AudioKit 下采样音频

Question

我有一些现有代码使用AVAudioEngine 从麦克风获取输入，对其进行下采样并将其写入AVAudioFile

internal func setupNodeChain() {
    guard let audioEngine = audioEngine else { return } // Fatal error ?
    
    let engineInputNode = audioEngine.inputNode
    
    let bus = 0
    let engineInputNodeFormat = engineInputNode.outputFormat(forBus: bus)
    
    // This attempts to down sample the audio from the microphone
    let downSampleMixerNode = AVAudioMixerNode()
    let mixerOutputFormat = AVAudioFormat(standardFormatWithSampleRate: 8000, channels: 1)
    
    // Input -> (volume) -> down sample -> (volume) -> Output
    
    let inputVolumeMixerNode = AVAudioMixerNode()
    inputVolumeMixerNode.volume = Float(10 * microphoneVolume)
    
    audioEngine.attach(inputVolumeMixerNode)
    audioEngine.attach(downSampleMixerNode)
    
    self.downSampleMixerNode = downSampleMixerNode
    self.inputVolumeMixerNode = inputVolumeMixerNode
    
    let silenceNode = AVAudioMixerNode()
    silenceNode.outputVolume = 0
    
    self.silenceNode = silenceNode
    
    audioEngine.connect(engineInputNode, to: inputVolumeMixerNode, format: engineInputNodeFormat)
    audioEngine.connect(inputVolumeMixerNode, to: downSampleMixerNode, format: engineInputNodeFormat)
    
    // Try and stop the microphone audio from going through to the speaker
    audioEngine.attach(silenceNode)
    audioEngine.connect(downSampleMixerNode, to: silenceNode, format: mixerOutputFormat)
    audioEngine.connect(silenceNode, to: audioEngine.outputNode, format: mixerOutputFormat)

    downSampleMixerNode.installTap(onBus: bus, bufferSize: 1024 * 16, format: mixerOutputFormat) { (buffer: AVAudioPCMBuffer, time: AVAudioTime) in
        guard let tap = self.audioTap else { return }
        // Write buffer to AVAudioFile          
        tap.drip(buffer: buffer, time: time)
    }
}

这主要是可行的，但我正在研究用AudioKit 替换它，但我遇到了问题，我不知道如何创建一种机制来对从麦克风到录音机的音频进行下采样。

    AKSettings.enableEchoCancellation = true
    AKSettings.allowAirPlay = true
    AKSettings.useBluetooth = true
    
    do {
        try AKSettings.setSession(category: .playAndRecord,
                                                            with: [
                                                                .allowBluetoothA2DP,
        ])
        
        AKSettings.defaultToSpeaker = true
        
        let audioFile = try self.makeAudioFile(named: "Recording")
        
        let mixerOutputFormat = AVAudioFormat(standardFormatWithSampleRate: 8000, channels: 1)!

        let microphone = AKMicrophone()
        let microphoneBooster = AKBooster(microphone)
        microphoneBooster.gain = 0
        
        let recorder = try AKNodeRecorder(node: microphoneBooster)
        //recorder.recordFormat = mixerOutputFormat
        
        let silence = AKMixer(microphoneBooster)
        silence.volume = 0
        
        self.microphone = microphone
        self.microphoneBooster = microphoneBooster
        self.recorder = recorder
        self.silence = silence
        
        AKManager.output = silence
        
        log(debug: "Start")
        try AKManager.start()
        
        log(debug: "Record")
        try recorder.record()

        DispatchQueue.main.async {
            self.state = .recording
            self.plot?.node = microphone
            self.callButton.setImage(#imageLiteral(resourceName: "EndCall"), for: [])
        }
    } catch let error {
        log(error: "Failed to establish play and record session: \(error)")
    }

所以，问题是 - 我将如何创建“下采样”节点/工作流程，它将麦克风链接到具有“默认”格式的“节点”，然后将“节点”链接到下一个需要AVAudioFormat的链中的节点？

麦克风 -> 向下采样（默认格式）

下采样->下一个节点（目标格式）->记录器

Answer 1

基本上，我必须创建自己的“tap”来挖掘数据

首先，我有一个“转换器”。这基本上需要来自另一个混音器的音频（通过“点击”）将其转换为目标格式并将其写入音频文件

class TapConverter: NodeTapperDelegate {
    
    let audioConfig: AudioConfig
    
    internal var inputFormat: AVAudioFormat?
    internal var converter: AVAudioConverter?
    
    var onError: ((Error) -> Void)?
    
    init(audioConfig: AudioConfig) {
        self.audioConfig = audioConfig
    }
    
    func open(format: AVAudioFormat) throws {
        inputFormat = format
        converter = AVAudioConverter(from: format, to: audioConfig.audioFormat)
    }
    
    func drip(buffer: AVAudioPCMBuffer, time: AVAudioTime) {
        guard let converter = converter else {
            return
        }
        guard let inputFormat = inputFormat else {
            return
        }
        
        let inputBufferSize = inputFormat.sampleRate
        let sampleRateRatio = inputBufferSize / audioConfig.audioFormat.sampleRate
        let capacity = Int(Double(buffer.frameCapacity) / sampleRateRatio)
        
        let bufferPCM16 = AVAudioPCMBuffer(pcmFormat: audioConfig.audioFormat, frameCapacity: AVAudioFrameCount(capacity))!
        var error: NSError? = nil

        converter.convert(to: bufferPCM16, error: &error) { inNumPackets, outStatus in
            outStatus.pointee = AVAudioConverterInputStatus.haveData
            return buffer
        }
        if let error = error {
            // Handle error in someway
        } else {
            let audioFile = audioConfig.audioFile
            do {
                log(debug: "Write buffer")
                try audioFile.write(from: bufferPCM16)
            } catch let error {
                log(error: "Failed to write buffer to audio file: \(error)")
                onError?(error)
            }
        }
    }
    
    func close() {
        converter = nil
        inputFormat = nil
        // ? we close the audio file
    }
}

