Use multiple voice processing IO audio unit in iOS

Question

I'm working on an VOIP project on iOS. As per suggested by Apple's documents, I use VoiceProcessingIO audio unit to get echo cancellation support.

As my app requires separated operations on render and capture sides (e.g., shutdown speaker but let microphone continue), so I create two audio units, one has capture port shutdown, while the other has render port shutdown.

The current code works well until I learn about how echo cancellation works: it requires comparing signals from both microphone and speaker. So my concern is: is it safe to use two voice processing audio unit like my approach? Also, as audio cancellation works mostly from capture side, is it possible to use an RemoteIO audio unit for rendering (connect to speaker)?

I'm not 100% confident as I just enter this area for a short time. I also tried from developer.apple.com, but all examples I found from developer.apple.com typically use only one audio unit.

Could anyone give some hints? Does my approach have any potential effects on features of VoiceProcessingIO unit?

Thanks, Fuzhou

robbie_c robbie_c · Accepted Answer · 2012-10-10T14:26:39

Firstly, VoiceProcessingIO is (as of me writing this) only echo suppression, not echo cancellation. It essentially just turns off the input if the output is too loud. This means that e.g. in a VOIP call, the far end won't be able to hear you while they are talking. Full echo cancellation would keep the input turned on, but attempt to subtract the echoes of the output.

I would suggest only using one unit, and handle the "shutdown speaker" case yourself programmatically. Your output callback might look like this:

OSStatus output_callback(
    void *inRefCon,
    AudioUnitRenderActionFlags  *ioActionFlags,
    const AudioTimeStamp        *inTimeStamp,
    UInt32                      inInputBusNumber,
    UInt32                      inNumberFrames,
    AudioBufferList             *ioData)
{
  my_context_t *context = inRefCon;
  audio_sample_t *dst = (audio_sample_t *)ioData->mBuffers[0].mData;

  if (context->muted) {
    memset(dst, 0, inNumberFrames * sizeof(audio_sample_t));
  } else {
    get_output_samples(context, dst, inNumberFrames);
  }
  return noErr;
}

Giving silence to echo suppression should not affect it adversely.

Use multiple voice processing IO audio unit in iOS

1 Answers