Distortion from output Audio Unit - core-audio

Question

I am hearing a very loud and harsh distortion sound when I run this simple application. I am simply instantiating a default output unit and assign a render callback. And letting the program run in the runloop. I have detected no errors from Core Audio and everything works as usual except for this distortion.
#import <AudioToolbox/AudioToolbox.h>
OSStatus render1(void *inRefCon,
AudioUnitRenderActionFlags *ioActionFlags,
const AudioTimeStamp *inTimeStamp,
UInt32 inBusNumber,
UInt32 inNumberFrames,
AudioBufferList * ioData)
{
return noErr;
}
int main(int argc, const char * argv[]) {
AudioUnit timerAU;
UInt32 propsize = 0;
AudioComponentDescription outputUnitDesc;
outputUnitDesc.componentType = kAudioUnitType_Output;
outputUnitDesc.componentSubType = kAudioUnitSubType_DefaultOutput;
outputUnitDesc.componentManufacturer = kAudioUnitManufacturer_Apple;
outputUnitDesc.componentFlags = 0;
outputUnitDesc.componentFlagsMask = 0;
//Get RemoteIO AU from Audio Unit Component Manager
AudioComponent outputComp = AudioComponentFindNext(NULL, &outputUnitDesc);
if (outputComp == NULL) exit (-1);
CheckError(AudioComponentInstanceNew(outputComp, &timerAU), "comp");
//Set up render callback function for the RemoteIO AU.
AURenderCallbackStruct renderCallbackStruct;
renderCallbackStruct.inputProc = render1;
renderCallbackStruct.inputProcRefCon = nil;//(__bridge void *)(self);
propsize = sizeof(renderCallbackStruct);
CheckError(AudioUnitSetProperty(timerAU,
kAudioUnitProperty_SetRenderCallback,
kAudioUnitScope_Global,
0,
&renderCallbackStruct,
propsize), "set render");
CheckError(AudioUnitInitialize(timerAU), "init");
// tickMethod = completion;
CheckError(AudioOutputUnitStart(timerAU), "start");
CFRunLoopRunInMode(kCFRunLoopDefaultMode, 1000, false);
}

Answer 1

Your question does not seem complete. I don't know about the side effects of silencing the output noise which is probably just undefined behavior. I also don't know what your code would serve for as such. There is an unfinished render callback on the kAudioUnitSubType_DefaultOutput which does nothing (it is not generating silence!). I know for two ways of silencing it.
In the callback the ioData buffers have to be explicitly filled with zeroes, because there's no guarantee they will be initialized empty:
Float32 * lBuffer0;
Float32 * lBuffer1;
lBuffer0 = (Float32 *)ioData->mBuffers[0].mData;
lBuffer1 = (Float32 *)ioData->mBuffers[1].mData;
memset(lBuffer0, 0, inNumberFrames*sizeof(Float32));
memset(lBuffer1, 0, inNumberFrames*sizeof(Float32));
Other possibility is to leave the unfinished callback as it is, but declare the timerAU to be of outputUnitDesc.componentSubType = kAudioUnitSubType_HALOutput; instead of
outputUnitDesc.componentSubType = kAudioUnitSubType_DefaultOutput;
and explicity disable I/O before setting the render callback by means of following code:
UInt32 lEnableIO = 0;
CheckError(AudioUnitSetProperty(timerAU,
kAudioOutputUnitProperty_EnableIO,
kAudioUnitScope_Output,
0, //output element
&lEnableIO,
sizeof(lEnableIO)),
"couldn't disable output");
I would strongly encourage into studying thoroughly the CoreAudio API and understanding how to set up an audio unit. This is crucial in understanding the matter. I've seen in your code a comment mentioning a RemoteIO AU. There is nothing like a RemoteIO AU in OSX. In case you're attempting a port from iOS code, please try learning the differences. They are well documented.