Question

I have a AUGraph to Play and Record Audio with this Setup:

RemoteIO      ->                                 -> RemoteIO
                  Mulichannel Mixer -> 3D Mixer 
Read Callback ->                                 -> Write Callback

My Problem is that i have crackle in the recorded Sound. Without the 3D Mixer
the sound is good. But i need him to adjust the Playbackrate…

Audio Format:

    memset(&_audioFormat, 0, sizeof(_audioFormat));
size_t bytesPerSample = sizeof (UInt16);
_audioFormat.mFormatID          = kAudioFormatLinearPCM;
_audioFormat.mFormatFlags       = kAudioFormatFlagsCanonical;
_audioFormat.mBytesPerPacket    = bytesPerSample;
_audioFormat.mFramesPerPacket   = 1;
_audioFormat.mBytesPerFrame     = bytesPerSample;
_audioFormat.mChannelsPerFrame  = 1;
_audioFormat.mBitsPerChannel    = 8 * bytesPerSample;
_audioFormat.mSampleRate        = 22100.0f;

I don’t know why the 3D Mixer adds the crackle -.-
Here is a small sample Audio: Click

Hope thats enough Information to find the solution.
Thanks 🙂

Some additional informations:

With Crackle

    AudioUnitGraph 0x7F8000:
  Member Nodes:
    node 1: 'auou' 'rioc' 'appl', instance 0x1cd66d40 O I
    node 2: 'aumx' 'mcmx' 'appl', instance 0x1e055b40 O I
    node 3: 'aumx' '3dem' 'appl', instance 0x1e05e7a0 O I
  Connections:
    node   1 bus   1 => node   2 bus   1  [ 1 ch,  22000 Hz, 'lpcm' (0x0000000C) 16-bit little-endian signed integer]
    node   2 bus   0 => node   3 bus   0  [ 1 ch,  22000 Hz, 'lpcm' (0x0000000C) 16-bit little-endian signed integer]
    node   3 bus   0 => node   1 bus   0  [ 2 ch,  44100 Hz, 'lpcm' (0x00000C2C) 8.24-bit little-endian signed integer, deinterleaved]
  Input Callbacks:
    {0x12d9c1, 0x1cd65f30} => node   2 bus   0  [1 ch, 22000 Hz]
  CurrentState:
    mLastUpdateError=0, eventsToProcess=F, isRunning=F

Without Crackle

    AudioUnitGraph 0x7E7000:
  Member Nodes:
    node 1: 'auou' 'rioc' 'appl', instance 0x1e888810 O I
    node 2: 'aumx' 'mcmx' 'appl', instance 0x1e88a0e0 O I
    node 3: 'aumx' '3dem' 'appl', instance 0x1e88b200 O  
  Connections:
    node   1 bus   1 => node   2 bus   1  [ 1 ch,  22000 Hz, 'lpcm' (0x0000000C) 16-bit little-endian signed integer]
    node   2 bus   0 => node   1 bus   0  [ 1 ch,  22000 Hz, 'lpcm' (0x0000000C) 16-bit little-endian signed integer]
  Input Callbacks:
    {0x1469b9, 0x1d5bffa0} => node   2 bus   0  [1 ch, 22000 Hz]
  CurrentState:
    mLastUpdateError=0, eventsToProcess=F, isRunning=F

Update:

I think the Problem only occures after a AudioGraph restart.

1.) CheckError(AUGraphStart(self.graph), "Error starting graph. - error code");
2.) Recording <- NO CRACKLE!
3.) CheckError(AUGraphStop(self.graph), "Error stopping graph. - error code");
4.) CheckError(AUGraphStart(self.graph), "Error starting graph. - error code");
5.) Recording <- CRACKLE!

Answer 1

I found a Workaround but not the solution.
Just replace the 3DMixer with the kAudioUnitSubType_Varispeed AU.

speedDescription.componentFlags = 0;
speedDescription.componentFlagsMask = 0;
speedDescription.componentType = kAudioUnitType_FormatConverter;
speedDescription.componentSubType = kAudioUnitSubType_Varispeed;
speedDescription.componentManufacturer = kAudioUnitManufacturer_Apple;

I hope this helps someone!