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An FM Synthesizer in Swift using AVAudioEngine
import AVFoundation
import Foundation
// The maximum number of audio buffers in flight. Setting to two allows one
// buffer to be played while the next is being written.
private let kInFlightAudioBuffers: Int = 2
// The number of audio samples per buffer. A lower value reduces latency for
// changes but requires more processing but increases the risk of being unable
// to fill the buffers in time. A setting of 1024 represents about 23ms of
// samples.
private let kSamplesPerBuffer: AVAudioFrameCount = 1024
// The single FM synthesizer instance.
private let gFMSynthesizer: FMSynthesizer = FMSynthesizer()
public class FMSynthesizer {
// The audio engine manages the sound system.
private let engine: AVAudioEngine = AVAudioEngine()
// The player node schedules the playback of the audio buffers.
private let playerNode: AVAudioPlayerNode = AVAudioPlayerNode()
// Use standard non-interleaved PCM audio.
let audioFormat = AVAudioFormat(standardFormatWithSampleRate: 44100.0, channels: 2)
// A circular queue of audio buffers.
private let audioBuffers: [AVAudioPCMBuffer] = [AVAudioPCMBuffer]()
// The index of the next buffer to fill.
private var bufferIndex: Int = 0
// The dispatch queue to render audio samples.
private let audioQueue: dispatch_queue_t = dispatch_queue_create("FMSynthesizerQueue", DISPATCH_QUEUE_SERIAL)
// A semaphore to gate the number of buffers processed.
private let audioSemaphore: dispatch_semaphore_t = dispatch_semaphore_create(kInFlightAudioBuffers)
public class func sharedSynth() -> FMSynthesizer {
return gFMSynthesizer
private init() {
// Create a pool of audio buffers.
for var i = 0; i < kInFlightAudioBuffers; i++ {
let audioBuffer = AVAudioPCMBuffer(PCMFormat: audioFormat, frameCapacity: kSamplesPerBuffer)
// Attach and connect the player node.
engine.connect(playerNode, to: engine.mainMixerNode, format: audioFormat)
var error: NSError? = nil
if !engine.startAndReturnError(&error) {
NSLog("Error starting audio engine: \(error)")
NSNotificationCenter.defaultCenter().addObserver(self, selector: "audioEngineConfigurationChange:", name: AVAudioEngineConfigurationChangeNotification, object: engine)
public func play(carrierFrequency: Float32, modulatorFrequency: Float32, modulatorAmplitude: Float32) {
let unitVelocity = Float32(2.0 * M_PI / audioFormat.sampleRate)
let carrierVelocity = carrierFrequency * unitVelocity
let modulatorVelocity = modulatorFrequency * unitVelocity
dispatch_async(audioQueue) {
var sampleTime: Float32 = 0
while true {
// Wait for a buffer to become available.
dispatch_semaphore_wait(self.audioSemaphore, DISPATCH_TIME_FOREVER)
// Fill the buffer with new samples.
let audioBuffer = self.audioBuffers[self.bufferIndex]
let leftChannel = audioBuffer.floatChannelData[0]
let rightChannel = audioBuffer.floatChannelData[1]
for var sampleIndex = 0; sampleIndex < Int(kSamplesPerBuffer); sampleIndex++ {
let sample = sin(carrierVelocity * sampleTime + modulatorAmplitude * sin(modulatorVelocity * sampleTime))
leftChannel[sampleIndex] = sample
rightChannel[sampleIndex] = sample
audioBuffer.frameLength = kSamplesPerBuffer
// Schedule the buffer for playback and release it for reuse after
// playback has finished.
self.playerNode.scheduleBuffer(audioBuffer) {
self.bufferIndex = (self.bufferIndex + 1) % self.audioBuffers.count
playerNode.pan = 0.8
@objc private func audioEngineConfigurationChange(notification: NSNotification) -> Void {
NSLog("Audio engine configuration change: \(notification)")
// Play a bell sound:
FMSynthesizer.sharedSynth().play(440.0, modulatorFrequency: 679.0, modulatorAmplitude: 0.8)

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michaeldorner commented Apr 6, 2016

Thanks for sharing your code. I updated your snippet to the current Swift version. Feel free to copy back. :-)

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