AVAudioUnit (OS X) render block only called for certain sample rates - audiounit

Question

I'm having trouble getting AVAudioEngine (OS X) to play nice with all sample rates.
Here's my code for building the connections:
- (void)makeAudioConnections {
auto hardwareFormat = [self.audioEngine.outputNode outputFormatForBus:0];
auto format = [[AVAudioFormat alloc] initStandardFormatWithSampleRate:hardwareFormat.sampleRate channels:2];
NSLog(#"format: %#", format);
#try {
[self.audioEngine connect:self.avNode to:self.audioEngine.mainMixerNode format:format];
[self.audioEngine connect:self.audioEngine.inputNode to:self.avNode format:format];
} #catch(NSException* e) {
NSLog(#"exception: %#", e);
}
}
On my audio interface, the render callback is called for 44.1, 48, and 176.4kHz. It is not called for 96 and 192 kHz. On the built-in audio, the callback is called for 44.1, 48, 88 but not 96.
My AU's allocateRenderResourcesAndReturnError is being called for 96kHz. No errors are returned.
- (BOOL) allocateRenderResourcesAndReturnError:(NSError * _Nullable *)outError {
if(![super allocateRenderResourcesAndReturnError:outError]) {
return NO;
}
_inputBus.allocateRenderResources(self.maximumFramesToRender);
_sampleRate = _inputBus.bus.format.sampleRate;
return YES;
}
Here's my AU's init method, which is mostly just cut & paste from Apple's AUv3 demo:
- (instancetype)initWithComponentDescription:(AudioComponentDescription)componentDescription options:(AudioComponentInstantiationOptions)options error:(NSError **)outError {
self = [super initWithComponentDescription:componentDescription options:options error:outError];
if (self == nil) {
return nil;
}
// Initialize a default format for the busses.
AVAudioFormat *defaultFormat = [[AVAudioFormat alloc] initStandardFormatWithSampleRate:44100. channels:2];
// Create the input and output busses.
_inputBus.init(defaultFormat, 8);
_outputBus = [[AUAudioUnitBus alloc] initWithFormat:defaultFormat error:nil];
// Create the input and output bus arrays.
_inputBusArray = [[AUAudioUnitBusArray alloc] initWithAudioUnit:self busType:AUAudioUnitBusTypeInput busses: #[_inputBus.bus]];
_outputBusArray = [[AUAudioUnitBusArray alloc] initWithAudioUnit:self busType:AUAudioUnitBusTypeOutput busses: #[_outputBus]];
self.maximumFramesToRender = 256;
return self;
}
To keep things simple, I'm setting the sample rate before starting the app.
I'm not sure where to begin tracking this down.
Update
Here's a small project which reproduces the issue I'm having:
Xcode project to reproduce issue
You'll get errors pulling from the input at certain sample rates.
On my built-in audio running at 96kHz the render block is called with alternating 511 and 513 frame counts and errors -10863 (kAudioUnitErr_CannotDoInCurrentContext) and -10874 (kAudioUnitErr_TooManyFramesToProcess) respectively. Increasing maximumFramesToRender doesn't seem to help.
Update 2
I simplified my test down to just connecting the input to the main mixer:
[self.audioEngine connect:self.audioEngine.inputNode to:self.audioEngine.mainMixerNode format:nil];
I tried explicitly setting the format argument.
This still will not play through at 96kHz. So I'm thinking this may be a bug in AVAudioEngine.

Answer 1

For play-through with AVAudioEngine, the input and output hardware formats and all the connection formats must be at the same sample rate. So the following should work.
AVAudioFormat *outputHWFormat = [self.audioEngine.outputNode outputFormatForBus:0];
AVAudioFormat *inputHWFormat = [self.audioEngine.inputNode inputFormatForBus:0];
if (inputHWFormat.sampleRate == outputHWFormat.sampleRate) {
[self.audioEngine connect:self.audioEngine.inputNode to:self.audioEngine.mainMixerNode format:inputHWFormat];
[self.audioEngine connect:self.audioEngine.mainMixerNode to:self.audioEngine.outputNode format:inputHWFormat];
}