Q:

Does an Audio Unit with kAudioUnitType_Output and kAudioUnitSubType_RemoteIO use a low cut filter by default?

I'm developing an app that sets an audio unit to record and play and use a callback for audio processing. I was testing the record and play functionality without the callback so I measured the signal with a software called smaart I send as input a pink noise and analyze the output and I observe a drop down behind 200 Hz that seems like a filter. I don't know if I setting wrong something in my Audio Unit or this filter is applied by default. I tried using the same configuration but with a kAudioUnitSubType_VoiceProcessingIO and this sub category has various filters and a anti howling applied so it seems possible that the kAudioUnitSubType_RemoteIO applies filters by default too. 

A:

By default the remoteIO uses Automatic Gain Control and a high-pass filter. These can be disabled by setting the audio session mode to AVAudioSessionModeMeasurement.  It's quieter but much more accurate.
[[AVAudioSession sharedInstance] setMode:AVAudioSessionModeMeasurement error:&error];

