How can I get AVAudioEngine to output PCM-16 from the Mic?

Question

I'm using AVAudioEngine and I'm trying to get it to output .pcmFormatInt16 at 16000Hz but I can't seem to get it to work. Here's what I'm doing:

let audioEngine = AVAudioEngine()
let mixer = AVAudioMixerNode()
let input = self.audioEngine.inputNode!

audioEngine.attach(mixer)
audioEngine.connect(input, to: mixer, format: input.outputFormat(forBus: 0))

let recordingFormat = AVAudioFormat(commonFormat: .pcmFormatInt16, sampleRate: 16000.0, channels: 1, interleaved: true)

mixer.installTap(onBus: 0, bufferSize: 2048, format: recordingFormat) { [weak self] (buffer, _) in
    // buffer here is all 0's!
}

self.audioEngine.prepare()
try! self.audioEngine.start()

As noted above, when I access the buffer it is always all 0, silence.

Answer 1

AVAudioEngine don't supports changing sample rate. You can use AVAudioConverter to change sample rate like that

let inputFormat = input.outputFormat(forBus: 0)
let recordingFormat = AVAudioFormat(commonFormat: .pcmFormatInt16, sampleRate: 16000.0, channels: 1, interleaved: true)
converter = AVAudioConverter(from: inputFormat, to: recordingFormat)

mixer.installTap(onBus: 0, bufferSize: 2048, format: inputFormat) { [weak self] (buffer, _) in
    let convertedBuffer = self?.converter.convertBuffer(additionalBuffer: buffer)
}