-使用AVAudioConverter转换PCM缓冲区时发生-50错误

Question

我试图将采样率为44100的AVAudioPCMBuffer转换为采样率为48000的AVAudioPCMBuffer，但在转换时总是会出现异常(-50错误(。这是代码：

guard let deviceFormat = AVAudioFormat(standardFormatWithSampleRate: 48000.0, channels: 1) else {
preconditionFailure()
}
// This file is saved as mono 44100
guard let lowToneURL = Bundle.main.url(forResource: "Tone220", withExtension: "wav") else {
preconditionFailure()
}
guard let audioFile = try? AVAudioFile(forReading: lowToneURL) else {
preconditionFailure()
}
let tempBuffer = AVAudioPCMBuffer(pcmFormat: audioFile.processingFormat,
frameCapacity: AVAudioFrameCount(audioFile.length))!
tempBuffer.frameLength = tempBuffer.frameCapacity
do { try audioFile.read(into: tempBuffer) } catch {
assertionFailure("*** Caught: (error)")
}
guard let converter = AVAudioConverter(from: audioFile.processingFormat, to: deviceFormat) else {
preconditionFailure()
}
guard let convertedBuffer = AVAudioPCMBuffer(pcmFormat: deviceFormat,
frameCapacity: AVAudioFrameCount(audioFile.length)) else {
preconditionFailure()
}
convertedBuffer.frameLength = tempBuffer.frameCapacity
do { try converter.convert(to: convertedBuffer, from: tempBuffer) } catch {
assertionFailure("*** Caught: (error)")
}

有什么想法吗？

Answer 1

一位苹果工程师在他们的开发论坛上回答了这个问题。我错过了AVAudioConverter的convert(to:from:)变体无法转换采样率，所以您必须使用withInputFrom变体。这方面的文档不太清楚，但我想出了：

private func pcmBufferForFile(filename: String, sampleRate: Float) -> AVAudioPCMBuffer {
guard let newFormat = AVAudioFormat(standardFormatWithSampleRate: Double(sampleRate), channels: 1) else {
preconditionFailure()
}
guard let url = Bundle.main.url(forResource: filename, withExtension: "wav") else {
preconditionFailure()
}
guard let audioFile = try? AVAudioFile(forReading: url) else {
preconditionFailure()
}
guard let tempBuffer = AVAudioPCMBuffer(pcmFormat: audioFile.processingFormat,
frameCapacity: AVAudioFrameCount(audioFile.length)) else {
preconditionFailure()
}
let conversionRatio = sampleRate / Float(tempBuffer.format.sampleRate)
let newLength = Float(audioFile.length) * conversionRatio
guard let newBuffer = AVAudioPCMBuffer(pcmFormat: newFormat,
frameCapacity: AVAudioFrameCount(newLength)) else {
preconditionFailure()
}
do { try audioFile.read(into: tempBuffer) } catch {
preconditionFailure()
}
guard let converter = AVAudioConverter(from: audioFile.processingFormat, to: newFormat) else {
preconditionFailure()
}
var error: NSError?
converter.convert(to: newBuffer, error: &error, withInputFrom: { (packetCount, statusPtr) -> AVAudioBuffer? in
statusPtr.pointee = .haveData
return tempBuffer
})
if error != nil {
print("*** Conversion error: (error!)")
}
return newBuffer
}