrely the MQA version of the album is not available on Tidal, depsite Amy Duncan being at one point prominently displayed on the MQA website as one of the artists embracing MQA.#

 

 

Even more strange is the MQA version no longer being available for sale in the 7digital store. It still seems available directly from Amy Duncan's website though.

[bcwang]

I get the feeling that the "deblur" apodizing upsampling filter is run primarily at playback side and nothing really apart from the folding/encoding is done at the source.

 

I don't think this is the case because they claim deblur being a benefit even without an MQA dac, which means legacy devices would get the same deblur without any MQA code running.

 

Now my thought is maybe the MQA code upsamples even 44.1 kHz MQA sources because the DAC actually always runs internally at top sample rate to get that MQA trait of short impulse response. So with low res sources the upsampler is used while with high resolution sources there are less levels of upsampling and instead using real musical data. This would be one reason why a 44.1khz mqa file can posses a technical advantage over a non-mqa file without resorting to unfolding.

 

One of the first MQA albums was the Amy Duncan one, and it was released as a 44.1khz MQA. As a showcase for MQA without even using the unfolding feature, it would have to benefit from the deblur and dac compensation to differentiate itself.

[bcwang]

I think it shouldn't be doing it... But maybe the decoder always outputs 2x rate, regardless of input. But the hardware decoder knows from the stream metadata that it originated from 44.1 and displays that in the DAC display while still in reality putting out 88.2.

 

So I have another data point for this. I got another version of tidal, some test version due to a bug I found in mine. This version doesn't seem to detect my Explorer 2 as an MQA dac like the release version. In this one, if I do not have "passthrough MQA" checked, then it will output 88.2 or 96khz to the explorer2 and the blue light will not light up.

 

I believe in the previous version, it detected the MQA dac and sent the MQA original data to it regardless of checkbox setting. Because people thought with the checkbox unchecked the dac was receiving 88.2 or 96khz data and still saw the blue light, they thought it preserved the MQA stream. But I think it was just the checkbox not taking effect with the explorer 2.

[bcwang]

It's graph time again. 2L-111 is a 352.8 kHz master with remarkably low high-frequency noise. Here's the spectrum of the original and the MQA reconstruction (decode + render):

 

[ATTACH=CONFIG]32642[/ATTACH]

 

According to my analysis of the renderer, this is the filter used by its 4x upsampling with this file:

 

[ATTACH=CONFIG]32643[/ATTACH]

 

It's a 24-tap minimum phase FIR filter. There is a notch centred at 88.2 kHz, which is clearly visible in the "rendered" music. Towards the high end of the spectrum, the filter again attenuates, and the shaped dither noise swamps whatever remains of the images there.

 

By render you mean the part of code that is adjusting the final signal to match the dac?

 

If this is just a digital analysis, one part that is missing is how the particular dac chip is handling an input that in digital form may look like this. Maybe the dac compensation process is to tailor the digital signal so the output on the analog side matches what came in on the analog side of this end to end "studio quality" process that is being claimed. To really know, an analog measurement of a MQA dac needs to be made to see resulting waveform rather than just what digital was being fed to it. Maybe at the end this particular dac distorts that waveform and it ends up looking like the pcm representation.

 

Did you post a digital representation of the post unfolding without the render portion? Maybe it is a closer match to the pcm equivalent.

[bcwang]

My recordings from analog output of a Meridian DAC with hardware decoding pretty closely match to what has been shown here for digital domain analysis...

 

Would you be able to post a pic of that? Just so it's easier to visualize. What about the flac version as well output from the same dac to see if it matches the theoretical? MQA vs theoretical analog measurement on the same chart.

 

By the way, what is the waveform measuring? Some noise level at a specific instant of the recording? It looks like near silence with the highest signal being -60db so it surprises me it doesn't match with the close correlation archimago's blog shows after decoding.

[bcwang]

If you look earlier in this thread, I posted some results.

 

 

 

Same noise is there throughout regardless of the position.

