使用AvaudioEngine iOS的低通滤波器+采样率转换 [英] Low Pass filter + sample rate conversion using Avaudioengine iOS

问题描述

我们正在进行一个项目，该项目允许我们使用低通滤波器和高通滤波器录制来自具有5k Hz采样率的麦克风的一些声音。

我们使用的内容

为此，我们使用AvaudioEngine。

我们正在使用AVAudioConverter降级采样率。

我们正在将AVAudioUnitEQ用于低通和高通过滤器。

代码

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)")
       }

问题

低通和高通过滤器不起作用。

替代方法

为了检查代码是否正常工作，我们添加了混响效果，而不是低通滤波器。 混响效果(使用AVAudioUnitReverb)使用相同的代码。

有人能帮我吗？我们在应用低通滤波器时哪里做错了？

推荐答案

我认为此代码的主要问题在于AVAudioConverter是在调用engine.prepare()之前创建的，engine.prepare()可以并将更改mainMixerNode输出格式。除此之外，还有mainMixerNode到outputNode的冗余连接，以及可能不正确的格式-记录为自动创建mainMixerNode并按需连接到输出节点&Quot；。分路器也不需要格式。

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)")
}

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