Disable input/output AGC from RemoteIO and VPIO on iOS

Question

CoreAudio is always a mystery due to lack of documentations. Recently I hit some stone again:

In my program, I invoke RemoteIO and VoiceProcessingIO (VPIO) back and forth, and also change AVAudiosession in between. I tried to turn off AGC on VPIO with the follwing code:

if (ASBD.componentSubType == kAudioUnitSubType_VoiceProcessingIO) {
    UInt32 turnOff = 0;
    status = AudioUnitSetProperty(_myAudioUnit,
                                  kAUVoiceIOProperty_VoiceProcessingEnableAGC,
                                  kAudioUnitScope_Global,
                                  0,
                                  &turnOff,
                                  sizeof(turnOff));
    NSAssert1(status == noErr, @"Error setting AGC status: %d", (int)status);
}

Well I'm still not sure if this code disables AGC on the microphone side or the speaker side on VPIO, but anyways, let's continue. Here's the sequence to reproduce the problem:

1. Create a RemoteIO output audio unit with PlayAndRecord audio session category, work with it and destroy the unit;

2. Switch audio session to Playback only category;

3. Switch audio session to PlayAndRecord again and create another VPIO, work with it and destroy it;

4. Switch audio session to Playback and then PlayAndRecord category;

After these steps, then whatever RemoteIO/VPIO created later will bear this amplified microphone signal (as if a huge AGC is always applied) and there's no way to go back until manually kill the app and start over.

Maybe it's my particular sequence that triggered this, wonder if anyone seen this before and maybe know a correct workaround?

Answer 1

Try setting the mode AVAudioSessionModeMeasurement, or AVAudioSession.Mode .measurement, when configuring your app's Audio Session.