CoreAudio get output sample rate - core-audio

Question

I'm creating a Mac OS X CoreAudio command-line program with proprietary rendering of some alphanumeric terminal input into a live audio signal, by means of AudioUnits, trying to stay as simple as possible. All works fine up to matching output sample rate.
As a starting point I'm using the Chapter 07 tutorial code of Addisson Wesley's "Learning Core Audio", CH07_AUGraphSineWave.
I initialize the AudioComponent "by the book":
void CreateAndConnectOutputUnit (MyGenerator *generator)
{
AudioComponentDescription theoutput = {0};
theoutput.componentType = kAudioUnitType_Output;
theoutput.componentSubType = kAudioUnitSubType_DefaultOutput;
theoutput.componentManufacturer = kAudioUnitManufacturer_Apple;
AudioComponent comp = AudioComponentFindNext (NULL, &theoutput);
if (comp == NULL) {
printf ("can't get output unit");
exit (-1);
}
CheckError (AudioComponentInstanceNew(comp, &generator->outputUnit),
"Couldn't open component for outputUnit");
AURenderCallbackStruct input;
input.inputProc = MyRenderProc;
input.inputProcRefCon = generator;
CheckError(AudioUnitSetProperty(generator->outputUnit,
kAudioUnitProperty_SetRenderCallback,
kAudioUnitScope_Input,
0,
&input,
sizeof(input)),
"AudioUnitSetProperty failed");
CheckError (AudioUnitInitialize(generator->outputUnit),
"Couldn't initialize output unit");
}
My main problem is in my not knowing how to retreive the output hardware sample rate for the rendering AURenderCallbackStruct
since it does play a vital part in the signal generating process. I can't afford having the sample rate hard-coded into the rendering callback, although knowing it's the easiest way, since rate mismatch causes the signal being played at wrong pitch.
Is there a way of getting the default output's sample rate on such a low-level API?
Is there a way of matching it somehow, without getting overly complicated?
Have I missed something?
Thanks in advance.
Regards,
Tom

Answer 1

When calling AudioUnitGetProperty, the 6th parameter must be a pointer to a variable that will get the size of the answer.
Float64 sampleRate;
UInt32 sampleRateSize;
CheckError(AudioUnitGetProperty(generator->outputUnit,
kAudioUnitProperty_SampleRate,
kAudioUnitScope_Input,
0,
&sampleRate,
&sampleRateSize),
"AudioUnitGetProperty failed");
However, as long as the sample rate has not been set, the function does not return a value (but there is also no error!)
You can however set the sample rate with for instance:
Float64 sampleRate = 48000;
CheckError(AudioUnitSetProperty(generator->outputUnit,
kAudioUnitProperty_SampleRate,
kAudioUnitScope_Input,
0,
&sampleRate,
sizeof(sampleRate)),
"AudioUnitGetProperty failed");
From now on you can also read the value with the Get-call.
This does not answer the question, what the default value is. As far as I know that is always 44100 Hz.

Answer 2

The sample-rate is a property of all AudioUnits - see kAudioUnitProperty_SampleRate (documentation here) - although ultimately it's the IO Unit (RemoteIO on iOS or HAL unit on MacOSX) that drives the sample-rate at the audio interface. This is not available in the call-back structure; you need to read this property with AudioUnitGetProperty() in your initialisation code.
In your case, the following would probably do it:
Float64 sampleRate;
CheckError(AudioUnitGetProperty(generator->outputUnit,
kAudioUnitProperty_SampleRate,
kAudioUnitScope_Input,
0,
&sampleRate,
sizeof(sampleRate)),
If you're targeting iOS, you also need to interact with the Audio Session.