AVAudio和脱机手动渲染模式-无法将频率较高的缓冲区写入输出文件

Question

我正在读取一个输入文件，在脱机手动渲染模式下，我想执行振幅调制，并将结果写入输出文件。

为了便于测试，我产生了纯正弦波-这对于低于6.000 Hz的频率很好。对于更高的频率(我的目标是使用大约20.000赫兹)，信号(因此收听输出文件)失真，频谱在8.000赫兹结束-不再是纯频谱，在0到8.000赫兹之间有多个峰值。

下面是我的代码片段：

    let outputFile: AVAudioFile

    do {
        let documentsURL = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask)[0]
        let outputURL = documentsURL.appendingPathComponent("output.caf")
        outputFile = try AVAudioFile(forWriting: outputURL, settings: sourceFile.fileFormat.settings)
    } catch {
        fatalError("Unable to open output audio file: \(error).")
    }

    var sampleTime: Float32 = 0

    while engine.manualRenderingSampleTime < sourceFile.length {
        do {
            let frameCount = sourceFile.length - engine.manualRenderingSampleTime
            let framesToRender = min(AVAudioFrameCount(frameCount), buffer.frameCapacity)
            
            let status = try engine.renderOffline(framesToRender, to: buffer)
            
            switch status {
            
            case .success:
                // The data rendered successfully. Write it to the output file.
                let sampleRate:Float = Float((mixer.outputFormat(forBus: 0).sampleRate))

                let modulationFrequency: Float = 20000.0
                
                for i in stride(from:0, to: Int(buffer.frameLength), by: 1) {
                    let val = sinf(2.0 * .pi * modulationFrequency * Float(sampleTime) / Float(sampleRate))
                    // TODO: perform modulation later
                    buffer.floatChannelData?.pointee[Int(i)] = val
                    sampleTime = sampleTime + 1.0
                }

                try outputFile.write(from: buffer)
                
            case .insufficientDataFromInputNode:
                // Applicable only when using the input node as one of the sources.
                break
                
            case .cannotDoInCurrentContext:
                // The engine couldn't render in the current render call.
                // Retry in the next iteration.
                break
                
            case .error:
                // An error occurred while rendering the audio.
                fatalError("The manual rendering failed.")
            @unknown default:
                fatalError("unknown error")
            }
        } catch {
            fatalError("The manual rendering failed: \(error).")
        }
    }

或者有谁知道如何产生具有更高频率的正弦波的输出文件？

我认为手动渲染模式不足以处理更高的频率。

更新:在此期间，我使用Audacity分析了输出文件。上面是1.000 Hz的波形，下面是20.000 Hz的波形：

​

当我放大时，我看到以下内容：

​

比较两个输出文件的频谱，我得到以下结果：

​

奇怪的是，频率越高，振幅就越接近于零。此外，我在第二个频谱中看到更多的频率。

与结果相关的一个新问题是以下算法的正确性：

// Process the audio in `renderBuffer` here
for i in 0..<Int(renderBuffer.frameLength) {
    let val = sinf(1000.0*Float(index) *2 * .pi / Float(sampleRate))
    renderBuffer.floatChannelData?.pointee[i] = val
    index += 1
}

我确实检查了采样率，它是48000 -我知道当采样频率大于被采样信号的最大频率的两倍时，原始信号可以被忠实地重建。

更新2：

    settings[AVFormatIDKey] = kAudioFormatAppleLossless
    settings[AVAudioFileTypeKey] = kAudioFileCAFType
    settings[AVSampleRateKey] = readBuffer.format.sampleRate
    settings[AVNumberOfChannelsKey] = 1
    settings[AVLinearPCMIsFloatKey] = (readBuffer.format.commonFormat == .pcmFormatInt32)
    settings[AVSampleRateConverterAudioQualityKey] = AVAudioQuality.max
    settings[AVLinearPCMBitDepthKey] = 32
    settings[AVEncoderAudioQualityKey] = AVAudioQuality.max

现在输出信号的质量更好了，但并不完美。我得到了更高的振幅，但在频谱分析仪中总是不止一个频率。也许解决方法可以包括应用高通滤波器？

在此期间，我确实使用了一种SignalGenerator，将处理过的缓冲区(使用正弦波)直接传输到扬声器-在这种情况下，输出是完美的。我认为将信号路由到文件会导致这样的问题。

