Modern Consumer Sound Appreciation

Over recent months, I have been racking my brain, trying to answer questions I have, about how sound that was compressed in the frequency-domain, may or may not be able to preserve phase-information. This does not mean that I, personally, can hear phase-information, nor that specific MP3 Files I have been listening to, would even be good examples of how well modern MP3s compress sound. I suspect that in order to stay in business, the developers of MP3 have in fact been improving their codec, so that when played back correctly, the quality of MP3s will stay in line with more-recent formats that exist, such as OGG Vorbis…

But I think that people under-appreciate my intellectual point of view.

For many months and years, I had my doubts, that MP3 Files can in fact encode ± 180⁰ phase-shifts, i.e. a stereo-difference channel that has the correct polarity with respect to the stereo-sum channel, over a range of frequencies. What my own musings have only taught me in recent days, is that in fact, MP3 is capable of ± 180⁰ phase-separation.

Further, similar types of compression should be capable of better phase-separation than that, If their bit-rates are set high enough, that not too many of their frequency-coefficients get chopped down – according to what I have reasoned out today.

What I also know, is that the sound-formats AC3 and AAC have as an explicit feature, to store surround-sound. MPEG-2 Video Files more-or-less require the use of the AC3 codec for sound, and MP4 Files absolutely require the use of the AAC codec. And, stored in its compressed format, the surround-effect only requires ± 180⁰ phase-accuracy.

This subject is orthogonal to debate which exists, about whether it is of benefit to human listeners, to have sound reproduced at very high sample-rates, or at great bit-depths. Furthermore, I do not fully know what good a very high sample-rate – such as “192kHz” – is supposed to do any listener, if his sound has been MP3-compressed. As far as I am concerned, ultra-high sample-rates have to do with lossless compression, or no compression, which also happen to produce the same file-sizes at that signal-format.

What I did was just check, in what format iTunes downloads music by default. And it downloads its music in AAC Format. All this does for me, is corroborate a claim a friend of mine made, that he can hear his music with full positioning, since that is also the main feature of AAC, and not of MP3.

I still listen to an unhealthy amount of my music as MP3 Files.

BTW, In order for my friend to be hearing his music with full positioning, there is an added bit of technology needed, which is called “Headphone Spatialization“. It is also known as ‘Binaural Sound’. This is because by default, the surround-sound effect is meant to be played through speakers.

How the stereophonic signal reaches the ears is also independent, of how spatialization is implemented. When spatialization is implemented, compressed sound is converted into stereo, but with an enhanced opportunity for the listener to make out directions.

If Apple has found a techier way to transmit stereo to the ears of the listener, then this will neither harm nor help the effect, which needs to be applied before the stereo is transmitted to the listener’s headphones.

Now, the question of whether the compression-scheme between the new iPhone 7 and its wireless headphones preserves phase information, is not substantial. When MP3s were traditionally compressed, this brought their bit-rate down to 128kbps. I imagine that the link to the wireless headphones operates at at least twice that bit-rate, so that this is no longer a question. At a hypothetical 384kbps, it no longer represents much of an obstacle, also to transmit phase-information. aptX can do it easily, because aptX is inherently low-latency.

The main danger I see in the sound that comes out of the headphone-jacks of Android smart-phones, is that when they perform their 2x over-sampling, they might be using a linear interpolation. I require better. A Daubechies Wavelet will do better, and a Sinc Filter will do even better than that. Most Android devices have neither.

In fact, we are advised against encoding OGG Files at 48kHz for playback under Android, because Android will convert it to 44.1Khz, with no low-pass filter !

None of my talk is asking for a 192kHz sample-rate here.

Dirk

 

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