Does iOS preprocess microphone data on modern iPhones?

[p-i]

Disable AGC and highpass filter on iOS

iOS has automatic gain control and a high pass filter on the mic input. You may want to disable this if doing an audio app. Adding this to the iOSAudioIODevice will disable it.  setSessionUInt32Property(kAudioSessionProperty_Mode, kAudioSessionMode_Measurement); And this will re-enable it. setSessionUInt32Property(kAudioSessionProperty_Mode, kAudioSessionMode_Default); It would be good if the AudioIODevice had a way to platform agnosticlly turn on or off any input processing.

^ this looks like the culprit.

However it’s very strange that it works on an iPhone 5S but not on an iPhone 6. All of the devices are using the latest iOS. And if I look at detailed comparisons between the two models I find no mention of anything related to microphone preprocessing.