 

If you mean the spectrograms, I unfortunately still haven't figured out how to interpret them to draw any conclusions and especially not how to compare those with the graphs mansr has provided. Would you be able to explain?

 

What I was looking for was output looking like what mansr showed as digital analysis but of the actual analog measurement from an mqa dac. One playing the mqa file and one the high rate flac, to see that it did indeed change the output noise. I browsed the whole thread again and didn't see anything like that.

[bcwang]

I see that know. If you read the comments at least one guy seems to have both blue and green led versions of one track : Computer Audiophile - My First 24 Hours With MQA

 

That one guy being the founder of this site. =)

 

You know who to ask but I don't know if you'll get what you're looking for.

[bcwang]

I'll try to check this on Monday or Tuesday. I need to send the digital capture of the software decoded file because Tidal application enforces hardware decoding when it detects known MQA DAC.

 

The new version of tidal released this week doesn't seem to detect the explorer 2 as mqa dac anymore and passthrough disable will force 2x mode and the mqa light will be off

[bcwang]

I thought it is apparent. Same information, but with one extra dimension - time. The Y-axis of mansr's graphics are color-coded and the frequency is swapped to the Y-axis because time is on X-axis.

 

I know how to read the spectrogram, but the conclusions you are able to draw from it and especially that you are able to state it matches the 2 dimension graph is what I'm unable to see at least at a glance. If I can view them both on a computer side by side maybe I can start to see but going back and forth on the phone it's pretty difficult so far.

[bcwang]

There is constant reference by Miska or Mansr to the rendering stage of MQA to be just upsampling. Can you explain this as this doesn't fit with what MQA claims.

 

1. Hardware decoded files show signal information above 48khz which the software decoded files don't. If it was simply upsampling 96khz I would expect a sharp cutoff at 48khz and nothing above. So his doesn't sound like upsampling

 

2. MQA claims hierarchical unfolding, where 96khz and above sound is encoded and buried in the noise floor of between 48-96khz band. Then that whole band is encoded and buried in the noise floor below 20khz. If you simply claim it's upsampling then that means you think MQA is lying about how their tech works?

 

So I'm not sure where this claim that rendering is simply upsampling with a specific digital filter comes from.

[bcwang]

The low 8 bits of a 24-bit MQA file contain a compressed representation of the 24-48 kHz frequency content. The decoder combines this with the base band information in the high 15 bits to recreate a 96 kHz stream. This part works more or less the way they say it does.

 

 

 

The claim comes from looking at the actual code that does it. It's a perfectly standard polyphase interpolation filter. Nothing more. The filters they use have horrible amounts of aliasing/imaging, which is why the output includes frequencies above 48 kHz.

 

So you are looking at actual source code for an MQA full decoder or just looking at output streams from it?

 

It seems if there was a specific upsampler tailored to work with the triangular sampling, the first unfold data would have to be tailored so this specific upsampling would recreate the frequencies precisely as originally encoded. However, this data if not upsampled would not sound right when played back as is. Thus when not doing a full decode to a specific dac which will upsample after the first unfold, the first unfold creates a different bitstream compatible with all DACs.

 

When I'm talking about sound information above 48khz, I'm not just talking about mirror image aliasing. From some of the graphs archimago blog shows, it matches the original hires content pretty closely up to 60khz.

 

Also it's said the render mode increases noise, so if that's not coming from the upsampling, there is also more going on here. The graphs you show are good but are only digital domain. You would want to correlate it with the actual analog output of an MQA dac to see nothing else is happening that you are not accounting for.

[bcwang]

can someone explain the de-blurring process mqa claims to do. It doesn't show up in measurements...at least those I've seen.

 

You'd need a measurement of an impulse in the time domain to visualize and compare. I haven't seen measurements of that sort yet.

[bcwang]

And you never will. There is no way MQA would permit such a test to be published.

 

You'd have to look for musical material that exists from the same mastering to do the measurement. A cymbal hit maybe, or any sound with a fast transient edge.