AVCaptureSession Audio preview - ios

Question

I apologise in advance for the "dumb" question, but I feel I have exhausted all resources. I have little to no experience with Swift and coding in general but I understand much based on past experience and use of object based programming such as MAX MSP.
I am attempting to develop a camera/microphone capture iOS app for the macOS QuickTime Player recording function (answering my own need for RAW camera access as I literally could not find the right thing out there!).
Having implemented AVCaptureSession video output successfully, I have tried many methods of sending audio to Quicktime (including AVAudioSessionPortUSBAudio) to no avail. This was before I realised that QuickTime automatically captures the iOS system audio output.
So my presumption was that I should be able to preview audio under AVCapture Session easily; not so! It seems AVCaptureAudioPreviewOutput in "not available" in swift4 or I am simple missing some basics. I have seen articles on stack mentions the need to STOP audio processing, so I'm hopeful it is easy to preview/monitor it.
Could any of you point me to a method of previewing audio in AVCaptureSession? I have an instantiated AVAudioSession still (my original attempt), and have also just managed (I hope) to successfully connect the mic to the AVCaptureSession. However, I am not sure what else to use! My aim: just to hear the Mic input on the system's audio output: the Quicktime connection should (hopefully) handle capturing from the USB port (music played on the phone goes over the usb when the iOS device is selected as the microphone).
let audioDevice = AVCaptureDevice.default(for: AVMediaType.audio)
do {
let audioInput = try AVCaptureDeviceInput(device: audioDevice!)
self.captureSession.addInput(audioInput)
} catch {
print("Unable to add Audio Device")
}
I have also attempted other things which I am becoming lost on;
captureSession.automaticallyConfiguresApplicationAudioSession = true
func showAudioPreview() -> Bool { return true }
Perhaps it is possible to use AVAudioSession alongside the capture? However, my basic knowledge points to the fact that there are problems running Capture and Audio Sessions together.
Any help would be sincerely appreciated, I am sure many of you will roll your eyes and be able to easily point out my mistakes!
Thanks,
Iwan

Answer 1

AVCaptureAudioPreviewOutput is only available on the mac, but you could instead use AVSampleBufferAudioRenderer. You have to manually enqueue audio CMSampleBuffers to it which an AVCaptureAudioDataOutput can provide:
import UIKit
import AVFoundation
class ViewController: UIViewController, AVCaptureAudioDataOutputSampleBufferDelegate {
let session = AVCaptureSession()
let bufferRenderSyncer = AVSampleBufferRenderSynchronizer()
let bufferRenderer = AVSampleBufferAudioRenderer()
override func viewDidLoad() {
super.viewDidLoad()
bufferRenderSyncer.addRenderer(bufferRenderer)
let audioDevice = AVCaptureDevice.default(for: .audio)!
let captureInput = try! AVCaptureDeviceInput(device: audioDevice)
let audioOutput = AVCaptureAudioDataOutput()
audioOutput.setSampleBufferDelegate(self, queue: DispatchQueue.main) // or some other dispatch queue
session.addInput(captureInput)
session.addOutput(audioOutput)
session.startRunning()
}
func captureOutput(_ output: AVCaptureOutput, didOutput sampleBuffer: CMSampleBuffer, from connection: AVCaptureConnection) {
bufferRenderer.enqueue(sampleBuffer)
if bufferRenderSyncer.rate == 0 {
bufferRenderSyncer.setRate(1, time: sampleBuffer.presentationTimeStamp)
}
}
}