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On the blurriness of MQA

199.46.250.152
Posted on November 2, 2016 at 12:38:32
Dave_K
 Something occurred to me as I skimmed the arguments below.

I think it would be more technically accurate to call Meridian's proposed ideal system impulse response a blurring filter. It has a slow roll-off response and its shape is designed to be as close as possible to a Gaussian. In image processing, Gaussian filtering is also called Gaussian blur. It is used to soften and smooth images.

In their 2014 AES paper, Stuart and Craven say that their ideal system response is equivalent to the dispersion of sound passing through 10m of air. They speak of it like it's a good thing. But how about zero meters? Why should we accept a system that adds any dispersion at all? Conventional high sample rate PCM systems (I would say 88.2k and above, but it depends on the source of music) can already pass the full musical spectrum without adding any dispersion. Compared to that, Stuart's system adds temporal dispersion and high frequency roll-off, smoothing over transients somewhat. I know it's "only" equivalent to 10m of air, which seems like it should be pretty benign, but why adopt a system whose design target is less transparent than what's already available?

Second, the idea of selecting a reconstruction filter on playback that's matched to the anti-aliasing filter used in recording was a fine idea when it was implemented for HDCD because of the limitations of the Redbook format, where both filters had to be right at the top of the audible band. It is completely unnecessary at higher sample rates where the filters' stop bands begin at a frequency higher than the musical content.

 

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Guys, pleeeze..., posted on November 2, 2016 at 18:46:13
13th Duke of Wymbourne
Audiophile

Posts: 2796
Location: Orange Co., Ca
Joined: September 19, 2001
 It seems many have ideological objections to a proprietary encoding format. Fair enough, but please don't second guess the technical aspects to justify the objection. That way madness lies. Bob Stuart's paper shows that the end-to-end dispersion of a 192kHz linear phase system is much more than MQA.

Interesting that MP3 is brought up in this thread. I think of them as similar, in broad terms: MP3 analyses the signal and throws away signal in as harmless a way as possible to fit a given data rate, MQA analyses the signal and throws away only unoccupied dynamic range to reduce the bit-rate. Of course, I might be wrong, but I like that analogy.

Regards
13DoW

 

I'll stick to the technical points then, posted on November 3, 2016 at 08:57:34
Dave_K
 I edited my post and deleted the last paragraph. It was kind of beside the point.

The main point I really wanted to convey was that when employed as an end to end technology such that the system impulse response described in the Stuart paper is achieved, the result is a blurring of the signal. Conventional anti-aliasing and reconstruction filters do not blur, and when employed at higher samples rates where the cutoff is above the musical spectrum they don't add any ringing either. Using a high enough sample rate eliminates the problem of filter artifacts that plagues Redbook. No Meridian special sauce is required. I find it ironic that Stuart & co. are marketing MQA as an end to end technology that minimizes temporal blur, when the reality is that they have chosen a target impulse response that adds blur.

 

RE: I'll stick to the technical points then, posted on November 3, 2016 at 15:07:29
13th Duke of Wymbourne
Audiophile

Posts: 2796
Location: Orange Co., Ca
Joined: September 19, 2001
 Hi Dave,

Of everything I understand and have read about MQA the raison d'etre is to have less time dispersion than present day hi-res PCM. I even double-checked the paper, that contains a graph comparing time dispersion of the two systems, before I posted.

Regards
13DoW

 

RE: I'll stick to the technical points then, posted on November 8, 2016 at 12:30:14
Dave_K
 I read through Stuart & Craven's AES paper a couple times and don't see anything like that.

The closest I can find is a claim in the text that "the end-to-end response, shown in Figure 15, introduces considerably less blur than transmission at 96 kHz using conventional filters, as shown in Figure 14 below." The term "blur" is not defined. Figures 14 and 15 are showing impulse responses. My guess is that the authors are trying to imply that a longer impulse response = more temporal blur. If so, it's disingenuous. A sinc for example, has an infinite impulse response and introduces zero time dispersion and provides the sharpest transient response possible within a given bandwidth.

The closer you get to a perfect brick wall response, the less time dispersion. The more gentle the filter slope, the more it blunts and smooths over transients, and that's what I would call blur. The fact that their proposed response is a Gaussian, which is a filter type normally used for image blurring, makes them sound silly when they talk about de-blurring. It's like they have the whole idea backwards.

