How to use an Audio Unit on the iPhone

Question

I'm looking for a way to change the pitch of recorded audio as it is saved to disk, or played back (in real time). I understand Audio Units can be used for this. The iPhone offers limited support for Audio Units (for example it's not possible to create/use custom audio units, as far as I can tell), but several out-of-the-box audio units are available, one of which is AUPitch.

How exactly would I use an audio unit (specifically AUPitch)? Do you hook it into an audio queue somehow? Is it possible to chain audio units together (for example, to simultaneously add an echo effect and a change in pitch)?

EDIT: After inspecting the iPhone SDK headers (I think AudioUnit.h, I'm not in front of a Mac at the moment), I noticed that AUPitch is commented out. So it doesn't look like AUPitch is available on the iPhone after all. weep weep

Apple seems to have better organized their iPhone SDK documentation at developer.apple.com of late - now its more difficult to find references to AUPitch, etc.

That said, I'm still interested in quality answers on using Audio Units (in general) on the iPhone.

Answer 1

There are some very good resources here (http://michael.tyson.id.au/2008/11/04/using-remoteio-audio-unit/) for using the RemoteIO Audio Unit. In my experience working with Audio Units on the iPhone, I've found that I can implement a transformation manually in the callback function. In doing so, you might find that solves you problem.

Answer 2

Regarding changing pitch on the iPhone, OpenAL is the way to go. Check out the SoundManager class available from www.71squared.com for a great example of an OpenAL sound engine that supports pitch.