Ah, I stand corrected! I wonder why they took that approach...

Not sure - although I would imagine that it would effectively double the storage and bandwidth/data requriement for Apple Music in general if they had to send two files with equal bitrate.

I stand corrected as well!

Wonder why they take this approach though, as it is clearly over-engineering (if I correctly understand that the goal is just to make vocals volume adjustable).

Maybe it's licensing? I can imagine copyright holders being squeamish about Apple processing, permanently storing, and serving heavily altered versions of their music. The difference is silly and pedantic, but by processing it in real-time during playback, one might argue it's just a filter effect like EQ.

> Wonder why they take this approach though, as it is clearly over-engineering (if I correctly understand that the goal is just to make vocals volume adjustable).

Depends what the other non-functional requirements were. i.e. if the NFRs were as follows:

* Cannot increase bandwidth / mobile data usage.

* Cannot impact music quality / bitrate.

* Has to work offline.

* Cannot increase on-device storage.

* Has to be responsive.

Then two audio streams might not work.

Another advantage of doing it on-device is that it doesn't actually change any of the backend architecture too. It might be a lot of change to a lot of systems for a feature which only adds a small amount of functionality - i.e. architecting your entire backend and streaming around seperating audio tracks might not be the right focus.