Buffer Size while iOS screen is off?

Hello,

I am having a similar problem. I want to configure a small buffer size (e.g. 256 for 0.005 ms) but iOS wouldn't let me. No matter what I do, it sets the outputNode buffer size to 4096. And I am not even locking the screen.

I am setting up the session like this:

  private let engine = AVAudioEngine()

In init() of my custom class:

        do {

            try AVAudioSession.sharedInstance().setCategory(AVAudioSessionCategoryPlayAndRecord)
            try AVAudioSession.sharedInstance().setPreferredIOBufferDuration(0.005)

            var result: OSStatus
            var framesPerSlice = UInt32(0)
            var size = UInt32(sizeof(UInt32))
            result = AudioUnitGetProperty(engine.outputNode.audioUnit, kAudioUnitProperty_MaximumFramesPerSlice, kAudioUnitScope_Global, 0, &framesPerSlice, &size);
            if result != noErr {
                 print("AudioUnitGetProperty for outputNode, result=\(result)")
            }
            print("outputNode bufferSize=\(framesPerSlice)")
        }
        catch {
            fatalError("Cannot set Audio Session category or buffer duration")
        }

This is the console output:

AVAudioSession buffer duration=0.0232199542224407 outputLatency=0.013650793582201
outputNode bufferSize=4096

Previously I tried to use AVAudioSessionCategoryPlayback but no change.

Any idea why the buffer size is so stubborn?

I need to play sounds using AVAudioUnitSampler so there is no need for recording. However, in the code above I picked AVAudioSessionCategoryPlayAndRecord because I have read that with output only, the system would set its buffer size to 1024. But I need to go as low as 256.