Filtering audio in AudioKit

Question

What i need to do:

• record audio file;
• as it's record from iPhone/iPad microphone it can be quiet, so i need to filter it to make it louder;
• save filtered record;

I'm new in audio programming, but as I understand so far I need "All Pass" filter (if not please correct me).

For this task I've found two libs: Novocaine and AudioKit, but Novocaine written in C, so it's harder to implement it in swift, and I decided to use AudioKit, but I didn't found "All Pass" filter there.

Does anybody know how to implement it in AudioKit and save filtered file? Thank you!

Answer 1

You have a few choices, for musical recordings I recommend AKBooster as it purely boosts the audio, you have to be careful how much you boost, otherwise you might cause clipping.

For spoken word audio I recommend AKPeakLimiter. It will give you the maximum volume without clipping. Set the attackTime and decayTime to lower values to hear a more pronounced effect.

The values of the sliders won't represent the values of the parameters until you move them.

import UIKit
import AudioKit

class ViewController: UIViewController {

    let mic = AKMicrophone()
    let boost = AKBooster()
    let limiter = AKPeakLimiter()

    override func viewDidLoad() {
        super.viewDidLoad()

        mic >>> boost >>> limiter
        AudioKit.output = limiter
        AudioKit.start()

        let inset: CGFloat = 10.0
        let width = view.bounds.width - inset * 2


        for i in 0..<4 {
            let y = CGFloat(100 + i * 50)
            let slider = UISlider(frame: CGRect(x: inset, y: y, width: width, height: 30))
            slider.tag = i
            slider.addTarget(self, action: #selector(sliderAction(slider:)), for: .valueChanged)
            view.addSubview(slider)
        }

        boost.gain = 1

    }

    @objc func sliderAction(slider: UISlider) {
        switch slider.tag {
        case 0:
            boost.gain = slider.value * 40
        case 1:
            limiter.preGain = slider.value * 40
        case 2:
            limiter.attackTime = max(0.001, slider.value * 0.03)
        case 4:
            limiter.decayTime = max(0.001, slider.value * 0.06)
        default: break

        }
    }

}