带Swift的流音频

Question

我正在开发一个应用程序，该应用程序应该记录用户的语音并通过MQTT协议将其传输到自定义设备。自定义设备的音频规范:小终端，无签名，16位LPCM，8khz采样率.每个数据包应该是1000个字节。

我不熟悉AudioEngine，我发现了这个代码示例，我认为它适合我的情况：

func startRecord() {
    audioEngine = AVAudioEngine()
    let bus = 0
    let inputNode = audioEngine.inputNode
    let inputFormat = inputNode.outputFormat(forBus: bus)
    
    var streamDescription = AudioStreamBasicDescription()
    streamDescription.mFormatID = kAudioFormatLinearPCM.littleEndian
    streamDescription.mSampleRate = 8000.0
    streamDescription.mChannelsPerFrame = 1
    streamDescription.mBitsPerChannel = 16
    streamDescription.mBytesPerPacket = 1000
    
    
    let outputFormat = AVAudioFormat(streamDescription: &streamDescription)!
    
    guard let converter: AVAudioConverter = AVAudioConverter(from: inputFormat, to: outputFormat) else {
        print("Can't convert in to this format")
        return
    }
    
    inputNode.installTap(onBus: 0, bufferSize: 1024, format: inputFormat) { (buffer, time) in
        print("Buffer format: \(buffer.format)")
        
        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)
        
        print("Converted buffer format:", convertedBuffer.format)
    }
    
    audioEngine.prepare()
    
    do {
        try audioEngine.start()
    } catch {
        print("Can't start the engine: \(error)")
    }
    
}

但是目前，转换器无法将输入格式转换为我的输出格式，我不明白为什么。如果我将输出格式更改为类似的内容：

let outputFormat = AVAudioFormat(commonFormat: .pcmFormatInt16, sampleRate: 8000.0, channels: 1, interleaved: false)!

那就成功了。

Answer 1

您的streamDescription是错误的，您没有填写所有字段，而mBytesPerPacket则是错误的--这与您的协议要求的数据包不同。对于未压缩音频(如LPCM)，AudioStreamBasicDescription要求此字段为1。如果您的协议要求样本在1000个组中，那么您必须这样做。

尝尝这个

var streamDescription = AudioStreamBasicDescription()
streamDescription.mSampleRate = 8000.0
streamDescription.mFormatID = kAudioFormatLinearPCM
streamDescription.mFormatFlags = kAudioFormatFlagIsSignedInteger // no endian flag means little endian
streamDescription.mBytesPerPacket = 2
streamDescription.mFramesPerPacket = 1
streamDescription.mBytesPerFrame = 2
streamDescription.mChannelsPerFrame = 1
streamDescription.mBitsPerChannel = 16
streamDescription.mReserved = 0