Answer 1

手动渲染模式的速度不是问题，因为手动渲染上下文中的速度有点无关紧要。

下面是从源文件到输出文件的手动渲染框架代码：

// Open the input file
let file = try! AVAudioFile(forReading: URL(fileURLWithPath: "/tmp/test.wav"))

let engine = AVAudioEngine()
let player = AVAudioPlayerNode()

engine.attach(player)

engine.connect(player, to:engine.mainMixerNode, format: nil)

// Run the engine in manual rendering mode using chunks of 512 frames
let renderSize: AVAudioFrameCount = 512

// Use the file's processing format as the rendering format
let renderFormat = AVAudioFormat(commonFormat: file.processingFormat.commonFormat, sampleRate: file.processingFormat.sampleRate, channels: file.processingFormat.channelCount, interleaved: true)!
let renderBuffer = AVAudioPCMBuffer(pcmFormat: renderFormat, frameCapacity: renderSize)!

try! engine.enableManualRenderingMode(.offline, format: renderFormat, maximumFrameCount: renderBuffer.frameCapacity)

try! engine.start()
player.play()

// The render format is also the output format
let output = try! AVAudioFile(forWriting: URL(fileURLWithPath: "/tmp/foo.wav"), settings: renderFormat.settings, commonFormat: renderFormat.commonFormat, interleaved: renderFormat.isInterleaved)

// Read using a buffer sized to produce `renderSize` frames of output
let readBuffer = AVAudioPCMBuffer(pcmFormat: file.processingFormat, frameCapacity: renderSize)!

// Process the file
while true {
    do {
        // Processing is finished if all frames have been read
        if file.framePosition == file.length {
            break
        }

        try file.read(into: readBuffer)
        player.scheduleBuffer(readBuffer, completionHandler: nil)

        let result = try engine.renderOffline(readBuffer.frameLength, to: renderBuffer)

        // Process the audio in `renderBuffer` here

        // Write the audio
        try output.write(from: renderBuffer)
        if result != .success {
            break
        }
    }
    catch {
        break
    }
}

player.stop()
engine.stop()

下面的代码片段显示了如何在整个引擎中设置相同的采样率：

// Replace:
//engine.connect(player, to:engine.mainMixerNode, format: nil)

// With:
let busFormat = AVAudioFormat(standardFormatWithSampleRate: file.fileFormat.sampleRate, channels: file.fileFormat.channelCount)

engine.disconnectNodeInput(engine.outputNode, bus: 0)
engine.connect(engine.mainMixerNode, to: engine.outputNode, format: busFormat)

engine.connect(player, to:engine.mainMixerNode, format: busFormat)

使用以下命令验证采样率是否始终相同：

NSLog("%@", engine)

________ GraphDescription ________
AVAudioEngineGraph 0x7f8194905af0: initialized = 0, running = 0, number of nodes = 3

     ******** output chain ********

     node 0x600001db9500 {'auou' 'ahal' 'appl'}, 'U'
         inputs = 1
             (bus0, en1) <- (bus0) 0x600001d80b80, {'aumx' 'mcmx' 'appl'}, [ 2 ch,  48000 Hz, 'lpcm' (0x00000029) 32-bit little-endian float, deinterleaved]

     node 0x600001d80b80 {'aumx' 'mcmx' 'appl'}, 'U'
         inputs = 1
             (bus0, en1) <- (bus0) 0x600000fa0200, {'augn' 'sspl' 'appl'}, [ 2 ch,  48000 Hz, 'lpcm' (0x00000029) 32-bit little-endian float, deinterleaved]
         outputs = 1
             (bus0, en1) -> (bus0) 0x600001db9500, {'auou' 'ahal' 'appl'}, [ 2 ch,  48000 Hz, 'lpcm' (0x00000029) 32-bit little-endian float, deinterleaved]

     node 0x600000fa0200 {'augn' 'sspl' 'appl'}, 'U'
         outputs = 1
             (bus0, en1) -> (bus0) 0x600001d80b80, {'aumx' 'mcmx' 'appl'}, [ 2 ch,  48000 Hz, 'lpcm' (0x00000029) 32-bit little-endian float, deinterleaved]
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