An FM Synthesizer in Swift using AVAudioEngine

gistfile1.swift

//Swift 4
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


public class FMSynthesizer {
    
    // The audio engine manages the sound system.
    private let engine = AVAudioEngine()
    
    // The player node schedules the playback of the audio buffers.
    private let playerNode = AVAudioPlayerNode()
    
    // Use standard non-interleaved PCM audio.
    let audioFormat = AVAudioFormat(standardFormatWithSampleRate: 44100.0, channels: 2)!
    
    // A circular queue of audio buffers.
    private var audioBuffers: [AVAudioPCMBuffer] = []
    
    // The index of the next buffer to fill.
    private var bufferIndex = 0
    
    // The dispatch queue to render audio samples.
    private let audioQueue = DispatchQueue(label: "FMSynthesizerQueue", attributes: [])
    
    // A semaphore to gate the number of buffers processed.
    private let audioSemaphore = DispatchSemaphore(value: kInFlightAudioBuffers)
    
    public static let shared = FMSynthesizer()
    
    private init() {
        // Create a pool of audio buffers.
        for _ in 0 ..< kInFlightAudioBuffers {
            let audioBuffer = AVAudioPCMBuffer(pcmFormat: audioFormat, frameCapacity: kSamplesPerBuffer)!
            audioBuffers.append(audioBuffer)
        }
        
        // Attach and connect the player node.
        engine.attach(playerNode)
        engine.connect(playerNode, to: engine.mainMixerNode, format: audioFormat)
        
        do {
            try engine.start()
        } catch let error as NSError {
            print("Error starting audio engine: \(error)")
        }

        NotificationCenter.default.addObserver(self, selector: #selector(audioEngineConfigurationChange(notification:)), name: NSNotification.Name.AVAudioEngineConfigurationChange, object: engine)
        
    }
    
    public func play(carrierFrequency: Float32, modulatorFrequency: Float32, modulatorAmplitude: Float32) {
        let unitVelocity = Float32(2.0 * Double.pi / audioFormat.sampleRate)
        let carrierVelocity = carrierFrequency * unitVelocity
        let modulatorVelocity = modulatorFrequency * unitVelocity
        
        audioQueue.async {
            var sampleTime: Float32 = 0
            while true {
                // Wait for a buffer to become available.
                _ = self.audioSemaphore.wait(timeout: DispatchTime.distantFuture)
                
                // Fill the buffer with new samples.
                let audioBuffer = self.audioBuffers[self.bufferIndex]
                let floatChannelData = audioBuffer.floatChannelData!
                let leftChannel = floatChannelData[0]
                let rightChannel = floatChannelData[1]
                for sampleIndex in 0 ..< Int(kSamplesPerBuffer) {
                    let sample = sin(carrierVelocity * sampleTime + modulatorAmplitude * sin(modulatorVelocity * sampleTime))
                    leftChannel[sampleIndex] = sample
                    rightChannel[sampleIndex] = sample
                    sampleTime += 1
                }
                audioBuffer.frameLength = kSamplesPerBuffer
                
                // Schedule the buffer for playback and release it for reuse after
                // playback has finished.
                self.playerNode.scheduleBuffer(audioBuffer) {
                    self.audioSemaphore.signal()
                    return
                }
                
                self.bufferIndex = (self.bufferIndex + 1) % self.audioBuffers.count
            }
        }
        
        playerNode.pan = 0.8
        playerNode.play()
    }
    
    @objc func audioEngineConfigurationChange(notification: NSNotification) -> Void {
        NSLog("Audio engine configuration change: \(notification)")
    }
    
}

FMSynthesizer.shared.play(carrierFrequency: 440.0, modulatorFrequency: 679.0, modulatorAmplitude: 0.8)