aptX Handles Polyphonic Sound Surprisingly Well.

Right now, as I am typing this, I am listening to Beethovens 9th Symphony on my real “LG Tone Pro HBS-750″ Bluetooth Headphones. The quality of sound is dramatically better, than what the fake HBS-730s had produced, simply because those were fake.

This recording of Beethoven is stored on my phone, as a series of FLAC files, and Android Lollipop devices are well-able to play back FLAC files. I did this, in order to test the fake headphones at first, because I was not sure whether their poor performance then was due to some interaction of aptX, with MP3 or OGG compression, rather than due to the implementation of aptX I was getting. Playing back a FLAC file is equivalent to playing back a raw audio file.

From what I read, aptX not only splits the uncompressed spectrum into 4 sub-bands, but then quantizes each sub-band. The 4 sub-bands are approximately from 0 to 5.5 kHz, from 5.5 to 11 kHz, from 11 kHz to 16.5 kHz, and finally from 16.5 kHz to 22 kHz. These sub-bands are then compressed using ADPCM, which allocates 8,4,2,2 bits to each.

This implies, that the first sub-band contains the bass and the mid-range, and that what I would call ‘melodic treble’ sounds, do not extend beyond sub-band 2, since treble notes with fundamental frequencies higher than 11 kHz are not usually played. And sub-bands 3 and 4 simply add texture to the sound. This means, that to allocate fewer bits of precision to sub-bands 3 and 4 ‘makes sense’, since our natural way of interpreting sound, already sees less detail at those frequencies.

A question which I had raised earlier, was if the act of quantizing the sub-bands 3 and 4 greatly – down to 2 bits in fact – will damage the degree of polyphony that can be achieved.

And now that I possess true headphones I am finding, that the answer is No. The sub-bands 3 and 4, are still capable of being played back in a multi-spectral way, even though their differentials have been quantized that much.

(Edit 06/25/2016 : ) Instead of receiving a regular sequence of +1, 0 and -1 data-points, it is possible to receive an atactic sequence of them. The first thing that happens when decoding that, is an integration, which will already emphasize lower, original frequency components that have been deemphasized. After that, the degree with which the analog signal can be reconstructed is only as good, as the interpolation. And in practice, interpolation is often provided by means of a linear filter which has more than two coefficients. Having a longer sequence of coefficients, such as maybe 6 or 8, provides better interpolation, even in sub-bands 3 and 4, which we supposedly hear less-well.

 

I do find though, now that the entire signal is much more clear, that when I listen closely, the highest frequencies belonging to Beethovens 9th, seem to have slightly less resolution than they are truly supposed to have. But not as much less resolution, than I am used to hearing, due to poor headphones, or due to MP3 compression.

It is already a dramatic improvement over what my past told me, that today, Some Bluetooth Headphones can play back high-quality music, in addition to being usable for telephony.

Now, Beethoven died before he finished his 9th symphony, and later artists officially completed it, by adding the 5th movement, which is actually “Shiller’s Ode To Joy”. According to what I am hearing, that 5th movement is compromised more by the aptX compression than the first 4 were, that were actually written by Beethoven.

The reason seems to be the fact, that Shiller’s work is more operatic, and has choruses singing very high notes, which results in a lot of the signal energy being in the 2nd, 3rd and 4th sub-bands. So when I hear that movement, I can hear the quantization quite clearly.

It is usually not a preference of mine, to listen to this 5th movement, because I don’t find it to be authentic Beethoven. Right now I am listening to it, and observing this effect with some fascination.

Dirk

 

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