Swift: iPhone's volume is low when trying to change speech to iPhone's voice in swift

Question

I am trying Speech recognition sample. If I started to recognise my speech via microphone, then I tried to get iPhone's voice of that recognised text. It is working. But, voice is too low. Can u guide me on this?

Rather than, if I am trying in simple button action, with AVSpeechUtterance code, volume is normal.

After that, If I go for startRecognise() method, volume is too low.

My Code

func startRecognise()
{
let audioSession = AVAudioSession.sharedInstance()  //2
    do
    {
        try audioSession.setCategory(AVAudioSessionCategoryPlayAndRecord)
        try audioSession.setMode(AVAudioSessionModeDefault)
        try audioSession.setMode(AVAudioSessionModeMeasurement)
        try audioSession.setActive(true, with: .notifyOthersOnDeactivation)
        try AVAudioSession.sharedInstance().overrideOutputAudioPort(AVAudioSessionPortOverride.speaker)
    }
    catch
    {
        print("audioSession properties weren't set because of an error.")
    }
    recognitionRequest = SFSpeechAudioBufferRecognitionRequest()
    guard let inputNode = audioEngine.inputNode else {
        fatalError("Audio engine has no input node")
    }
    guard let recognitionRequest = recognitionRequest else {
        fatalError("Unable to create an SFSpeechAudioBufferRecognitionRequest object")
    }
    recognitionRequest.shouldReportPartialResults = true
    recognitionTask = speechRecognizer.recognitionTask(with: recognitionRequest, resultHandler: { (result, error) in
        if result != nil
        {
            let lastword = result?.bestTranscription.formattedString.components(separatedBy: " ").last
            if lastword == "repeat" || lastword == "Repeat"{
                self.myUtterance2 = AVSpeechUtterance(string: "You have spoken repeat")
                self.myUtterance2.rate = 0.4
                self.myUtterance2.volume = 1.0
                self.myUtterance2.pitchMultiplier = 1.0
                self.synth1.speak(self.myUtterance2)
                // HERE VOICE IS TOO LOW. 
            }
        }
    })
    let recordingFormat = inputNode.outputFormat(forBus: 0)  //11
    inputNode.installTap(onBus: 0, bufferSize: 1024, format: recordingFormat) { (buffer, when) in
    self.recognitionRequest?.append(buffer)
    }
    audioEngine.prepare()
    do 
    {
        try audioEngine.start()
    } 
    catch 
    {
        print("audioEngine couldn't start because of an error.")
    }
}

My Button Action

func buttonAction()
{
   self.myUtterance2 = AVSpeechUtterance(string: "You are in button action")
   self.myUtterance2.rate = 0.4
   self.myUtterance2.volume = 1.0
   self.myUtterance2.pitchMultiplier = 1.0
   self.synth1.speak(self.myUtterance2)
   // Before going for startRecognise() method, 
   //I tried with buttonAction(), 
   //this time volume is normal. 
   //After startRecognise() method call, volume is too low in both methods.
}

Answer 1

After digging into the technical details its observed that, overrideOutputAudioPort() temporarily changes the current audio route.

func overrideOutputAudioPort(_ portOverride: AVAudioSession.PortOverride) throws

If your app uses the playAndRecord category, calling this method with the AVAudioSession.PortOverride.speaker option causes audio to be routed to the built-in speaker and microphone regardless of other settings.

This change remains in effect only until the current route changes or you call this method again with the AVAudioSession.PortOverride.none option.

try audioSession.setMode(AVAudioSessionModeDefault)

If you would prefer to permanently enable this behavior, you should instead set the category's defaultToSpeaker option. Setting this option will always route to the speaker rather than receiver if no other accessory such as headphones are in use.

In Swift 5.x above code looks like -

let audioSession = AVAudioSession.sharedInstance()
do {
  try audioSession.setCategory(.playAndRecord)
  try audioSession.setMode(.default)
  try audioSession.setActive(true, options: .notifyOthersOnDeactivation)
  try audioSession.overrideOutputAudioPort(.speaker)
} catch {
  debugPrint("Enable to start audio engine")
  return
}

By setting mode to the measurement, its responsible for minimize the amount of system-supplied signal processing to input and output signals.

try audioSession.setMode(.measurement)

By commenting this mode and using default mode responsible for permanently enabling the audio route to the built-in speaker and microphone.

Thanks @McDonal_11 for you answer. Hope this will helps to understand the technical details.

Answer 2

Finally, I got Solution.

func startRecognise()
{
let audioSession = AVAudioSession.sharedInstance()  //2
    do
    {
        try audioSession.setCategory(AVAudioSessionCategoryPlayAndRecord)
        try audioSession.setMode(AVAudioSessionModeDefault)
        //try audioSession.setMode(AVAudioSessionModeMeasurement)
        try audioSession.setActive(true, with: .notifyOthersOnDeactivation)
        try AVAudioSession.sharedInstance().overrideOutputAudioPort(AVAudioSessionPortOverride.speaker)
    }
    catch
    {
        print("audioSession properties weren't set because of an error.")
    }

    ... 
}

Once I comment this line, try audioSession.setMode(AVAudioSessionModeMeasurement), volume is working normal.