AVFoundation / C5 note is not playing properly

Question

Created Oct ’20

Replies 1

Boosts 0

Views 1.1k

Participants 1

Hello,

I'm trying to play some waves I'm downloading from a seismometer and the sound is not good.

I decided to create a simple wave (C5 note, 523.25 Hz) and play it and it does not work too.

Here is my code:

Code Block swiftimport AVFoundation
import Combine
class ContentViewModel: ObservableObject {
    let audioEngine: AVAudioEngine
    let player: AVAudioPlayerNode
    
    let data: [Double]
    let sampleRate: Double
    init() {
        let sinFrequency: Double = 523.25  /* C5 */
        let sampleRate: Double = 44100
        let seconds: Double = 5
        
        let range = 0 ..< Int(seconds * sampleRate)
        self.data = range.map { sin(2.0 * .pi * Double($0) * sinFrequency / sampleRate) }
        self.sampleRate = sampleRate
        
        audioEngine = AVAudioEngine()
        let _ = audioEngine.mainMixerNode
        audioEngine.prepare()
        try! audioEngine.start()
        try! AVAudioSession.sharedInstance().setCategory(.playback)
        
        self.player = AVAudioPlayerNode()
        audioEngine.attach(player)
    }
    func copyBuffer<T: FixedWidthInteger>(data: [Double], buffer: AVAudioPCMBuffer, channelData: UnsafePointer<UnsafeMutablePointer<T>>) {
        buffer.frameLength = buffer.frameCapacity
        let buffData = data.map { T(Double(T.max) * $0) }
        memcpy(channelData[0], buffData, Int(buffer.frameCapacity) * MemoryLayout<T>.size)
    }
    enum BufferType {
        case int16
        case int32
    }
    
    func createBuffer(for type: BufferType) -> AVAudioPCMBuffer {
        switch type {
        case .int16:
            guard
                let inputFormat = AVAudioFormat(commonFormat: .pcmFormatInt16, sampleRate: sampleRate, channels: 1, interleaved: false),
                let buffer = AVAudioPCMBuffer(pcmFormat: inputFormat, frameCapacity: UInt32(data.count)),
                let channelData = buffer.int16ChannelData
            else {
                fatalError()
            }
            copyBuffer(data: data, buffer: buffer, channelData: channelData)
            return buffer
        case .int32:
            guard
                let inputFormat = AVAudioFormat(commonFormat: .pcmFormatInt16, sampleRate: sampleRate, channels: 1, interleaved: false),
                let buffer = AVAudioPCMBuffer(pcmFormat: inputFormat, frameCapacity: UInt32(data.count)),
                let channelData = buffer.int16ChannelData
            else {
                fatalError()
            }
            copyBuffer(data: data, buffer: buffer, channelData: channelData)
            return buffer
        }
    }
    
    func play(for type: BufferType) {
        let buffer = createBuffer(for: type)
        
        let linkFormat = AVAudioFormat(standardFormatWithSampleRate: sampleRate, channels: 1)
        audioEngine.connect(player, to: audioEngine.mainMixerNode, format: linkFormat)
        audioEngine.prepare()
        audioEngine.mainMixerNode.outputVolume = 0.5
        player.scheduleBuffer(buffer, at: nil, options: .interrupts, completionHandler: nil)
        if !player.isPlaying {
            player.play()
        }
    }
}

You can listen to the note looking for "Middle C Sine Wave for Ten Hours - 261.6 hertz" on YouTube (the title is wrong, this video is for C5).

Could you please tell me why my sound does not sound like the real C5 note?

Thanks!!!

You can create a simple ContentView in Swift with this code:

Code Block swiftimport SwiftUI
struct ContentView: View {
    @StateObject var viewModel = ContentViewModel()
    
    var body: some View {
        VStack {
            Spacer()
            HStack {
                Button("Play Int16") {
                    viewModel.play(for: .int16)
                }
                Button("Play Int32") {
                    viewModel.play(for: .int32)
                }
            }
            Spacer()
        }
    }
}

Answered by heltena in 638480022

I changed AVAudioPlayerNode by AVAudioSourceNode (check the BuildingASignalGenerator example).

