MQA technical analysis

[mansr]

As we know, the Bluesound software contains an MQA decoder. All the MQA code resides in a shared library, libbluos_ssc.so, which is called by various other parts of the playback software. There are three main parts to the library: decode, render, and preserve.

 

Decode

The decode portion accepts audio data and outputs decoded data at twice the sample rate of the input. It also provides feedback regarding the MQA status of the data being processed: none, MQA (green), or MQA Studio (blue). Non-MQA input is also upsampled to double rate. I speculate that this corresponds to the MQA Core decoding done by the Tidal player.

 

Render

The render stage takes the output of the decode stage and upsamples it further. Gain can also be applied here. Again, non-MQA input is accepted.

 

Preserve

This function appears to transfer some MQA signal encoded in the LSB from one audio stream to another. The Bluesound player uses it together with software volume and tone controls between the decode and render stages.

 

Having deduced the programming interface of the library, I wrote some simple test programs to process audio files and save the output. Attached below is a snippet from the output of running the decoder on the 2L-048 sample (recorded at 96 kHz). I'd be thankful if someone who has an MQA DAC could play it and report whether the indicator LED comes on.

2L-048-mqadec.zip

[Miska]

Having deduced the programming interface of the library, I wrote some simple test programs to process audio files and save the output. Attached below is a snippet from the output of running the decoder on the 2L-048 sample (recorded at 96 kHz). I'd be thankful if someone who has an MQA DAC could play it and report whether the indicator LED comes on.

 

What happens if you just use the default ALSA plugin setup and store the output?

 

No blue light on Meridian DAC with this file.

[mansr]

What happens if you just use the default ALSA plugin setup and store the output?

 

It's not all done with ALSA plugins. That was my mistake.

 

No blue light on Meridian DAC with this file.

 

Does it light up when using the Tidal software decoder?

[Miska]

Does it light up when using the Tidal software decoder?

 

Well, it lights up with Tidal player, but Tidal application detects the DAC and forces the hardware decoding. So the driver is indicating 48k sampling rate when playing MQA track... Regardless if the pass-through setting is enabled or disabled in the Tidal application.

[mansr]

Well, it lights up with Tidal player, but Tidal application detects the DAC and forces the hardware decoding. So the driver is indicating 48k sampling rate when playing MQA track... Regardless if the pass-through setting is enabled or disabled in the Tidal application.

 

No way to force software decoding in Tidal?

 

Would you be able to capture a short sample of Tidal passthrough and software decode of the same track? It would be most helpful if you, or someone else, could.

[Miska]

Here's spectrogram of digital capture of the Tidal software decoded 2L-048 test track. I tried to use same spectrogram parameters you used before.

 

 

Here's spectrogram of the difference to the original, palette switched to highcolor which I mostly use to make the difference better visible.

 

 

Looking at the background noise from the spectrums, there's about 6 dB increase in background noise in MQA version, thus a 6 dB drop in the SNR.

[mansr]

Here's spectrogram of digital capture of the Tidal software decoded 2L-048 test track.

 

Could you share that capture, please?

[mansr]

Could you share that capture, please?

 

With the exception of two short bursts in the first 0.2 seconds, my decoded file matches Miska's capture bit for bit. I guess that's some kind of success.

[mansr]

Well, it lights up with Tidal player, but Tidal application detects the DAC and forces the hardware decoding. So the driver is indicating 48k sampling rate when playing MQA track... Regardless if the pass-through setting is enabled or disabled in the Tidal application.

 

If you play the captured software-decoded file, does the Explorer indicate MQA? Is it supposed to do this?

[mm67]

If you play the captured software-decoded file, does the Explorer indicate MQA? Is it supposed to do this?

You mean the one in first post ? Blue light doesn't lit up with that one on my Explorer. If I capture something from Tidal and play it in Foobar mqa indicator does lit up

[Miska]

If you play the captured software-decoded file, does the Explorer indicate MQA? Is it supposed to do this?

 

No it doesn't, it plays it just as it would play any other 96k file. I don't think there is supposed to be any difference...

 

I'll make some analog recordings today in different ways...

[mansr]

You mean the one in first post ? Blue light doesn't lit up with that one on my Explorer. If I capture something from Tidal and play it in Foobar mqa indicator does lit up

 

You mean capture an MQA pass-through from Tidal? I'd expect that to work.

[mm67]

You mean capture an MQA pass-through from Tidal? I'd expect that to work.

 

That's what I meant, yes

[mansr]

Since I'm now confident the decoder is working correctly, here's a spectrum plot of 2L-048:

 

Here we can see that the decoding process removes the characteristic hump and restores some of the high-frequency content. In this plot, the graphs start diverging noticeably around 30 kHz, the decoded MQA having somewhat higher noise level.

 

Spectrogram of the aligned difference:

 

The larger differences correlate with the louder portions of the audio.

