Clicks / Distortion in AudioKit - ios

Question

When I add a bunch (20-40) samples playing and overlapping eachother simultaneously sometimes it starts getting distorted and then some waving, oscillating, and clicking begins to happen. A similar sound happens when the samples are playing the the app crashes - sounds like an abrupt, crunchy halt.
Notice the waviness begins between 0:05 and 0:10; nasty clicks start around 0:15.
Listen Here
How can I make it smoother? I am spawning AKPlayer objects (from 4.1) that play 4-8 second .wav files. Those go into AKBoosters which go into AKMixers which go into the final AKMixer for output.
Edit:
Many PenAudioNodes get plugged into the mixer of the AudioReceiver singleton.
Here's my AudioReceiver singleton:
class AudioReceiver {
static var sharedInstance = AudioReceiver()
private var audioNodes = [UUID : AudioNode]()
private let mixer = AKMixer()
private let queue = DispatchQueue(label: "audio-queue")
//MARK: - Setup & Teardown
init() {
AudioKit.output = mixer //peakLimiter
AudioKit.start()
}
//MARK: - Public
func audioNodeBegan(_ message : AudioNodeMessage) {
queue.async {
var audioNode: AudioNode?
switch message.senderType {
case .pen:
audioNode = PenAudioNode()
case .home:
audioNode = LoopingAudioNode(with: AudioHelper.homeLoopFile())
default:
break
}
if let audioNode = audioNode {
self.audioNodes[message.senderId] = audioNode
self.mixer.connect(input: audioNode.output)
audioNode.start(message)
}
}
}
func audioNodeMoved(_ message : AudioNodeMessage) {
queue.async {
if let audioNode = self.audioNodes[message.senderId] {
audioNode.update(message)
}
}
}
func audioNodeEnded(_ message : AudioNodeMessage) {
queue.async {
if let audioNode = self.audioNodes[message.senderId] {
audioNode.stop(message)
}
self.audioNodes[message.senderId] = nil
}
}
}
Here's my PenAudioNode:
class PenAudioNode {
fileprivate var mixer: AKMixer?
fileprivate var playersBoosters = [AKPlayer : AKBooster]()
fileprivate var finalOutput: AKNode?
fileprivate let file: AKAudioFile = AudioHelper.randomBellSampleFile()
//MARK: - Setup & Teardown
init() {
mixer = AKMixer()
finalOutput = mixer!
}
}
extension PenAudioNode: AudioNode {
var output: AKNode {
return finalOutput!
}
func start(_ message: AudioNodeMessage) {
}
func update(_ message: AudioNodeMessage) {
if let velocity = message.velocity {
let newVolume = Swift.min((velocity / 50) + 0.1, 1)
mixer!.volume = newVolume
}
if let isClimactic = message.isClimactic, isClimactic {
let player = AKPlayer(audioFile: file)
player.completionHandler = { [weak self] in
self?.playerCompleted(player)
}
let booster = AKBooster(player)
playersBoosters[player] = booster
booster.rampTime = 1
booster.gain = 0
mixer!.connect(input: booster)
player.play()
booster.gain = 1
}
}
func stop(_ message: AudioNodeMessage) {
for (_, booster) in playersBoosters {
booster.gain = 0
}
DispatchQueue.global().asyncAfter(deadline: DispatchTime.now() + 1) {
self.mixer!.stop()
self.output.disconnectOutput()
}
}
private func playerCompleted(_ player: AKPlayer) {
playersBoosters.removeValue(forKey: player)
}
}

Answer 1

This sounds like you are not releasing objects and you are eventually overloading the audio engine with too many instances of processing nodes connected in the graph. In particular not releasing AKBoosters will cause an issue like this. I can't really tell what your code is doing, but if you are spawning objects without releasing them properly, it will lead to garbled audio.
You want to conserve objects as much as possible and make sure you are using the absolute minimum amount of AKNode based processing.
There are various ways to debug this, but you can start by printing out the current state of the AVAudioEngine:
AudioKit.engine.description
That will show how many nodes you have connected in the graph at any given moment.