iPhone Remote IO Issues

Question

I've been playing around with the SDK recently, and I had an idea to just build a personal autotuner (because I am just as awesome as T-Pain).

Intro aside, I wanted to attach a high-quality microphone into the headphone jack, and I wanted my audio to be processed in a callback, and then copied to the output buffer. This has several implications:

1. When my audio-in is being routed through the built-in microphone, I need to be able to process this input, and send it once my input has stopped (this works).

2. When my audio-in is being routed through the microphone-in input from the headset jack, I want the output to be sent immediately.

Routing, however, doesn't seem to work properly when using AudioSession modes and overrides, which technically should allow you to reroute output to the iPhone speakers, no matter where the input is coming from. This is documented to work, but in practice, doesn't really work.

Remote IO, however, is not documented at all. Anyone with experience using Remote IO audio units, can you give me a reasonable high-level overview on how to do this properly? I have been using the aurioTouch example code, but I am running into errors where I get error codes like -50 and -10863, none of which are documented.

Thanks in advance.

Answer 1

The aurioTouch example implements remoteIO play through.
You could modify the samples before passing them on.

It simply calls AudioUnitRender in the output render callback.

NB this trick does not seem to work if you port the code
to OSX style CoreAudio. There, 99% of the time, you need
to create two AUHALs (RemoteIO-a-likes) and pass
the samples between them.