Setting audio pan programmatically on iPhone not working

Question

In my code, I set up an audio processing graph with two audio units: an I/O unit, and a multichannel mixer unit. First the I/O unit:

bool RtApiIos::setupAU(void *handle, AURenderCallbackStruct inRenderProc, AudioStreamBasicDescription &outFormat)
{

AudioUnitHandle *auHandle = (AudioUnitHandle *)handle;

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 );

if (AudioComponentInstanceNew(comp, &auHandle->audioUnit))
    return false;

if (AudioUnitSetProperty(auHandle->audioUnit, kAudioUnitProperty_SetRenderCallback, kAudioUnitScope_Input, 0, &inRenderProc, sizeof(inRenderProc)))
    return false;

Then the multichannel mixer unit:

    AudioComponentDescription mixerDesc;
    mixerDesc.componentType = kAudioUnitType_Mixer;
    mixerDesc.componentSubType = kAudioUnitSubType_MultiChannelMixer;
    mixerDesc.componentManufacturer = kAudioUnitManufacturer_Apple;
    mixerDesc.componentFlags = 0;
    mixerDesc.componentFlagsMask = 0;

    comp = AudioComponentFindNext( NULL, &mixerDesc );

    if (AudioComponentInstanceNew(comp, &auHandle->mixerUnit))
        return false;

    UInt32 busCount = 1;
    if( AudioUnitSetProperty(auHandle->mixerUnit, kAudioUnitProperty_ElementCount, kAudioUnitScope_Input, 0, &busCount, sizeof(busCount)) )
        return false;

    if (AudioUnitSetProperty(auHandle->mixerUnit, kAudioUnitProperty_SetRenderCallback, kAudioUnitScope_Input, 0, &inRenderProc, sizeof(inRenderProc)))
        return false;

    size = sizeof(localFormat);
    if (AudioUnitGetProperty(auHandle->mixerUnit, kAudioUnitProperty_StreamFormat, kAudioUnitScope_Input, 0, &localFormat, &size ))
        return false;

    if( AudioUnitSetProperty(auHandle->mixerUnit, kAudioUnitProperty_SampleRate, kAudioUnitScope_Output, 0, &localFormat.mSampleRate, sizeof(localFormat.mSampleRate)))
        return false;

I start up the processing graph, and off we go. The audio plays OK, but the last call of the routine has no audible effect on the pan (and "err" == 0).

    // Declare and instantiate an audio processing graph
    NewAUGraph(&auHandle->processingGraph);

    AUNode mixerNode;
    AUGraphAddNode(auHandle->processingGraph, &mixerDesc, &mixerNode);

    AUNode ioNode;
    AUGraphAddNode(auHandle->processingGraph, &desc, &ioNode);

    // Indirectly performs audio unit instantiation
    if (AUGraphOpen(auHandle->processingGraph))
        return false;

    if( AUGraphConnectNodeInput(auHandle->processingGraph, mixerNode,  0, ioNode, 0) )
        return false;

    if (AUGraphInitialize(auHandle->processingGraph))
        return false;

    AudioUnitParameterValue panValue = 0.9; // panned almost dead-right. possible values are between -1 and 1
    OSStatus err = AudioUnitSetParameter(auHandle->mixerUnit, kMultiChannelMixerParam_Pan, kAudioUnitScope_Output, 0, panValue, 0);
    if (err != noErr) {
        //error setting pan
    }
}

What am I doing wrong? Thanks!!

Answer 1

Got this working, with a lot of help from Apple's old MixerHost sample project.

First created a structure to bundle my resources:

struct AudioUnitHandle {
    AudioUnit audioUnit;
    AUGraph processingGraph;       
    AudioUnit mixerUnit;
};

Specified my audio unit descriptions, created an audio processing graph, and added two nodes - one for my I/O node, and one for the multichannel mixer:

bool setupAU(void *handle, AURenderCallbackStruct inRenderProc, AudioStreamBasicDescription &outFormat)
{
AudioUnitHandle *auHandle = (AudioUnitHandle *)handle;

// I/O Unit
AudioComponentDescription desc;
desc.componentType = kAudioUnitType_Output;
desc.componentSubType = kAudioUnitSubType_RemoteIO;
desc.componentManufacturer = kAudioUnitManufacturer_Apple;
desc.componentFlags = 0;
desc.componentFlagsMask = 0;

// Multichannel mixer unit
AudioComponentDescription mixerDesc;
mixerDesc.componentType = kAudioUnitType_Mixer;
mixerDesc.componentSubType = kAudioUnitSubType_MultiChannelMixer;
mixerDesc.componentManufacturer = kAudioUnitManufacturer_Apple;
mixerDesc.componentFlags = 0;
mixerDesc.componentFlagsMask = 0;

