Question

I have a AUGraph setup fairly simply with a multichannel mixer connected to an I/O unit. The playback is accessed through a callback function and everything works nicely.

I am trying to switch over to the 3D Mixer instead of the Multichannel mixer. So I switched the parameter from kAudioUnitSubType_MultiChannelMixer to kAudioUnitSubType_AU3DMixerEmbedded and left all the other setup the same.

The result was sort of a high pitched whine that seemed to start sounding like something then became just whine-ish. I have gone through each of the 3D Mixer unit’s parameters and set them to their defaults but there was no change. Flipping on and off the k3DMixerParam_Enable parameter did work at muting and unmuting the playback though.

What setup I might have missed? or know where to find an example of a working 3d Mixer?

Answer 1

As already pointed out the 3d mixer needs mono inputs. But you also have to use UInt16 as the input sample data type. This is a working AudioStreamBasicDescription:

AudioStreamBasicDescription streamFormat = {0};
size_t bytesPerSample = sizeof (UInt16);

streamFormat.mFormatID          = kAudioFormatLinearPCM;
streamFormat.mFormatFlags       = kAudioFormatFlagsCanonical;
streamFormat.mBytesPerPacket    = bytesPerSample;
streamFormat.mFramesPerPacket   = 1;
streamFormat.mBytesPerFrame     = bytesPerSample;
streamFormat.mChannelsPerFrame  = 1;
streamFormat.mBitsPerChannel    = 8 * bytesPerSample;
streamFormat.mSampleRate        = graphSampleRate;

// Set the input stream format of the desired 3D mixer unit audio bus
AudioUnitSetProperty (
                      mixerUnit,
                      kAudioUnitProperty_StreamFormat,
                      kAudioUnitScope_Input,
                      audioBus,
                      &streamFormat,
                      sizeof (streamFormat)
                      );