MQA technical analysis

[mansr]

Actually its a pretty accurate way of talking about it. If they are using noise-shaped subtractive dither (the patent mentions this) then its possible to increase apparent resolution while reducing the coding-space for dynamic range. Bob never goes into the details but its clear they are relying on something like this to regain the apparent dynamic range. And I believe they analyze the noise floor of the recording to ensure that there is sufficient bit depth once MQA has been applied. The stereophile Q&A goes into this a little more than the Q&A done here with Bob in April.

 

The renderer applies noise-shaped subtractive dither in the final step before sending the data to the DAC. It works reasonably here since the sample rate is 176.4 kHz or higher, meaning there's somewhere to put the error noise. I don't really see the point in doing this, however, since any half-decent DAC supports 24-bit input anyway.

 

In the 44.1 kHz MQA container, no matter how you shape the dither noise, some of it will be below 20 kHz. For this reason, an undecoded file may well provide audibly degraded sound.

[Maldur]

only the top 4 bits are passed intact, so while the music would be recognisable, it would sound awful. I don't know what might enable this as I have not seen it with any of the samples I've tested (hardly surprising). Explain how this isn't DRM.

Let's say, if, if MQA decides to enable some "pay-per-hearing" or "play-only-on-authenticated-system" feature, then it acts like DRM. Even in so called demo mode (for stores) this 4bit track is lower quality than lossy demo files on today's stores...

[tmtomh]

The renderer applies noise-shaped subtractive dither in the final step before sending the data to the DAC. It works reasonably here since the sample rate is 176.4 kHz or higher, meaning there's somewhere to put the error noise. I don't really see the point in doing this, however, since any half-decent DAC supports 24-bit input anyway.

 

I agree 100% - there are and will be zero MQA-capable DACs that don's support 24-bit input. And I would venture that there are approximately zero existing DACs that are MQA-upgradable BUT not capable of 24-bit inout.

[miguelito]

So are you saying that the better sound supplied by MQA has a form of EQ applied to it?

MeQA

[Miska]

I won't stop what I'm doing, but if Chris desires, I have no choice but to talk less about it here.

 

Just let us know if you take the discussion elsewhere...

 

AFAIK, reverse engineering is legal in most places for interoperability purposes:

https://en.wikipedia.org/wiki/Reverse_engineering#Legality

[Miska]

The result is mathematically a different signal compared to the original PCM master. But it may be a better sounding signal, or even a more "accurate" representation of the original sound.

 

I guess you could say that MQA tries to anticipate the damage caused by the ADC and DAC and then applies DSP at various stages in the digital chain in order to remove that noise and reproduce the original analog-like sound.

 

Does it work? We have to decide ourselves. That's the consumer's task So far, Tidal Masters are sounding rather nice.

 

I would rather take an objective view of the performance, rather than subjective.

[mansr]

Just let us know if you take the discussion elsewhere...

 

AFAIK, reverse engineering is legal in most places for interoperability purposes:

https://en.wikipedia.org/wiki/Reverse_engineering#Legality

This isn't my first foray into reverse engineering. I've done nothing illegal, but I understand if Chris is cautious.

[abrxx]

I agree 100% - there are and will be zero MQA-capable DACs that don's support 24-bit input. And I would venture that there are approximately zero existing DACs that are MQA-upgradable BUT not capable of 24-bit inout.

 

If I understand you correctly, you have discovered that MQA has some handling of decoding a 16bit mqa file? If so, that scenerio has been discussed by Bob. MQA is meant to partially work even if the signal has been truncated to 20 or 16 bits.

[Miska]

Thanks, much appreciated.

 

I just checked and the decoded MQA stream from Tidal doesn't trigger the render mode (noise) on the Meridian DAC... So it is played just like plain normal hires file.

[mansr]

I just checked and the decoded MQA stream from Tidal doesn't trigger the render mode (noise) on the Meridian DAC... So it is played just like plain normal hires file.

 

Good to know. I guess the Explorer2 doesn't operate as a separate renderer, which is presumably what the Dragonfly will do when (if?) they release the update.

[tmtomh]

If I understand you correctly, you have discovered that MQA has some handling of decoding a 16bit mqa file? If so, that scenerio has been discussed by Bob. MQA is meant to partially work even if the signal has been truncated to 20 or 16 bits.

Thanks, but this is sort of becoming a tail-chasing exercise. I did not discover this, mansr did. I responded to him saying essentially what you have just written here - that Stuart/MQA already has noted that decoding data is located within the first 16 bits. As for any evaluation of this fact, I will leave that to you and mansr, and anyone else who wishes to chime in.

