Programmatically turn off VoiceProcessingIO AGC

Question

I'm using Apple's CoreAudio framework for recording my microphone feed. Automatic Gain Control seems to be enabled by default: https://developer.apple.com/library/mac/#documentation/AudioUnit/Reference/AudioUnitPropertiesReference/Reference/reference.html

kAUVoiceIOProperty_VoiceProcessingEnableAGC
Indicates whether automatic gain control is enabled (any nonzero value) or disabled (a value of 0). Automatic gain control is enabled by default.
Value is a read/write UInt32 valid on the global audio unit scope.
Available in OS X v10.7 and later.
Declared in AudioUnitProperties.h.

How do I programmatically turn off the AGC in CoreAudio?

Answer 1

Assuming you're using an AUVoiceProcessor Audio Unit called voiceProcessor

UInt32 turnOff = 0;
AudioUnitSetProperty(voiceProcessor,
                     kAUVoiceIOProperty_VoiceProcessingEnableAGC,
                     kAudioUnitScope_Global,
                     0,
                     &turnOff,
                     sizeof(turnOff));

Quick explanation: What this is doing is setting a property on the audio unit to 0, which in this case disables the AGC. Audio Units typically have two sets of controllable values called properties and parameters. You can set/get these values by using AudioUnitSetProperty() / AudioUnitGetProperty() and AudioUnitSetParameter() / AudioUnitGetParameter() accordingly.

Note: You should probably check the OSStatus code that AudioUnitSetProperty() returns (it'll be equal to noErr if there wasn't an error).