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Re: custom AudioUnits and processing iTunes library on iPhone Chris Adamson Tue Apr 03 13:00:14 2012

On Apr 3, 2012, at 2:49 PM, Kevin Dixon wrote:

> Quick set of questions regarding feasibility of music processing on
> the iPhone/iPod:
> 
> 1. Can an app install AudioUnits? (Are custom AudioUnits supported?)

No. There was a custom Audio Units API in iPhone OS 3. It didn't actually work, 
and was removed in iOS 4.

Any custom processing needs to be handled in render callbacks to your own code. 
It's not unreasonably difficult, and saves you the hassles of testing and 
packaging a custom unit that you don't intend to offer to third-parties anyways.

> 2. Are "Media Player" apps allowed?

There are already many apps that use the songs and other audio in the user's 
music library.

> If not, is there a plug-in
> architecture for the iPhone's iTunes or whatever its called?

The Media Player framework is how you get at items in the user's music library.

> 3. Is there any way to process the user's playing music in real time?
> AudioUnits or OpenAL?

You can get sample-level access to the user's music by using the 
MPMediaItemPropertyAssetURL, using it to open the song as an AVURLAsset with 
the AV Foundation framework, then using the AVAssetReader API to read samples. 
Once you have samples, you could run them through an AUGraph and perform 
whatever processing you like. There are certain threading and buffering 
considerations that make this harder than you might be ready for, to say 
nothing of having to juggle three media APIs, but that should be the basic idea.

--Chris


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