This commonly held belief is now being challenged. A recent meta-analysis found a statistically significant, better-than-average rate of correct identification that could not have been produced by chance among listeners who were trained to recognise high resolution audio. For listeners who weren't trained or chosen at random the effect was absent.
I've done some brief listening tests comparing a high resolution album I bought with the iTunes version (which is admittedly less than CD quality) and the difference is like night and day. I can correctly identify it almost 100% of the time even without my high quality DAC. And if you don't believe me I can send you example tracks and you can convert/upscale the iTunes ones and strip both files of all metadata so they look exactly the same (use tools like "Touch" in Terminal even so they look exactly the same) and you can send them back and I will tell you which is which. I can even tell if you send only only one file back whether it's the high res version or Apple iTunes.
Try me!
For this reason (and others) I stopped buying from iTunes a long time ago. Mostly because there are now plenty of other online retailers (or from the artist directly so they get more) that sell lossless or high resolution albums for the same price or less than iTunes. If someone offered you more for less you'd take it right?
iTunes is only marginally cheaper even than buying new release physical CDs in a shop and CD stores often discount older albums significantly while iTunes rarely does. With iTunes you're paying for (minor) convenience and sacrificing almost everything else.
"who were trained to recognise high resolution audio"
If you need to "train" to recognize the difference between normal iTunes downloads and high fidelity sources, then there isn't that much of a difference and it will never catch on as a mainstream thing.
"and the difference is like night and day"
You are lying, unless there were different masterings, which would be why there would be any difference to begin with.
No I don't believe you, where are your ABX test results? There isn't anyone that could realistically recognize the difference between regular AAC 256 kbps files from so-called Hi.Res tracks, providing that they are the same file with the same mastering, etc.
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There is no comparison between a high res recording and an mp3, the same thing goes to compare a Blu-ray against an ITunes version of the movie (sound and audio)
Except there is, a well encoded MP3 or an AAC file compared to a Hi.Res track is virtually indistinguishable, providing that they are the same master and mix.
There is no difference, learn how your ears work.