How to set speaker configuration programatically using Core Audio API on Mac OS X?

Question

I have a 7.1 channel audio output device and a custom kext to drive that. My custom application needs to send 7.1 rear channel audio data to the device but the device receives only 2 channel audio data. I checked "Configure Speaker" option in "Audio MIDI setup" application and it's set to stereo. When I set it to "7.1 Rear Surround" everything works fine. In my final product, I don't want the user to have to do all of this manually. So, the question is - Is there any Core Audio API or any other means of doing this programatically?

Answer 1

Ok, after playing around with some Core Audio APIs, finally I could get this done.

1. Get the AudioDeviceID:

   AudioDeviceID audioDevice = getMyAwesomeDeviceID();
   

2. Create an AudioObjectPropertyAddress:

   AudioObjectPropertyAddress propertyAddress;
   propertyAddress.mSelector = kAudioDevicePropertyPreferredChannelLayout;
   propertyAddress.mScope = kAudioDevicePropertyScopeOutput;
   propertyAddress.mElement = kAudioObjectPropertyElementMaster;
   

3. Query if the audio object has this property:

   AudioObjectHasProperty(audioDevice, &propertyAddress)
   

4. Get the data size of this property and create AudioChannelLayout:

   UInt32 propSize(0);
   AudioObjectGetPropertyDataSize(audioDevice, &propertyAddress, 0, NULL, &propSize);
   AudioChannelLayout* layout = (AudioChannelLayout*)malloc(propSize);
   

5. Configure your AudioChannelLayout structure (eg: stereo layout):

   AudioChannelLabel labels[2] = {kAudioChannelLabel_Right, kAudioChannelLabel_Left};
   
   layout->mNumberChannelDescriptions = 2;
   for (UInt32 i = 2; i < layout->mNumberChannelDescriptions; i++) {
       layout->mChannelDescriptions[i].mChannelLabel = labels[i];
       layout->mChannelDescriptions[i].mChannelFlags = kAudioChannelFlags_AllOff;
   }
   

6. Set the AudioObject property data:

   AudioObjectSetPropertyData(audioDevice, &propertyAddress, 0, NULL, propSize, layout);