[abrxx]

Good to know. I guess the Explorer2 doesn't operate as a separate renderer, which is presumably what the Dragonfly will do when (if?) they release the update.

 

Is the Explorer 2 running latest firmware? Very curious finding. I guess for a USB DAC there is no need for it to act as a standalone renderer IF it can be used as a full decoder.

[Miska]

Is the Explorer 2 running latest firmware?

 

Yes it is...

[HpW]

If you are looking for good source recorded by DxD and with various converted DSD/Wave/MQA formats

http://www.2l.no/hires/

 

As:

- Finzi: Come Away, Death Marianne Beate Kielland, mezzo soprano Sergej Osadchuk, piano

- Bøhren/Åserud: Blågutten HOFF ensemble

- Groven: Undring Sigmund Groven, harmonica Iver Kleive, organ....

 

Hp

[Miska]

If you are looking for good source recorded by DxD and with various converted DSD/Wave/MQA formats

http://www.2l.no/hires/

 

As:

- Finzi: Come Away, Death Marianne Beate Kielland, mezzo soprano Sergej Osadchuk, piano

- Bøhren/Åserud: Blågutten HOFF ensemble

- Groven: Undring Sigmund Groven, harmonica Iver Kleive, organ....

 

Unfortunately not many contain notable amount of high frequency content that would really push MQA into noise modulation (running short on HF encoding bandwidth/stealing bits from lower frequencies). For that you want synth music or rock/blues/pop with close mic'ed crash cymbals and such.

[HpW]

Unfortunately not many contain notable amount of high frequency content that would really push MQA into noise modulation (running short on HF encoding bandwidth/stealing bits from lower frequencies). For that you want synth music or rock/blues/pop with close mic'ed crash cymbals and such.

Well, most recordings, as I name this, have a nice tinnitus (square wave) on 15khz... may would be nice to see what MQA does with them, while I have no MQA gear..

 

 

Hp

[audiventory]

Well, most recordings, as I name this, have a nice tinnitus (square wave) on 15khz... may would be nice to see what MQA does with them, while I have no MQA gear..

 

In the DXD high frequency noise should be removed before playback or coding. So there will not issues for coding, I suppose.

[mansr]

The MQA data stream can contain embedded metadata. The 2L samples include ID3v2 tags. The ID3v2 data is split into chunks of no more than 256 bytes delivered in a haphazard order. Only two of the samples contain a complete tag. Very odd. One of the files, 2L-120, additionally contains metadata packets in another format. In this file, it contains the string "44100Hz" variously padded and abbreviated, which isn't enough information to deduce the meaning of these packets.

[mansr]

As an experiment, I took the MQA version of 2L-048 and zapped the high 15 bits preserving only the sign while leaving the MQA control bitstream in bit 8 and the lower bits intact. The decoder still reports this as "blue-light" MQA and tries it's best to decode it. Part of the decoding process depends on the input sample values, so that part will obviously not be working correctly. Nevertheless, the average spectrum for "decoded" output from this file (green) alongside the original (blue) and the normally decoded MQA (red) looks like this:

 

 

The dashed vertical line marks 24 kHz (the input has 48 kHz sample rate). While lower in level, the high-frequency output here largely follows the same shape as the "proper" spectrum, including the small spike near the top end.

 

It should also be noted that the decoder calculates a checksum over the input samples, and I don't know what effect a mismatch here has.

[Abtr]

Could it be the result of some error correction algorithm? How does it sound?

[mansr]

Could it be the result of some error correction algorithm? How does it sound?

I haven't spotted any error correction in the decoder. Those frequencies at those levels are inaudible.

[Abtr]

... Those frequencies at those levels are inaudible.

That doesn't exactly answer how it sounds overall..

[mansr]

That doesn't exactly answer how it sounds overall..

How what sounds?

[Abtr]

How what sounds?

The MQA version of 2L-048 where you zapped the high 15 bits preserving only the sign while leaving the MQA control bitstream in bit 8 and the lower bits intact.

[mansr]

The MQA version of 2L-048 where you zapped the high 15 bits preserving only the sign while leaving the MQA control bitstream in bit 8 and the lower bits intact.

I haven't listened to it, but it's too low level to be audible (one could of course apply digital gain; there's some 90 dB headroom). Looking at the spectrogram, there is some correlation with the original.