ios - How to write output of AUGraph to a file?

Question

I am trying to write (what should be) a simple app that has a bunch of audio units in sequence in an AUGraph and then writes the output to a file. I added a callback using AUGraphAddRenderNotify. Here is my callback function:

OSStatus MyAURenderCallback(void *inRefCon,
                        AudioUnitRenderActionFlags *actionFlags,
                        const AudioTimeStamp *inTimeStamp,
                        UInt32 inBusNumber,
                        UInt32 inNumberFrames,
                        AudioBufferList *ioData) {
    if (*actionFlags & kAudioUnitRenderAction_PostRender) {
        ExtAudioFileRef outputFile = (ExtAudioFileRef)inRefCon;
        ExtAudioFileWriteAsync(outputFile, inNumberFrames, ioData);
    }
}

This sort of works. The file is playable and I can hear what I recorded but there is horrible amounts of static that makes it barely audible.

Does anybody know what is wrong with this? Or does anyone know of a better way to record the AUGraph output to a file?

Thanks for the help.

Answer 1

I had a epiphany with regards to Audio Units just now which helped me solve my own problem. I had a misconception about how audio unit connections and render callbacks work. I thought they were completely separate things but it turns out that a connection is just short hand for a render callback.

Doing an kAudioUnitProperty_MakeConnection from the output of audio unit A to the input of audio unit B is the same as doing kAudioUnitProperty_SetRenderCallback on the input of unit B and having the callback function call AudioUnitRender on the output of audio unit A.

I tested this by doing a make connection after setting my render callback and the render callback was no longer invoked.

Therefore, I was able to solve my problem by doing the following:

AURenderCallbackStruct callbackStruct = {0};
callbackStruct.inputProc = MyAURenderCallback;
callbackStruct.inputProcRefCon = mixerUnit;

AudioUnitSetProperty(ioUnit,
                     kAudioUnitProperty_SetRenderCallback,
                     kAudioUnitScope_Input,
                     0,
                     &callbackStruct,
                     sizeof(callbackStruct));

And them my callback function did something like this:

OSStatus MyAURenderCallback(void *inRefCon,
                        AudioUnitRenderActionFlags *actionFlags,
                        const AudioTimeStamp *inTimeStamp,
                        UInt32 inBusNumber,
                        UInt32 inNumberFrames,
                        AudioBufferList *ioData) {

    AudioUnit mixerUnit = (AudioUnit)inRefCon;

    AudioUnitRender(mixerUnit,
                    actionFlags,
                    inTimeStamp,
                    0,
                    inNumberFrames,
                    ioData);

    ExtAudioFileWriteAsync(outputFile, 
                           inNumberFrames, 
                           ioData);

    return noErr;
}

This probably should have been obvious to me but since it wasn't I'll bet there are others that were confused in the same way so hopefully this is helpful to them too.

I'm still not sure why I had trouble with the AUGraphAddRenderNotify callback. I will dig deeper into this later but for now I found a solution that seems to work.