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Note that the difference is not large. A 128 kbps opus or AAC might be comparible to a 160 or 192 kbps MP3. So it's less than 2x improvement of file size.

AAC has an additional advantage though, which is that many phones and receivers can transmit AAC files over Bluetooth without reencoding. this is technically possible for MP3 too, but very few devices implement it.

the loss of quality from transcoding lossy to lossy is usually a lot worse than the difference in quality between codecs and bitrates (within reason).

Interesting, didn't know the Bluetooth fact. I don't usually deal with AAC myself, since opus is so close in every quality/feature, and the AAC patent license is sometimes costly to use commercially ($0.98 per software sale).

Even though it's theoretically possible to send over Bluetooth without reencoding, I wonder if it happens in practice. The audio pipeline has too many stages and each of them would have to retain the encoding.

good point. the developer settings on my pixel 2 allow me to set the preferred codec, but I've never dug into it enough to know whether the setting is actually honored. all my music is MP3 anyway so it's going to sound awful over Bluetooth no matter what.

Similar developer settings on the S8+/Note 9 - as soon as you connect to a device that doesn't support your chosen codec, it'll reset. I can tell the difference between APT-X and AAC, but I've got no idea if the AAC is being re-encoded.

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