Dr. AIX's POSTS HD-AUDIO — 06 December 2014

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You can call if what you want, but the new technology that Meridian announced last week is a data reduction technique that requires both encoding and decoding. In Robert Stuart’s AES paper he uses the term “lossy” and “lossless”…so you decide whether this new methodology is a codec or not.

But it very certainly is a technique for maintaining the benefits of high sample rate audio within a much smaller container. And the container can be a traditional PCM or FLAC file infused with metadata that allows for the expansion of the smaller file back to the original 192 kHz source file…the decoding side of the process.

Just how this works and the magnitude of improvement in terms of bandwidth and storage capacity is described in the AES paper, “A Hierarchical Approach to Archiving and Distribution” authored by Robert Stuart and Peter Craven. The bottom line on the distribution system is that a 192 kHz/24-bit original requiring 9.2 Mbps can be “encapsulated” using an average data rate of 922 kbps, which is well below the rate of a stereo 44.1 kHz/16-bit compact disc!

Getting high-resolution audio fidelity down to bitrates associated with CDs is quite a feat. From the discussion that Robert gave at the AES convention in Los Angeles, a careful reading of the previously mentioned paper, and the discussions/videos available on the Meridian website, I think I have a pretty good handle on MQA.

It’s all about switching from rectangles to triangles. I’ve included lots of graphics in previous posts that show diminishing intensity of signals as they increase in frequency (except using DSD). Take a look at the diagram below:

141206_plot

Figure 1 – This is a typical plot of intensity vs. frequency for a 96 kHz/24-bit PCM recording. [Click to enlarge]

I plotted one of my tracks to show that amplitude decrease as frequency increases. However, our current digital encoding schemes allocate the same number of samples and bits to each octave regardless of whether there is information there or not. Many customers of high-resolution digital music sites are spending their money on a bunch of digital zeros! The only plots that I’ve ever seen that have rising amounts of ultrasonic information were those derived from DSD recordings and even DXD tracks! And of course, that noise is the result of the in band noise generated by the 1-bit DSD system.

Meridian looked at a lot of these amplitude vs. frequency plots and said, “Why don’t we try to encode just the signal part of the music and ignore the empty space?” Instead of having a rectangular frequency vs. amplitude box generating large amount of data…much of which is empty…they came up with a system that focuses on just the audio. Of course, there’s a lot more to it than just using filter to remove the blank part of the plot, but this is a key component of the MQA technology. They call this “structured encoding” and “encapsulation filtering”.

In the final paragraphs of the AES paper, they “recommend that modern digital recordings should employ a wideband encoding system which places specific emphasis on time and frequency and sampling at no less than 384 kHz.

More coming…

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I’m still looking to raise the $3700 needed to fund a booth at the 2015 International CES. I’ve received some very generous contributions but still need to raise additional funds. Please consider contributing any amount. I write these posts everyday in the hopes that readers will benefit from my network, knowledge and experience. I hope you consider them worth a few dollars. You can get additional information at my post of December 2, 2014. Thanks.

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