Play audio buffers generated by AVSpeechSynthesizer directly

Question

We have a requirement for audio processing on the output of AVSpeechSynthesizer. So we started with using the write method of AVSpeechSynthesizer class to apply processing on top. of it. What we currently have:

var synthesizer = AVSpeechSynthesizer()
var playerNode: AVAudioPlayerNode = AVAudioPlayerNode()
fun play(audioCue: String){
   let utterance = AVSpeechUtterance(string: audioCue)
   synthesizer.write(utterance, toBufferCallback: {[weak self] buffer in
      // We do our processing including conversion from pcmFormatFloat16 format to pcmFormatFloat32 format which is supported by AVAudioPlayerNode
      self.playerNode.scheduleBuffer(buffer as! AVAudioPCMBuffer, completionCallbackType: .dataPlayedBack)
   }
}

All of it was working fine before iOS 16 but with iOS 16 we started getting this exception:

[AXTTSCommon] TTSPlaybackEnqueueFullAudioQueueBuffer: error -66686 enqueueing buffer

Not sure what this exception means exactly. So we are looking for a way of addressing this exception or may be a better way of playing the buffers.

UPDATE:

Created an empty project for testing and it turns out the write method if called with an empty bloc generates these logs:

Answer 1

Code I have used for Swift project :

let synth = AVSpeechSynthesizer()    
let myUtterance = AVSpeechUtterance(string: message)
myUtterance.rate = 0.4
synth.speak(myUtterance)

Can move let synth = AVSpeechSynthesizer() out of this method and declare on top for this class and use.

Settings to enable for Xcode14 & iOS 16 : If you are using XCode14 and iOS16, it may be voices under spoken content is not downloaded and you will get an error on console saying identifier, source, content nil. All you need to do is, go to accessiblity in settings -> Spoken content -> Voices -> Select any language and download any profile. After this run ur voice and you will be able to hear the speech from passed text.

It is working for me now.