使用AvaudioengineiOS的低通濾波器 采樣率轉換

我們正在開展一個專案，該專案允許我們使用一些低通濾波器和高通濾波器以 5k Hz 采樣率錄制來自麥克風的一些聲音。

我們正在使用什么

為此，我們正在使用AvaudioEngine。

我們正在使用AVAudioConverter來降低采樣率。

我們將AVAudioUnitEQ用于低通和高通濾波器。

代碼

let bus = 0
let inputNode = engine.inputNode

let equalizer = AVAudioUnitEQ(numberOfBands: 2)

equalizer.bands[0].filterType = .lowPass
equalizer.bands[0].frequency = 3000
equalizer.bands[0].bypass = false

equalizer.bands[1].filterType = .highPass
equalizer.bands[1].frequency = 1000
equalizer.bands[1].bypass = false
engine.attach(equalizer) //Attach equalizer

// Connect nodes
engine.connect(inputNode, to: equalizer, format: inputNode.inputFormat(forBus: 0))
engine.connect(equalizer, to: engine.mainMixerNode, format: inputNode.inputFormat(forBus: 0))
engine.connect(engine.mainMixerNode, to: engine.outputNode, format: inputNode.inputFormat(forBus: 0))

let outputFormat = AVAudioFormat(commonFormat: .pcmFormatInt16,
                                            sampleRate: 5000,
                                            channels: 1,
                                            interleaved: false)!

// Converter to downgrade sample rate
guard let converter: AVAudioConverter = AVAudioConverter(from: inputNode.inputFormat(forBus: 0), to: outputFormat) else {
           print("Can't convert in to this format")
           return
       }

engine.mainMixerNode.installTap(onBus: bus, bufferSize: 2688, format: engine.mainMixerNode.outputFormat(forBus: 0)) { (buffer, time) in
           
     var newBufferAvailable = true
           
     let inputCallback: AVAudioConverterInputBlock = { inNumPackets, outStatus in
           if newBufferAvailable {
                outStatus.pointee = .haveData
                newBufferAvailable = false
                return buffer
           } else {
                outStatus.pointee = .noDataNow
                return nil
           }
     }
           
           
     let convertedBuffer = AVAudioPCMBuffer(pcmFormat: outputFormat, frameCapacity: AVAudioFrameCount(outputFormat.sampleRate) * buffer.frameLength / AVAudioFrameCount(buffer.format.sampleRate))!

           var error: NSError?
           let status = converter.convert(to: convertedBuffer, error: &error, withInputFrom: inputCallback)
           assert(status != .error)

           
           if status == .haveData {
             // Process with converted buffer
           }
            
       }
       
       engine.prepare()
       
       do {
           try engine.start()
       } catch {
           print("Can't start the engine: \(error)")
       }

問題

低通和高通濾波器不起作用。

替代方法

為了檢查代碼是否有效，我們添加了混響效果而不是低通濾波器。混響效果（使用AVAudioUnitReverb）使用相同的代碼。

任何人都可以幫助我在應用低通濾波器時我們做錯了什么嗎？

uj5u.com熱心網友回復：

我認為這段代碼的主要問題AVAudioConverter是在呼叫engine.prepare()which 可以并且將更改mainMixerNode輸出格式之前被創建。除此之外，還有mainMixerNodeto的冗余連接outputNode，以及可能不正確的格式 -mainMixerNode記錄為自動創建并“按需”連接到輸出節點。水龍頭也不需要格式。

let bus = 0
let inputNode = engine.inputNode

let equalizer = AVAudioUnitEQ(numberOfBands: 2)

equalizer.bands[0].filterType = .lowPass
equalizer.bands[0].frequency = 3000
equalizer.bands[0].bypass = false

equalizer.bands[1].filterType = .highPass
equalizer.bands[1].frequency = 1000
equalizer.bands[1].bypass = false
engine.attach(equalizer) //Attach equalizer

// Connect nodes
engine.connect(inputNode, to: equalizer, format: inputNode.inputFormat(forBus: 0))
engine.connect(equalizer, to: engine.mainMixerNode, format: inputNode.inputFormat(forBus: 0))

// call before creating converter because this changes the mainMixer's output format
engine.prepare()

let outputFormat = AVAudioFormat(commonFormat: .pcmFormatInt16,
                                 sampleRate: 5000,
                                 channels: 1,
                                 interleaved: false)!

// Downsampling converter
guard let converter: AVAudioConverter = AVAudioConverter(from: engine.mainMixerNode.outputFormat(forBus: 0), to: outputFormat) else {
    print("Can't convert in to this format")
    return
}

engine.mainMixerNode.installTap(onBus: bus, bufferSize: 2688, format: nil) { (buffer, time) in
    var newBufferAvailable = true
    
    let inputCallback: AVAudioConverterInputBlock = { inNumPackets, outStatus in
        if newBufferAvailable {
            outStatus.pointee = .haveData
            newBufferAvailable = false
            return buffer
        } else {
            outStatus.pointee = .noDataNow
            return nil
        }
    }
    
    
    let convertedBuffer = AVAudioPCMBuffer(pcmFormat: outputFormat, frameCapacity: AVAudioFrameCount(outputFormat.sampleRate) * buffer.frameLength / AVAudioFrameCount(buffer.format.sampleRate))!
    
    var error: NSError?
    let status = converter.convert(to: convertedBuffer, error: &error, withInputFrom: inputCallback)
    assert(status != .error)
    
    
    if status == .haveData {
        // Process with converted buffer
    }
}

do {
    try engine.start()
} catch {
    print("Can't start the engine: \(error)")
}

標籤：迅速 频率 音频引擎 低通滤波器 高通滤波器

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