 

This is the difference spectrum of the loud portion from 190 to 250 seconds:

 

Same thing with linear frequency scale:

 

Difference spectrum of the silence at the end:

 

And again with linear frequency scale:

[scintilla]

So, does the blusound decoder include a renderer? Will it unfold a 24/48 file to 24/192? Since you have detected an activation bitsteam in the least significant bits, that implies that it is used to switch the renderer on and perhaps apply a specific filter profile. That last stage is of import to those of us with the PWM dacs that currently do not support blusound, since Roon can do the first unfold decode but not the final render with appropriately selected filter.

[mansr]

So, does the blusound decoder include a renderer? Will it unfold a 24/48 file to 24/192? Since you have detected an activation bitsteam in the least significant bits, that implies that it is used to switch the renderer on and perhaps apply a specific filter profile. That last stage is of import to those of us with the PWM dacs that currently do not support blusound, since Roon can do the first unfold decode but not the final render with appropriately selected filter.

 

It includes what appears to be a renderer. There obviously isn't any reference available to compare the output against.

[Miska]

Here's same track, recorded at 192k sampling rate from line output of the Explorer2. I tried to get as low noise output from the DAC as possible, using iFi iUSB as a power supply. There's still some amount of USB packet noise (8 kHz intervals) visible.

 

Original 96k file:

 

MQA file decoded by the DAC:

 

For comparison, here is self-noise of the ADC with cable just disconnected:

[mansr]

Here's same track, recorded at 192k sampling rate from line output of the Explorer2. I tried to get as low noise output from the DAC as possible, using iFi iUSB as a power supply. There's still some amount of USB packet noise (8 kHz intervals) visible.

 

Original 96k file:

[ATTACH=CONFIG]32541[/ATTACH]

 

MQA file decoded by the DAC:

[ATTACH=CONFIG]32539[/ATTACH]

 

Here's the average spectrum of what the Bluesound MQA renderer outputs:

 

[Miska]

Here's the average spectrum of what the Bluesound MQA renderer outputs:

 

[ATTACH=CONFIG]32543[/ATTACH]

 

So it largely matches. The average doesn't show noise increase at the bands where there is also music content, minimum spectrum would probably show that thing better.

 

I have a feeling that some regular hires files can trigger this rendering mode too. I have one regular hires FLAC that seems to trigger this noise and upsampling filter behavior (without the blue light though). Running that FLAC through upsampling before sending it to the DAC takes out the noise and makes it much quieter, and also removes the 30 kHz filter cut. So it seems like there are hires files that are sort of "encoded-decoded" MQA...

 

Seems like the 30 kHz cut-off is more on the rendering phase then. I really fail to see the point in this "rendering" part... The upsampling filter is really slow roll-off, so it leaks a lot of image above Nyquist, visible on tracks that really hit the Nyquist limit.

[mansr]

I have a feeling that some regular hires files can trigger this rendering mode too. I have one regular hires FLAC that seems to trigger this noise and upsampling filter behavior (without the blue light though). Running that FLAC through upsampling before sending it to the DAC takes out the noise and makes it much quieter, and also removes the 30 kHz filter cut. So it seems like there are hires files that are sort of "encoded-decoded" MQA...

 

Does it have the bit pattern described above? What happens if you dither it to 22 bits or so? That ought to ruin any signatures the decoder is looking for.

 

Seems like the 30 kHz cut-off is more on the rendering phase then. I really fail to see the point in this "rendering" part... The upsampling filter is really slow roll-off, so it leaks a lot of image above Nyquist, visible on tracks that really hit the Nyquist limit.

 

I fail to see the point of any of it.

[Jud]

I fail to see the point of any of it.

 

That's because you are blurry and need de-blurring.

[lmitche]

That's because you are blurry and need de-blurring.

Spoken like the BlurMeister, or is it De BlurMeister!

[smartin]

I have a feeling that some regular hires files can trigger this rendering mode too. I have one regular hires FLAC that seems to trigger this noise and upsampling filter behavior (without the blue light though). Running that FLAC through upsampling before sending it to the DAC takes out the noise and makes it much quieter, and also removes the 30 kHz filter cut. So it seems like there are hires files that are sort of "encoded-decoded" MQA...

 

I have only a rudimentary understanding of the analysis y'all have been doing but this paragraph struck me as odd. Wouldn't this suggest that the noise you are seeing with this MQA decode might be a factor of the track, and not be related to MQA at all. It seems that if you see this behavior with non-MQA content, it would be more likely to be due to some attribute of the track itself (that was carried over into the MQA version) than something inherent to MQA?

[audiventory]

By graphs above, looks like MQA work like 24 bit uncompressed format.

 

But MQA lossless or lossy?

 

I read about a probable "frequency-amplitude response correction", but real implementation of encoder/decoder is unknown.

[mansr]

I have only a rudimentary understanding of the analysis y'all have been doing but this paragraph struck me as odd. Wouldn't this suggest that the noise you are seeing with this MQA decode might be a factor of the track, and not be related to MQA at all. It seems that if you see this behavior with non-MQA content, it would be more likely to be due to some attribute of the track itself (that was carried over into the MQA version) than something inherent to MQA?

 

I took it to mean he's seeing the noise characteristic of the MQA renderer when playing some non-MQA 96 kHz files. No DAC I've ever seen normally generates such noise. Even the built-in upsamplers in the DAC chips are better than that.