swift - How to apply audio effect to a file and write to filesystem - iOS

Question

I'm building an app that should allow user to apply audio filters to a recorded audio, such as Reverb, Boost.

I was unable to find any viable source of information on how to apply filters to a file itself, because it's needed to upload processed file to server later.

I'm currently using AudioKit for visualization, and I'm aware that it's capable of doing audio processing, but only for playback. Please give any suggestions for further research.

Answer 1

AudioKit has an offline render node that doesn't require iOS 11. Here's an example, the player.schedule(...) and player.start(at.) bits are required as AKAudioPlayer's underlying AVAudioPlayerNode will block on the calling thread waiting for the next render if you start it with player.play().

import UIKit
import AudioKit

class ViewController: UIViewController {

    var player: AKAudioPlayer?
    var reverb = AKReverb()
    var boost = AKBooster()
    var offlineRender = AKOfflineRenderNode()

    override func viewDidLoad() {
        super.viewDidLoad()

        guard let url = Bundle.main.url(forResource: "theFunkiestFunkingFunk", withExtension: "mp3") else {
            return
        }
        var audioFile: AKAudioFile?
        do {
            audioFile = try AKAudioFile.init(forReading: url)
            player = try AKAudioPlayer.init(file: audioFile!)
        } catch {
            print(error)
            return
        }
        guard let player = player else {
            return
        }


        player >>> reverb >>> boost >>> offlineRender

        AudioKit.output = offlineRender
        AudioKit.start()


        let docs = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask).first!
        let dstURL = docs.appendingPathComponent("rendered.caf")

        offlineRender.internalRenderEnabled = false
        player.schedule(from: 0, to: player.duration, avTime: nil)
        let sampleTimeZero = AVAudioTime(sampleTime: 0, atRate: AudioKit.format.sampleRate)
        player.play(at: sampleTimeZero)
        do {
            try offlineRender.renderToURL(dstURL, seconds: player.duration)
        } catch {
            print(error)
            return
        }
        offlineRender.internalRenderEnabled = true

        print("Done! Rendered to " + dstURL.path)
    }
}