How to play a signal with AudioUnit (iOS)?

Question

I need to generate a signal and play it with iPhone's speakers or a headset.

To do so I generate an interleaved signal. Then i need to instantiate an AudioUnit inherited class object with the next info: 2 channels, 44100 kHz sample rate, some buffer size to store a few frames.

Then I need to write a callback method which will take a chink of my signal and pit it into iPhone's output buffer.

The problem is that I have no idea how to write an AudioUnit inherited class. I can't understand Apple's documentation regarding it, and all the examples I could find either read from file and play it with huge lag or use depricated constructions.

I start to think I am stupid or something. Please, help...

Answer 1

To play audio to the iPhone's hardware with an AudioUnit , you don't derive from the AudioUnit as CoreAudio is ac framework - instead you give it a render callback in which you feed the unit your audio samples. The following code sample shows you how. You need to replace the assert s with real error handling and you'll probably want to change or at least inspect the audio unit's sample format using the kAudioUnitProperty_StreamFormat selector. My format happens to be 48kHz floating point interleaved stereo.

static OSStatus
renderCallback(
               void* inRefCon,
               AudioUnitRenderActionFlags* ioActionFlags,
               const AudioTimeStamp* inTimeStamp,
               UInt32 inBusNumber,
               UInt32 inNumberFrames,
               AudioBufferList* ioData)
{
    // inRefCon contains your cookie

    // write inNumberFrames to ioData->mBuffers[i].mData here

    return noErr;
}

AudioUnit
createAudioUnit() {
    AudioUnit   au;
    OSStatus err;

    AudioComponentDescription desc;
    desc.componentType = kAudioUnitType_Output;
    desc.componentSubType = kAudioUnitSubType_RemoteIO;
    desc.componentManufacturer = kAudioUnitManufacturer_Apple;
    desc.componentFlags = 0;
    desc.componentFlagsMask = 0;

    AudioComponent comp = AudioComponentFindNext(NULL, &desc);
    assert(0 != comp);

    err = AudioComponentInstanceNew(comp, &au);
    assert(0 == err);


    AURenderCallbackStruct input;
    input.inputProc = renderCallback;
    input.inputProcRefCon = 0;  // put your cookie here

    err = AudioUnitSetProperty(au, kAudioUnitProperty_SetRenderCallback, kAudioUnitScope_Input, 0, &input, sizeof(input));
    assert(0 == err);

    err = AudioUnitInitialize(au);
    assert(0 == err);

    err = AudioOutputUnitStart(au);
    assert(0 == err);

    return au;
}