斯威夫特 - 如何以 8kHz 采样率从 iPhone 上的麦克风获取音频输出，缓冲区大小为 16ms 和 1 通道？

问：

对于 iphone 上的演示应用程序，我想以 8khz 的频率从麦克风获取音频输出，缓冲区大小为 16ms，以便它一次提供 128 字节的帧长度。

我通过在输入节点上安装一个水龙头来尝试这个 AVAudioEngine，但无法设置采样率和缓冲区大小。关于如何实现 128 字节输出的任何建议？

采样率可以不同，但我需要 128 字节和 16 毫秒。

谢谢

private func startRecording() throws {
    checkPermission()
    if !self.hasPermissionToRecord {
        return
    }
    if engine == nil {
        engine = AVAudioEngine()
    }
    let inputNode = engine.inputNode
    let microphoneOutputFormat: AVAudioFormat = inputNode.outputFormat(forBus: 0)
    let microphoneAudioStream = microphoneOutputFormat.formatDescription.audioStreamBasicDescription
    let microphoneSampleRate = microphoneAudioStream!.mSampleRate

    var mulawDescription = AudioStreamBasicDescription(
        mSampleRate: 8000,
        mFormatID: kAudioFormatULaw,
        mFormatFlags: 0,
        mBytesPerPacket: 1,
        mFramesPerPacket: 1,
        mBytesPerFrame: 1,
        mChannelsPerFrame: 1,
        mBitsPerChannel: 8,
        mReserved: 0
    )
        
    let mulawFormat = AVAudioFormat(streamDescription: &mulawDescription)!
    converter = AVAudioConverter(from: microphoneOutputFormat, to: mulawFormat)
        
    inputNode.installTap(onBus: 0, bufferSize: defaultUlawBufferSize, format: microphoneOutputFormat) { [self] (buffer: AVAudioPCMBuffer!, time: AVAudioTime!) in
            
    let secondsInBuffer:Float64 = Float64(buffer.frameLength) / Float64(microphoneSampleRate);
    let ulawBufferSize = UInt32(8000 * secondsInBuffer) // 800
            
    // conversion - downsample
    guard let mulawBuffer = AVAudioPCMBuffer(pcmFormat: converter.outputFormat, frameCapacity: 800) else {
        return
    }
    var outError: NSError? = nil
    let microphoneInputBlock: AVAudioConverterInputBlock = { (inNumbPackets, outStatus) -> AVAudioBuffer? in
        outStatus.pointee = AVAudioConverterInputStatus.haveData
        return buffer
    }
    self.converter.convert(to: mulawBuffer, error: &outError, withInputFrom: microphoneInputBlock)
    let mulawData = Data(buffer:mulawBuffer)
    if (!(receivedAudioCallback?(mulawData) ?? false)) {
         stop()
     }
    }
    engine.prepare()
    try engine.start()
}

我试过这段代码，在以 8khz 下采样后，它给了我 800bytes