How can I specify the format of AVAudioEngine Mic-Input?

Question

I'd like to record the some audio using AVAudioEngine and the users Microphone. I already have a working sample, but just can't figure out how to specify the format of the output that I want...

My requirement would be that I need the AVAudioPCMBuffer as I speak which it currently does...

Would I need to add a seperate node that does some transcoding? I can't find much documentation/samples on that problem...

And I am also a noob when it comes to Audio-Stuff. I know that I want NSData containing PCM-16bit with a max sample-rate of 16000 (8000 would be better)

Here's my working sample:

private var audioEngine = AVAudioEngine()

func startRecording() {

  let format = audioEngine.inputNode!.inputFormatForBus(bus)

  audioEngine.inputNode!.installTapOnBus(bus, bufferSize: 1024, format: format) { (buffer: AVAudioPCMBuffer, time:AVAudioTime) -> Void in

     let audioFormat = PCMBuffer.format
     print("\(audioFormat)")
  }

  audioEngine.prepare()
  do {
     try audioEngine.start()
  } catch { /* Imagine some super awesome error handling here */ }
}

If I changed the format to let' say

let format = AVAudioFormat(commonFormat: AVAudioCommonFormat.PCMFormatInt16, sampleRate: 8000.0, channels: 1, interleaved: false)

then if will produce an error saying that the sample rate needs to be the same as the hwInput...

Any help is very much appreciated!!!

EDIT: I just found AVAudioConverter but I need to be compatible with iOS8 as well...

Answer 1

You cannot change audio format directly on input nor output nodes. In the case of the microphone, the format will always be 44KHz, 1 channel, 32bits. To do so, you need to insert a mixer in between. Then when you connect inputNode > changeformatMixer > mainEngineMixer, you can specify the details of the format you want.

Something like:

var inputNode = audioEngine.inputNode
var downMixer = AVAudioMixerNode()

//I think you the engine's I/O nodes are already attached to itself by default, so we attach only the downMixer here:
audioEngine.attachNode(downMixer)

//You can tap the downMixer to intercept the audio and do something with it:
downMixer.installTapOnBus(0, bufferSize: 2048, format: downMixer.outputFormatForBus(0), block:  //originally 1024
            { (buffer: AVAudioPCMBuffer!, time: AVAudioTime!) -> Void in
                print(NSString(string: "downMixer Tap"))
                do{
                    print("Downmixer Tap Format: "+self.downMixer.outputFormatForBus(0).description)//buffer.audioBufferList.debugDescription)

        })

//let's get the input audio format right as it is
let format = inputNode.inputFormatForBus(0)
//I initialize a 16KHz format I need:
let format16KHzMono = AVAudioFormat.init(commonFormat: AVAudioCommonFormat.PCMFormatInt16, sampleRate: 11050.0, channels: 1, interleaved: true)

//connect the nodes inside the engine:
//INPUT NODE --format-> downMixer --16Kformat--> mainMixer
//as you can see I m downsampling the default 44khz we get in the input to the 16Khz I want 
audioEngine.connect(inputNode, to: downMixer, format: format)//use default input format
audioEngine.connect(downMixer, to: audioEngine.outputNode, format: format16KHzMono)//use new audio format
//run the engine
audioEngine.prepare()
try! audioEngine.start()

I would recommend using an open framework such as EZAudio, instead, though.

Answer 2

The only thing I found that worked to change the sampling rate was

AVAudioSettings.sharedInstance().setPreferredSampleRate(...)

You can tap off engine.inputNode and use the input node's output format:

engine.inputNode.installTap(onBus: 0, bufferSize: 2048,
                            format: engine.inputNode.outputFormat(forBus: 0))

Unfortunately, there is no guarantee that you will get the sample rate that you want, although it seems like 8000, 12000, 16000, 22050, 44100 all worked.

The following did NOT work:

1. Setting the my custom format in a tap off engine.inputNode. (Exception)
2. Adding a mixer with my custom format and tapping that. (Exception)
3. Adding a mixer, connecting it with the inputNode's format, connecting the mixer to the main mixer with my custom format, then removing the input of the outputNode so as not to send the audio to the speaker and get instant feedback. (Worked, but got all zeros)
4. Not using my custom format at all in the AVAudioEngine, and using AVAudioConverter to convert from the hardware rate in my tap. (Length of the buffer was not set, no way to tell if results were correct)

This was with iOS 12.3.1.