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An FM Synthesizer in Swift using AVAudioEngine
//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)
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