AudioConfig 只是一个基本占位符，它包含已写入（必须已创建）的audioFile 和目标AVAudioFormat

struct AudioConfig {
    let url: URL
    let audioFile: AVAudioFile
    let audioFormat: AVAudioFormat
}

创作可能看起来像...

let settings: [String: Any] = [
    AVFormatIDKey: NSNumber(value: kAudioFormatMPEG4AAC),
    AVSampleRateKey: NSNumber(value: 8000),
    AVNumberOfChannelsKey: NSNumber(value: 1),
    AVEncoderBitRatePerChannelKey: NSNumber(value: 16),
    AVEncoderAudioQualityKey: NSNumber(value: AVAudioQuality.min.rawValue)
]
let audioFile = try AVAudioFile(forWriting: sourceURL, settings: settings)

let audioConfig = AudioConfig(url: sourceURL, audioFile: audioFile, audioFormat: audioFormat)

从那里，我需要一种方法来点击节点（获取它的数据）并将其传递给我的转换器，为此，我使用了类似...

import Foundation
import AudioKit

protocol NodeTapperDelegate: class {
    func open(format: AVAudioFormat) throws
    func drip(buffer: AVAudioPCMBuffer, time: AVAudioTime)
    func close()
}

class NodeTapper: NSObject {
    // MARK: - Properties
    
    // The node we record from
    private(set) var node: AKNode?
    
    /// True if we are recording.
    @objc private(set) dynamic var isTapping = false
    
    /// The bus to install the recording tap on. Default is 0.
    private var bus: Int = 0
    
    /// Used for fixing recordings being truncated
    private var recordBufferDuration: Double = 16_384 / AKSettings.sampleRate
    
    weak var delegate: NodeTapperDelegate?
    
    // MARK: - Initialization
    
    /// Initialize the node recorder
    ///
    /// Recording buffer size is defaulted to be AKSettings.bufferLength
    /// You can set a different value by setting an AKSettings.recordingBufferLength
    ///
    /// - Parameters:
    ///   - node: Node to record from
    ///   - bus: Integer index of the bus to use
    ///
    @objc init(node: AKNode? = AKManager.output,
                         bus: Int = 0) throws {
        self.bus = bus
        self.node = node
    }
    
    // MARK: - Methods
    
    /// Start recording
    @objc func start() throws {
        if isTapping == true {
            return
        }
        
        guard let node = node else {
            return
        }
        
        guard let delegate = delegate else {
            return
        }
        
        let bufferLength: AVAudioFrameCount = AKSettings.recordingBufferLength.samplesCount
        isTapping = true
        
        // Note: if you install a tap on a bus that already has a tap it will crash your application.
        let nodeFormat = node.avAudioNode.outputFormat(forBus: 0)
        try delegate.open(format: nodeFormat)

        // note, format should be nil as per the documentation for installTap:
        // "If non-nil, attempts to apply this as the format of the specified output bus. This should
        // only be done when attaching to an output bus which is not connected to another node"
        // In most cases AudioKit nodes will be attached to something else.
        node.avAudioUnitOrNode.installTap(onBus: bus,
                                                                            bufferSize: bufferLength,
                                                                            format: nil, // Might need to the input node's format :/
                                                                            block: process(buffer:time:))
    }
    
    private func process(buffer: AVAudioPCMBuffer, time: AVAudioTime) {
        guard let sink = delegate else { return }
        sink.drip(buffer: buffer, time: time)
    }
    
    /// Stop recording
    @objc func stop() {
        if isTapping == false {
            return
        }
        
        isTapping = false
        
        if AKSettings.fixTruncatedRecordings {
            //  delay before stopping so the recording is not truncated.
            let delay = UInt32(recordBufferDuration * 1_000_000)
            usleep(delay)
        }
        node?.avAudioUnitOrNode.removeTap(onBus: bus)
        delegate?.close()
    }
}

然后，不知何故，完全绑定它

let microphone = AKMicrophone()
microphone?.volume = 10 * volume

let monoToStereo = AKStereoFieldLimiter(microphone, amount: 1)
let microphoneMixer = AKMixer(monoToStereo)

// This is where we're converting the audio from
// the microphone and dripping it into the audio file
let converter = TapConverter(audioConfig: audioConfig)
// handleError is basically just a func in this case
converter.onError = handleError
// Here we tap the mixer/node and output to the converter
let tapper = try NodeTapper(node: microphoneMixer)
tapper.delegate = converter

// Silence the output from the microphone, so it's not
// fed back into the microphone
let silence = AKMixer(microphoneMixer)
silence.volume = 0

self.microphoneMixer = microphoneMixer
self.converter = converter
self.tapper = tapper
self.microphone = microphone
self.silence = silence

AKManager.output = silence

log(debug: "Start")
try AKManager.start()

log(debug: "Record")
try tapper.start()

其中大部分来自网络上不同帖子的不同想法，那么这是最佳选择吗？我不知道，但它做了我需要它做的事情