Why can't I change the number of elements / buses in the input scope of AU multi channel mixer?

Question

UPDATE: I'm changing my code to illustrate the issue in a more streamlined way. Also I had a little bug which, while not deterring from the problem, did add some confusion.

I'm instantiating a Multi Channel Mixer AU in iOS (kAudioUnitSubType_MultiChannelMixer) and I do the following:

OSStatus status = noErr;

//  Set component type:
AudioComponentDescription cd = {0};
cd.componentType = kAudioUnitType_Mixer;
cd.componentSubType = kAudioUnitSubType_MultiChannelMixer;
cd.componentManufacturer = kAudioUnitManufacturer_Apple;

//  Alloc unit:
AudioComponent defaultOutput = AudioComponentFindNext(NULL, &cd);
AudioUnit mixer;
status = AudioComponentInstanceNew(defaultOutput, &mixer);
NSLog(@"AU init status: %li", status);


//  You can try initializing the unit before or after the attempt to set its input bus count.
//  It doesn't matter; it won't work either way.
AudioUnitInitialize(mixer);

//  Get number of inputs
UInt32 busSize = sizeof(UInt32);
UInt32 busCount = 0;
status = AudioUnitGetProperty(mixer,
                              kAudioUnitProperty_ElementCount,
                              kAudioUnitScope_Input,
                              0,
                              &busCount,
                              &busSize);
NSLog(@"AU get input bus count | status: %li | count: %lu", status, busCount);


//  Try setting number of inputs
busCount = 3;
status = AudioUnitSetProperty(mixer,
                              kAudioUnitProperty_ElementCount,
                              kAudioUnitScope_Input,
                              0,
                              &busCount,
                              busSize);

NSLog(@"AU set input status: %li", status);

//  Get number of inputs
busCount = 0;
status = AudioUnitGetProperty(mixer,
                              kAudioUnitProperty_ElementCount,
                              kAudioUnitScope_Input,
                              0,
                              &busCount,
                              &busSize);

NSLog(@"AU get input bus count | status: %li | count: %lu", status, busCount);


//  Try setting number of inputs
busCount = 10;
status = AudioUnitSetProperty(mixer,
                              kAudioUnitProperty_ElementCount,
                              kAudioUnitScope_Input,
                              0,
                              &busCount,
                              busSize);

NSLog(@"AU set input status: %li", status);

//  Get number of inputs
busCount = 0;
status = AudioUnitGetProperty(mixer,
                              kAudioUnitProperty_ElementCount,
                              kAudioUnitScope_Input,
                              0,
                              &busCount,
                              &busSize);

NSLog(@"AU get input bus count | status: %li | count: %lu", status, busCount);

And I get the following output:

2013-10-11 19:54:32.248 AUMultiChannelRadar[3996:a0b] AU init status: 0
2013-10-11 19:54:32.249 AUMultiChannelRadar[3996:a0b] AU get input bus count | status: 0 | count: 8
2013-10-11 19:54:32.250 AUMultiChannelRadar[3996:a0b] AU set input status: 0
2013-10-11 19:54:32.250 AUMultiChannelRadar[3996:a0b] AU get input bus count | status: 0 | count: 8
2013-10-11 19:54:32.250 AUMultiChannelRadar[3996:a0b] AU set input status: 0
2013-10-11 19:54:32.251 AUMultiChannelRadar[3996:a0b] AU get input bus count | status: 0 | count: 10

I'd expect the bus count to not be 8 by default (since the Audio Unit documentation doesn't say there is a default value on instantiation) and I'd expect to be able to change the number of input elements to both less and more than 8 for that matter (since the docs say it can have "Any" number of inputs). However, from the output, attempting to set the input count to less than 8 does nothing; I can only set it to more than 8.

Answer 1

I talked to an Apple engineer who is part of the CoreAudio team and showed him my code and he agreed it is most certainly a bug. He even tried it out himself and got the same results as me: the unit seems to be instantiated with 8 inputs by default and you can only set the number to more than 8 but not less (yes, my code originally showed 2 inputs cause I was messing up and dividing 8/4 -- thanks).

He told me to file a radar so I did. # 15214291.

Answer 2

I think there is no need to divide the number of elements on UInt32 size.

UInt32 numElements = 0;
UInt32 size = sizeof(numElements);
AudioUnitGetProperty(_audioUnit, kAudioUnitProperty_ElementCount, kAudioUnitScope_Input, 0, &numElements, &size);

So, numElements will contain number of elements.