Using multiple AudioUnits with MTAudioProcessingTap on iOS - ios

Question

I'm attempting to play audio files from the users' iPod library on an iOS device, while using AudioUnit to apply a parametric EQ effect. I have been using this sample as a guide: https://developer.apple.com/library/ios/samplecode/AudioTapProcessor/Introduction/Intro.html
I have the EQ effect working, but I need to add multiple EQ effects.
In my 'process' callback, I tried running AudioUnitRender multiple times, on multiple AudioUnit effects (all of type Parametric EQ).
status = AudioUnitRender(audioUnit, 0, &audioTimeStamp, 0, (UInt32)numberFrames, bufferListInOut);
With any more than 1 AudioUnitRender call, the audio skips and cuts out.
How can I use multiple Parametric EQ effects at once?
Thanks

Answer 1

I'm still not sure what the reason of the glitch is, but I've found the solution and it's probably what AUGraph does internally.
The trick is to call AudioUnitRender() of the next AU from within the render callback function of the previous AU. The last render callback calls MTAudioProcessingTapGetSourceAudio(). Assuming you have a single render callback for all your AU's and assuming you have an array with all the AU's you created:
UInt64 processedFrames;
UInt32 curAudioUnit;
UInt32 audioUnitCount;
AudioUnit audioUnits[MAX_AUDIO_UNITS];
OSStatus AU_RenderCallback(void *tap, AudioUnitRenderActionFlags *ioActionFlags, const AudioTimeStamp *inTimeStamp, UInt32 inBusNumber, UInt32 inNumberFrames, AudioBufferList *ioData)
{
curAudioUnit++;
if (curAudioUnit == audioUnitCount)
return MTAudioProcessingTapGetSourceAudio(tap, inNumberFrames, ioData, NULL, NULL, NULL);
AudioTimeStamp audioTimeStamp;
audioTimeStamp.mSampleTime = processedFrames;
audioTimeStamp.mFlags = kAudioTimeStampSampleTimeValid;
return AudioUnitRender(audioUnits[curAudioUnit], 0, &audioTimeStamp, 0, inNumberFrames, ioData);
}
void tap_ProcessCallback(MTAudioProcessingTapRef tap, CMItemCount inNumberFrames, MTAudioProcessingTapFlags flags, AudioBufferList *bufferListInOut, CMItemCount *numberFramesOut, MTAudioProcessingTapFlags *flagsOut)
{
if (audioUnitCount)
{
curAudioUnit = 0;
AudioTimeStamp audioTimeStamp;
audioTimeStamp.mSampleTime = processedFrames;
audioTimeStamp.mFlags = kAudioTimeStampSampleTimeValid;
AudioUnitRender(audioUnits[curAudioUnit], 0, &audioTimeStamp, 0, inNumberFrames, bufferListInOut);
}
else
MTAudioProcessingTapGetSourceAudio(tap, inNumberFrames, bufferListInOut, flagsOut, NULL, numberFramesOut);
processedFrames += inNumberFrames;
}