Re: newbie asking what's OK to do on realtime audio thread

Subject: Re: newbie asking what's OK to do on realtime audio thread
From: Mark Wise <email@hidden>
Date: Tue, 27 Jan 2015 11:29:03 -0600

Michael Tyson, author of TheAmazingAudioEngine, recommended this article to me: http://www.rossbencina.com/code/real-time-audio-programming-101-time-waits-for-nothing

It doesn't address your question directly, but it's a good baseline for explaining the types of things that should be avoided.

I'm not an expert, but I would suspect that the AudioUnitGetProperty() is safe to call, since it is a C function and satisfies the "your audio thread code should be only C - never Obj-C" dictum. That said, it's possible that I'm completely wrong, so take that with a grain of salt!

Mark Wise
646.241.7126

On Tue, Jan 27, 2015 at 10:07 AM, Leo Thiessen <email@hidden> wrote:

Hi folks,

I’m new to coreaudio programming and audio programming in general. Having read through some online materials, I’m trying to wrap my head around what is safe to do on in a realtime audio thread render callback and what’s not. As far as I can determine I should really only be calling functions that have guarantees about the way they behave: fast, no memory allocations/freeing, consistent cpu demand vs high spikes of demand, etc.

This is maybe really obvious, but help me out: how do I know/find out what’s safe to do? For example, can I call AudioUnitGetProperty() in a render thread? I’m suspecting the answer might be it a “that depends…” type; but any pointers to help me locate the answers would be much appreciated. Included here is an example of what I’m doing in a render thread I’m working on. Is this sane/OK to do?

I’m using theamazingaudioengine.com, running a Mac OS X 10.9+ and a iOS 7+ target executable.

static OSStatus _renderCallback2(__unsafe_unretained VEAETrack *THIS,
__unsafe_unretained AEAudioController *audioController,
const AudioTimeStamp *time,
UInt32 frameCount,
AudioBufferList *audio) {

// Do the main audio processing
THIS->_superclassRenderCallback(THIS, audioController, time, frameCount, audio); // the superclass uses an AUAudioFilePlayer to render the audio, then applies a gain and pan filter using Apple’s vdsp functions

// Get our current time data
if(noErr==AudioUnitGetProperty(THIS->_au,
kAudioUnitProperty_CurrentPlayTime,
kAudioUnitScope_Global,
0,
&THIS->_audioTimeStamp,
&THIS->_audioTimeStampSize)) {
UInt32 currLoopCount = floor(THIS->_audioTimeStamp.mSampleTime / THIS->_mFramesToPlay);
THIS->_currentTime = (THIS->_audioTimeStamp.mSampleTime - ((float)currLoopCount * THIS->_mFramesToPlay)) / THIS->_outSampleRate;

// Check for callbacks to be done
if(THIS->_completionBlock) {
if(THIS->_isLooping) {
// If we are on a new loop number, trigger completion callback
if(currLoopCount > THIS->_numLoopsCompleted) {
THIS->_numLoopsCompleted++;
AEAudioControllerSendAsynchronousMessageToMainThread(audioController, _notifyCompletion, &THIS, sizeof(VEAETrack*)); // does not lock/block the realtime thread
}
} else {
// If we're in the last renderCallback of a non-looping channel, trigger the completion callback
UInt32 remainderPlusFramesThisRender = ((UInt32)THIS->_audioTimeStamp.mSampleTime % THIS->_mFramesToPlay) + frameCount;
if(remainderPlusFramesThisRender >= THIS->_mFramesToPlay) {
AEAudioControllerSendAsynchronousMessageToMainThread(audioController, _notifyCompletion, &THIS, sizeof(VEAETrack*));
}
}
}
}

return noErr;
}

The part I’m wondering about is my call to AudioUnitGetProperty(…) - how would I know or find out if that’s OK to do realtime like above? How about other coreaudio functions such as calling MusicDeviceMIDIEvent() on an Apple AUSampler instrument audio unit, from the realtime audio thread? Or must I run my own, separate thread parallel to the realtime audio thread to do these types of things?

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Follow-Ups:
- Re: newbie asking what's OK to do on realtime audio thread
  - From: Gregory Wieber <email@hidden>

References:
	>newbie asking what's OK to do on realtime audio thread (From: Leo Thiessen <email@hidden>)

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