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RE: So barely above CD bit depth.

Posted by Charles Hansen (M) on May 18, 2017 at 19:38:54

In Reply to: So barely above CD bit depth. posted by jusbe on May 18, 2017 at 08:19:33:

>> Is there another source for this calculation, for one to corroborate? <<

Sure. Please refer to the slides presented to Stereophile in their first article on MQA:

https://www.stereophile.com/content/ive-heard-future-streaming-meridians-mqa

More difficult to find is the presentation Bob Stuart made to the Japanese Audio Society. It contains the slides from the Stereophile article, plus at least one additional one, reproduced here:

This shows the spectrum after the first MQA "fold", from 192kHz to 96kHz. The quad-rate information in region "C" uses lossy compression to store that information in the area with purple squares designated by the arrow labeled "Encapsulation". (Presumably "encapsulation" sounds better to the customer than does "lossy compression".) The grey area directly above is reserved for the double-rate information and is clearly labeled "96k/17.2b".

When the second fold to 48kHz occurs, the quad-rate information is shifted from the dual-rate region to the single-rate (baseband) region, still leaving only 17.2 bits of resolution in the baseband audio. These other slides are shown in the Stereophile link noted above.

Please note that these graphs were all made with a string quartet playing a composition by Ravel. Now we turn to an article by James Boyk, noted pianist and professor at Caltech. His article "There's Life Above 20kHz" is linked in the URL below. Scroll past the 3/4 mark on the page to Table 1 and we can see that a violin has a maximum of 0.04% of its power in the band beyond 20kHz. While he doesn't specifically measure either viola or cello, we can be confident that they don't have *more* energy past 20kHz than a violin. In contrast cymbals have 40% (!) of their total acoustical power above 20kHz. For power this ratio of 1000:1 represents a 30dB difference.

The normally accepted calculation between bits and dB is 6dB per bit. Therefore program material that had a lot of cymbals (almost all modern pop, rock, jazz, plus much orchestral music) would require as much as 5 more bits of uncompressed space in the 24-bit FLAC container.

NB: In my previous post I subtracted 6 bits from the 17.2 bit resolution available with a string quartet to reach a possible minimum resolution of 11.2 bits. This was a typo and should have been 5 bits and 12.2 bits respectively. And as I am writing this, I also now realize that the energy above 20 kHz will be compressed (dual-rate information losslessly, and quad-rate information using lossy techniques). Therefore there may not be a full 5 bit reduction is dynamic range when MQA encoding is applied to music with lots of high-frequency energy (eg, cymbals). But when already reducing 24-bit data to a maximum resolution of 17.2 bits, there isn't much room for further reduction before the decoded MQA file cannot even achieve CD-standard 16-bit resolution.

The fact that MQA specifically chose a musical example with what is likely the least amount of high-frequency energy *possible* is interesting (to say the least). Unfortunately it is extremely difficult to cipher exactly how much resolution is lost for each specific MQA-encoded track. That would require special MQA-encoded test discs and the like.

It's unfortunate that MQA seems to have deliberately obfuscated the true costs of their encoding/decoding process, and only focused on the benefits. I suppose that is only natural for anybody trying to sell a something, but when it comes to physics there is simply no free lunch. In other words, if it sounds too good to be true, it probably is. All engineering is made of a series of compromises. If streaming bandwidth is a critical issue, then in some cases reduction of file size may be worth the cost of reduced resolution. However true high-res 96/24 FLAC files are only about 20% larger than MQA files. Even 192/24 FLAC files are about half the size of streaming video files. Most audiophiles have sufficient internet bandwidth to easily stream true hi-res audio files.

For whatever reason, MQA has chosen to only provide full (192kHz) MQA decoding via hardware. This requires the customer to purchase a new DAC for full decoding. Clearly it is possible to perform the decoding in software, as there are at least two software apps that will fully decode 96kHz files and (I believe) partially decode 192kHz files (to 96kHz).

The ultimate choice is up to each consumer. Is it worth purchasing a new DAC in order to play a few hundred titles that are realistically available from only one streaming service? To answer that question would require a crystal ball. While I have no doubt that it is possible for an MQA file to sound better than a Redbook file (44/16), my experience is that the orginal hi-res file sounds better still. How long before there is a streaming service that offers streaming of true high-res files?

Or before there is a streaming service that uses OraStream's adaptive technology? (If there is sufficient bandwidth at the playback device, OraStream will stream full 192/24 resolution - about 3500kb/s in FLAC. If there isn't enough bandwidth it automatically and seamlessly scales back to 96/24 - about 1800kb/s in FLAC. If less than that is available it scales back to 48/24 - about 1200kB/s in FLAC. The process continues all the way down to bitrates comparable to that used by YouTube, allowing for uninterrupted music playback even when using wireless services. By comparison MQA requires a constant 1500kb/s in FLAC.

Hope this helps. As always the opinions in my posts are my personal ones and do not reflect those of my employers, co-workers, friends, family, or enemies.

• Article by James Boyk   (Open in New Window)

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