The traditional complaint about brick wall filters in digital audio is based on experience with the 44.1k sampling rate. At that sample rate, the signal content coming off the mic feed usually extends above fs/2 , and therefore there is an interaction between the signal and the anti-aliasing and reconstruction filters. Also the filters are operating around the top of the human hearing range. If you just double the sample rate to 88.2k or better, the filters are operating far away from the human hearing range and in most cases, above the input signal bandwidth too. A brick wall filter at 42-44 will pass the signal with no time dispersion and no ringing. Stuart & Craven's filter will roll it off and smear the transients a little bit.

 

RE: On the blurriness of MQA, posted on November 2, 2016 at 12:46:44
mkuller
Audiophile

Posts: 38130
Location: SF Bay Area
Joined: April 22, 2003
 ..why? If it sounds better.

Lossy? No, compressed which is different.

 

MQA compression is lossy, posted on November 2, 2016 at 14:59:27
Dave_K
 It's a different kind of lossy than MP3. It's not using a predictive psycho-acoustic model to dynamically decide where to lose information. But MQA encoding & decoding does involve some loss of bit depth. Just how much has been a subject of debate.

 

RE: MQA compression is lossy, posted on November 2, 2016 at 16:41:25
mkuller
Audiophile

Posts: 38130
Location: SF Bay Area
Joined: April 22, 2003
 ...so that's the kind of loss I get with my Wadia 850 cd player when I set the digital volume control below 75.

Very different than the MP3 codec lossy.

 

RE: On the blurriness of MQA, posted on November 2, 2016 at 13:08:13
BubbaMike
Audiophile

Posts: 650
Location: Left Coast of the USA
Joined: January 4, 2002
 Compromised is more like it.




When they discover the center of the universe, a lot of people will be disappointed to discover they are not it. ~ Bernard Bailey

 

The point of these points is lost on me., posted on November 3, 2016 at 01:16:04
PAR
 Seemingly endless objections to MQA on AA and elsewhere. What is the point?

1. Most objectors have never heard MQA.

2. It has not yet reached the consumer market except in a very limited way.

3. Its primary purpose at this juncture seems to relate to streaming media where , whatever its limitations, it promises better sound than exists is this medium now.

4. Do people think that perhaps their arguments (often technically incorrect) may influence Bob Stuart to change it?

Why waste time now on this subject when in there is a real and present problem with digitally distributed recordings - the use of watermarking. Yet the comparative amount of forum postings on this subject is fractional.

 

The point is to discuss the merits of an emerging product that is starting to penetrate the market, posted on November 3, 2016 at 11:07:23
Dave_K
 If this was just some standalone product I wouldn't care. MQA deserves extra scrutiny since it aims to replace current delivery formats, and its end goal is to control the whole digital audio chain from recording to playback. It has the potential, if widely adopted, to affect every consumer of digital audio. That's why so many people are talking about it. Beyond that, the marketing of MQA has been confusing, sometimes contradictory, dare I say a bit shifty. So that's leading to some skepticism.

 

RE: The point is to discuss the merits of an emerging product that is starting to penetrate the market, posted on November 3, 2016 at 12:06:10
fmak
Audiophile

Posts: 13158
Location: Kent
Joined: June 1, 2002
 I have not actually seen any vigorous examination of the postulations behind the model. Instead, I have seen the assertions behind it.

We should recognise that human responses to acoustical factors are not uniform, and that the only way to validate such models is to expose statistically significant numbers of respondents to them. Even then, the results may well be dependent on ethnic origins and their exposes to what may be regarded as 'musical'.

 

What is the point, posted on November 3, 2016 at 07:31:32
fmak
Audiophile

Posts: 13158
Location: Kent
Joined: June 1, 2002
 The point is simply that MQA is being promoted thru media push and not through rational discussion of pros and cons at a technical and prospective consumer demonstration level.

MQA is a psychoacoustic software and hardware system. It is closed and web retailers are trying to sell MQA music at £16 a go, without the ability for users to compare file 'before and after'. So the sales model seems to be: let journalists write about the merits and let consumers be tempted towards its widespread adoption.

The 2L stuff is a red herring as, if you audition the same files at different resolution levels, they sound different in themselves anyway. And I have not seen comparisons of the original DXD files with the MQA ones using the same dac.

Take a recording like Kinds of Blue being written about wrt the merits of MQA. If you reverse the phase of the music being played, you can end up with 'an opening of the soundstage' and the other superlatives that have been bestowed on the format.

What is needed is a much more clear headed and valid approach to MQA v hires comparisons.

 

Hear, hear!, posted on November 3, 2016 at 07:34:23
Doug Schneider
Reviewer

Posts: 881
Location: North America
Joined: April 16, 2005
 Hear, hear! (meant in more ways than one...)

Doug Schneider
SoundStage!

 

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