Re. the Iphone / Android, AAC/Aptx question and which sounds better; I haven't had a chance to compare. However there are a few observations I can share:
- Android, like Windows PCs, does a degree of audio processing buried deep in the OS kernel, this includes a mixer and sample rate converter and allows multiple sources to play at the same time, i.e. notifications can play over the music you're listening to. However this processing tends to affect audio quality, sometimes quite seriously. I don't know if this is also true of Iphones.
- Years ago I freelanced as an engineer getting a well known radio station live on air. We were using a networked audio player to play encoded promos jingles and commercials to air and were worried about how transcoding the audio from one codec to another would affect audio quality. We transcoded between AAC, MP3 and Aptx. AptX came out as the definite winner on grounds of audio quality, transcoding to AptX had a far lower impact on audio quality than to any of the other formats.
On Iphones it makes sense to use AAC, this being the format used by Itunes so the likely source of your music. On Android it makes sense to use Aptx as there is likely to be a wider range of source codecs. I haven't tested which sounds best when encoding FLAC, ALAC or uncompressed audio however.
On my Android devices, using a player that bypasses the internal processing and provides bit-perfect playback makes a big difference. I recently bought a pair of Cambridge Audio Melomania 1+ tws earphones to replace a brokem pair of Sennheisers and was initally rather disappointed in the quality, however listening via a bit perfect player they sounded far better, more definition between instruments and notes, better sense of room acoustics etc. This was true on my old Sony X compact as well as my work Galaxy Note 20 ultra. BTW, this improvement was also obvious on my Sennheisers as well, especially via the headphone jack on the Sony when using wired cans.
Re. 256k being indistinguishable from uncompressed audio: On a good recording I can most definitely hear a difference, however I need to be listening on a good system with good source material. It is fairly obvious when listening on HD650s via an MDAC, not so much via a phone and the HD650s using a bit perfect player.
A couple of years back I attended a conference on Broadcast Audio and the new R128 audio control standard, part of which makes provision for lossy compression which can increase peak audio levels due to ringing in the band pass filters used in the compression codecs. One of the speakers demonstrated the effect of lossy compression by playing uncompressed then compressed audio starting at 320k and going down to 96k via a pair of studio monitors, the difference was certainly noticeable even at 320K. He then played what had been removed from the original audio during compression. It was fascinating, listening to what was removed as the compression was increased you could hear the correlation between the removed audio and what was missing in the compressed output. If anyone's It's possible to try this at home using recording software such as Audacity. Place the source file on tracks 1 & 2 and compressed tracks on 3 & 4, reverse the phase of one pair of tracks and then mix track 1 & 3 and tracks 2 & 4, this will subtract one stereo pair from the other leaving the difference signal. You will need to play with levels and synchronisation to get the best null. You will be left with the difference between compressed and uncompressed, i.e. what the compression has removed, which was described i the conference as sounding like 'space monkeys'.
Summary: If using Iphone, stick to AAC as this is likely your source codec format, If using Android, Aptx is likely best as it is the least destructive when transcoding form other compressed formats.
When using Android, if possible use a bit perfect player for serious listening (i.e. not speech), especially important if using an external DAC. I use USB Audio player PRO but I'm sure there are others. Not sure whether this is necessary with Iphones.
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