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.