My new code for your records:

Code Block swiftimport AVFoundation
import Combine
class ContentViewModel: ObservableObject {
    let audioEngine: AVAudioEngine
    var sourceNode: AVAudioSourceNode?
    let downMixer: AVAudioMixerNode
    let data: [Float]
    let sampleRate: Float
    
    init() {
        let sinFrequency: Float = 523.25  /* C5 */
        let sampleRate: Float = 44100
        let seconds: Float = 10
        
        let range = 0 ..< Int(seconds * sampleRate)
        self.data = range.map { sin(2.0 * .pi * Float($0) * sinFrequency / 44100) }
        self.sampleRate = sampleRate
        
        audioEngine = AVAudioEngine()
        downMixer = AVAudioMixerNode()
        
        let _ = audioEngine.mainMixerNode
        audioEngine.prepare()
        try! audioEngine.start()
        
        audioEngine.attach(downMixer)
        audioEngine.connect(downMixer, to: audioEngine.mainMixerNode, format: nil)
    }
   
    var isPlaying: Bool { sourceNode != nil }
    @Published private(set) var totalSeconds: Float = 0
    @Published private(set) var currentTime: Float = 0
    
    func stopCurrent() {
        if let sourceNode = sourceNode {
            audioEngine.detach(sourceNode)
            self.sourceNode = nil
        }
        self.totalSeconds = 0
        self.currentTime = 0
        self.objectWillChange.send()
    }
    
    func play(speedUpFactor: Float) {
        let playingSampleRate = sampleRate * speedUpFactor
        guard
            let dataFormat = AVAudioFormat(commonFormat: .pcmFormatFloat32, sampleRate: Double(playingSampleRate), channels: 1, interleaved: false)
        else {
            fatalError()
        }
        stopCurrent()
    
        self.totalSeconds = Float(data.count) / Float(playingSampleRate)
        self.currentTime = 0
        var index: Int = 0
        let sourceNode = AVAudioSourceNode(format: dataFormat) { [self] _, audioTimeStamp, frameCount, audioBufferList -> OSStatus in
            let ablPointer = UnsafeMutableAudioBufferListPointer(audioBufferList)
            
            let remainValues = min(Int(frameCount), data.count - index)
            for frame in 0 ..< remainValues {
                let value = index < data.count ? data[index] : 0.0
                index += 1
                
                for buffer in ablPointer {
                    let buf: UnsafeMutableBufferPointer<Float> = UnsafeMutableBufferPointer(buffer)
                    buf[frame] = value
                }
            }
            
            DispatchQueue.main.async {
                self.currentTime += Float(remainValues) / Float(playingSampleRate)
            }
            if remainValues < Int(frameCount) {
                for frame in remainValues ..< Int(frameCount) {
                    for buffer in ablPointer {
                        let buf: UnsafeMutableBufferPointer<Float> = UnsafeMutableBufferPointer(buffer)
                        buf[frame] = 0.0
                    }
                }
                DispatchQueue.main.async {
                    stopCurrent()
                }
            }
            return noErr
        }
        
        self.sourceNode = sourceNode
        audioEngine.attach(sourceNode)
        audioEngine.connect(sourceNode, to: downMixer, format: nil)
        self.objectWillChange.send()
    }
}

And the SwiftUI view:

Code Block swiftimport SwiftUI
struct ContentView: View {
    @StateObject var viewModel = ContentViewModel()
    
    var body: some View {
        VStack {
            Spacer()
            HStack {
                Button("Play x1") {
                    viewModel.play(speedUpFactor: 1)
                }
                Button("Play x2") {
                    viewModel.play(speedUpFactor: 2)
                }
                Button("Stop") {
                    viewModel.stopCurrent()
                }
            }
            if viewModel.isPlaying {
                Text("Playing: \(viewModel.currentTime) of \(viewModel.totalSeconds)")
            }
            Spacer()
        }
    }
}
struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView()
    }
}

Boost

Answer 1

heltena OP

Oct ’20

Accepted Answer

I changed AVAudioPlayerNode by AVAudioSourceNode (check the BuildingASignalGenerator example).

