5.1 system and 3D sound with AVAudioEngine

Hi,

I'm starting to get confused about the proper configuration of AVAudio engine to use for 3D sound with surround system.

My set up is an iPad Air 2 running iOS 9.2 with 5.1 system connected through the USB. I tried to set-up a simple sound scene with one sound source changing it's position relative to the listener. Somehow it seems that AVAudioEngine treats the output as stereo system (i.e. independent on where I place the sound source in the scene, there is no sound output through the rear speakers).

Do I miss some step required to enable 5.1 output for AVAudioEngine?

The most relevant parts of code follow:

  let kOutputNumberOfChannels = "outputNumberOfChannels"
  func setupAudioSession() {
    let sess = AVAudioSession.sharedInstance()
    sess.addObserver(self, forKeyPath: kOutputNumberOfChannels,
                     options: NSKeyValueObservingOptions.New, context: nil)
    let nc = NSNotificationCenter.defaultCenter()
    nc.addObserver(self, selector: "audioRouteChanged:",
                   name: AVAudioSessionRouteChangeNotification,
                   object: nil)
    reconfigureSession()
    do {
      try sess.setActive(true)
      try sess.setCategory(AVAudioSessionCategoryPlayback)
    } catch {
      NSLog("error when setting up audio session \(error)")
      return
    }
    engine = AVAudioEngine()
    environment = AVAudioEnvironmentNode()
    environment.listenerPosition = AVAudioMake3DPoint(0, 0, 0)
    environment.listenerVectorOrientation =
      AVAudioMake3DVectorOrientation(AVAudioMake3DVector(0, 0, -1),
                                     AVAudioMake3DVector(0, 1, 0))
    engine.attachNode(environment)
    engine.connect(environment, to: engine.outputNode,
                   format: outputFormatForEnvironment())
    do {
      try engine.start()
    } catch {
      NSLog("Failed to start engine: \(error)")
      return
    }
    player = AVAudioPlayerNode()
    let path = NSBundle.mainBundle()
               .pathForResource("sound", ofType:"caf")!
    do {
      let file =
        try AVAudioFile(forReading: NSURL(fileURLWithPath:path))
      let audioBuffer = AVAudioPCMBuffer(PCMFormat:file.processingFormat,
                                         frameCapacity: AVAudioFrameCount(file.length))
      try file.readIntoBuffer(audioBuffer)
      player.renderingAlgorithm = .SoundField
      engine.attachNode(player)
      setSoundSourcePosition()
      engine.connect(player, to: environment,
                     format: audioBuffer.format)
      player.scheduleBuffer(audioBuffer, atTime: nil,
                            options: .Loops,
                            completionHandler: nil)
      player.play()
    } catch {
      NSLog("player initialization failed: \(error)")
    }
  }
  func outputFormatForEnvironment() -> AVAudioFormat {
    let nChannels = engine.outputNode.outputFormatForBus(0)
                    .channelCount
    let sampleRate = engine.outputNode.outputFormatForBus(0)
                     .sampleRate
    var layoutTag : AudioChannelLayoutTag
    if nChannels > 5 {
      layoutTag = kAudioChannelLayoutTag_MPEG_5_1_A
    } else if nChannels < 3 {
      return AVAudioFormat(standardFormatWithSampleRate: sampleRate,
                         channels: 2)
    } else {
      layoutTag = kAudioChannelLayoutTag_Stereo
    }
    let layout = AVAudioChannelLayout(layoutTag: layoutTag)
    return AVAudioFormat(standardFormatWithSampleRate: sampleRate,
                         channelLayout: layout)
  }

  func reconfigureSession() {
    let sess = AVAudioSession.sharedInstance()
    let reqChannels = sess.maximumOutputNumberOfChannels
    if sess.outputNumberOfChannels != reqChannels {
      NSLog("Requesting \(reqChannels) output channels")
      do {
        try sess.setPreferredOutputNumberOfChannels(reqChannels)
      } catch {
        NSLog("request failed: \(error)")
      }
    }
    lblNumberOfChannels.text = String(format:
      "Current number of channels: %d",
      sess.outputNumberOfChannels)
  }

  // Used to manipulate sound source position by the user.
  func setSoundSourcePosition() {
    let coords : [Float] = [ -5.0, 0.0, 5.0 ]
    let x = coords[xSegment.selectedSegmentIndex]
    let y = coords[ySegment.selectedSegmentIndex]
    let z = coords[zSegment.selectedSegmentIndex]
    NSLog("Setting sound source position to (\(x), \(y), \(z))")
    player.position = AVAudioMake3DPoint(x, y, z)
  }