//............................................................................
// Create a new audio processing graph.
if( NewAUGraph(&auHandle->processingGraph) )
    return false;

//............................................................................
// Add nodes to the audio processing graph.
AUNode ioNode;
if( AUGraphAddNode(auHandle->processingGraph, &desc, &ioNode) )
    return false;

AUNode mixerNode;
if( AUGraphAddNode(auHandle->processingGraph, &mixerDesc, &mixerNode) )
    return false;

//............................................................................
// Open the audio processing graph
// Following this call, the audio units are instantiated but not initialized
//    (no resource allocation occurs and the audio units are not in a state to
//    process audio).
if( AUGraphOpen(auHandle->processingGraph) )
    return false;

Then quite a bit of setup for the multichannel mixer itself:

// Obtain the mixer unit instance from its corresponding node.
if( AUGraphNodeInfo(auHandle->processingGraph, mixerNode, NULL, &auHandle->mixerUnit) )
    return false;

//............................................................................
// Multichannel Mixer unit Setup
UInt32 busCount = 1;    // bus count for mixer unit input
UInt16 busNumber = 0;
if( AudioUnitSetProperty(auHandle->mixerUnit, kAudioUnitProperty_ElementCount, kAudioUnitScope_Input, busNumber, &busCount, sizeof(busCount)) )
    return false;

// Increase the maximum frames per slice allows the mixer unit to accommodate the
//    larger slice size used when the screen is locked.
UInt32 maximumFramesPerSlice = 4096;
if( AudioUnitSetProperty(auHandle->mixerUnit, kAudioUnitProperty_MaximumFramesPerSlice, kAudioUnitScope_Global, busNumber, &maximumFramesPerSlice, sizeof(maximumFramesPerSlice)) )
    return false;

// Set a callback for the mixer node's specified input
if( AUGraphSetNodeInputCallback(auHandle->processingGraph, mixerNode, busNumber, &inRenderProc) )
    return false;

// Set the format of bus 0 of the mixer unit
AudioStreamBasicDescription localFormat = {0};
UInt32 size = sizeof(localFormat);

if (AudioUnitGetProperty(auHandle->mixerUnit, kAudioUnitProperty_StreamFormat, kAudioUnitScope_Input, busNumber, &localFormat, &size ))
    return false;

localFormat.mSampleRate = outFormat.mSampleRate;
localFormat.mChannelsPerFrame = outFormat.mChannelsPerFrame;
localFormat.mFormatID = outFormat.mFormatID;
localFormat.mFormatFlags = kAudioFormatFlagIsFloat | kAudioFormatFlagIsPacked | kAudioFormatFlagIsNonInterleaved ;
localFormat.mBitsPerChannel = outFormat.mBitsPerChannel;
localFormat.mFramesPerPacket = outFormat.mFramesPerPacket;
localFormat.mBytesPerFrame = outFormat.mBytesPerFrame;
localFormat.mBytesPerPacket = outFormat.mBytesPerPacket;

OSStatus err = 0;
err = AudioUnitSetProperty(auHandle->mixerUnit, kAudioUnitProperty_StreamFormat, kAudioUnitScope_Input, busNumber, &localFormat, size);
if(noErr != err)            //kAudioUnitErr_FormatNotSupported        = -10868,
    return false;

// Set the mixer unit's output sample rate format. This is the only aspect of the output stream
//    format that must be explicitly set.
if( AudioUnitSetProperty(auHandle->mixerUnit, kAudioUnitProperty_SampleRate, kAudioUnitScope_Output, 0, &localFormat.mSampleRate, sizeof(localFormat.mSampleRate)) )
    return false;

Finally, connected the nodes and initialized the processing graph:

// Connect the nodes of the audio processing graph
//  Connect the mixer output to the input of the I/O unit output element
if( AUGraphConnectNodeInput(auHandle->processingGraph,
                            mixerNode,                  //Source node
                            0,                          //Source node output bus number
                            ioNode,                     //Destination node
                            0) )                        //Destination node input bus number
    return false;

//............................................................................
// Initialize the audio processing graph, configure audio data stream formats for
//    each input and output, and validate the connections between audio units.
if (AUGraphInitialize(auHandle->processingGraph))
    return false;

return true;

}

Very simple matter from here to start and stop this whole mess ala:

//Start
OSStatus err = AUGraphStart( auHandle->processingGraph );

//Stop
OSStatus err = AUGraphStop( auHandle->processingGraph );

Whew!