My new code for your records:

Code Block swiftimport AVFoundation
import Combine
class ContentViewModel: ObservableObject {
    let audioEngine: AVAudioEngine
    var sourceNode: AVAudioSourceNode?
    let downMixer: AVAudioMixerNode
    let data: [Float]
    let sampleRate: Float
    
    init() {
        let sinFrequency: Float = 523.25  /* C5 */
        let sampleRate: Float = 44100
        let seconds: Float = 10
        
        let range = 0 ..< Int(seconds * sampleRate)
        self.data = range.map { sin(2.0 * .pi * Float($0) * sinFrequency / 44100) }
        self.sampleRate = sampleRate
        
        audioEngine = AVAudioEngine()
        downMixer = AVAudioMixerNode()
        
        let _ = audioEngine.mainMixerNode
        audioEngine.prepare()
        try! audioEngine.start()
        
        audioEngine.attach(downMixer)
        audioEngine.connect(downMixer, to: audioEngine.mainMixerNode, format: nil)
    }
   
    var isPlaying: Bool { sourceNode != nil }
    @Published private(set) var totalSeconds: Float = 0
    @Published private(set) var currentTime: Float = 0
    
    func stopCurrent() {
        if let sourceNode = sourceNode {
            audioEngine.detach(sourceNode)
            self.sourceNode = nil
        }
        self.totalSeconds = 0
        self.currentTime = 0
        self.objectWillChange.send()
    }
    
    func play(speedUpFactor: Float) {
        let playingSampleRate = sampleRate * speedUpFactor
        guard
            let dataFormat = AVAudioFormat(commonFormat: .pcmFormatFloat32, sampleRate: Double(playingSampleRate), channels: 1, interleaved: false)
        else {
            fatalError()
        }
        stopCurrent()
    
        self.totalSeconds = Float(data.count) / Float(playingSampleRate)
        self.currentTime = 0
        var index: Int = 0
        let sourceNode = AVAudioSourceNode(format: dataFormat) { [self] _, audioTimeStamp, frameCount, audioBufferList -> OSStatus in
            let ablPointer = UnsafeMutableAudioBufferListPointer(audioBufferList)
            
            let remainValues = min(Int(frameCount), data.count - index)
            for frame in 0 ..< remainValues {
                let value = index < data.count ? data[index] : 0.0
                index += 1
                
                for buffer in ablPointer {
                    let buf: UnsafeMutableBufferPointer<Float> = UnsafeMutableBufferPointer(buffer)
                    buf[frame] = value
                }
            }
            
            DispatchQueue.main.async {
                self.currentTime += Float(remainValues) / Float(playingSampleRate)
            }
            if remainValues < Int(frameCount) {
                for frame in remainValues ..< Int(frameCount) {
                    for buffer in ablPointer {
                        let buf: UnsafeMutableBufferPointer<Float> = UnsafeMutableBufferPointer(buffer)
                        buf[frame] = 0.0
                    }
                }
                DispatchQueue.main.async {
                    stopCurrent()
                }
            }
            return noErr
        }
        
        self.sourceNode = sourceNode
        audioEngine.attach(sourceNode)
        audioEngine.connect(sourceNode, to: downMixer, format: nil)
        self.objectWillChange.send()
    }
}

And the SwiftUI view:

Code Block swiftimport SwiftUI
struct ContentView: View {
    @StateObject var viewModel = ContentViewModel()
    
    var body: some View {
        VStack {
            Spacer()
            HStack {
                Button("Play x1") {
                    viewModel.play(speedUpFactor: 1)
                }
                Button("Play x2") {
                    viewModel.play(speedUpFactor: 2)
                }
                Button("Stop") {
                    viewModel.stopCurrent()
                }
            }
            if viewModel.isPlaying {
                Text("Playing: \(viewModel.currentTime) of \(viewModel.totalSeconds)")
            }
            Spacer()
        }
    }
}
struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView()
    }
}

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