如何更改 iPhone 的麦克风采样率

Question

我正在努力向现有应用程序添加一项功能，以从设备麦克风获取音频输入，通过 FFT 将其转换为频域并将其发送到 coreML 模型。我正在使用标准 AVCaptureDevice：

guard let microphone = AVCaptureDevice.default(.builtInMicrophone,
                                                   for: .audio,
                                                   position: .unspecified),
let microphoneInput = try? AVCaptureDeviceInput(device: microphone) else {
                fatalError("Can't create microphone.")
}

问题是，我需要为麦克风定义自定义采样率。按照 Apple 的文档，setPreferredSampleRate (link) 应该能够在 8000-48000 Hz 范围内做到这一点。然而，无论我选择哪个值，采样率都不会改变，也不会抛出错误：

print("Microphone sample rate: ", AVAudioSession.sharedInstance().sampleRate)

do { var flag = try AVAudioSession.sharedInstance().setPreferredSampleRate(20000) }
catch { print("Unable to set microphone sampling rate!") }

print("Microphone sample rate: ", AVAudioSession.sharedInstance().sampleRate)

输出：

Microphone sample rate: 48000.0
Microphone sample rate: 48000.0

How can I Define the Sample Rate for iOS devices?

编辑：

遵循使用<的建议code>AVAudioConverter 来重新采样麦克风输入，考虑到我正在使用 AVCaptureAudioDataOutputSampleBufferDelegate 和相应的，最有效的方法是什么？从麦克风收集原始音频输入的 captureOutput 方法：

extension AudioSpectrogram: AVCaptureAudioDataOutputSampleBufferDelegate {

    public func captureOutput(_ output: AVCaptureOutput,
                              didOutput sampleBuffer: CMSampleBuffer,
                              from connection: AVCaptureConnection) {

        var audioBufferList = AudioBufferList()
        var blockBuffer: CMBlockBuffer?

        CMSampleBufferGetAudioBufferListWithRetainedBlockBuffer(
            sampleBuffer,
            bufferListSizeNeededOut: nil,
            bufferListOut: &audioBufferList,
            bufferListSize: MemoryLayout.stride(ofValue: audioBufferList),
            blockBufferAllocator: nil,
            blockBufferMemoryAllocator: nil,
            flags: kCMSampleBufferFlag_AudioBufferList_Assure16ByteAlignment,
            blockBufferOut: &blockBuffer)

        guard let data = audioBufferList.mBuffers.mData else {
            return
        }

        if self.rawAudioData.count < self.sampleCount * 2 {
            let actualSampleCount = CMSampleBufferGetNumSamples(sampleBuffer)

            let ptr = data.bindMemory(to: Int16.self, capacity: actualSampleCount)
            let buf = UnsafeBufferPointer(start: ptr, count: actualSampleCount)

            rawAudioData.append(contentsOf: Array(buf))
        }

        while self.rawAudioData.count >= self.sampleCount {
            let dataToProcess = Array(self.rawAudioData[0 ..< self.sampleCount])
            self.rawAudioData.removeFirst(self.hopCount)
            self.processData(values: dataToProcess)
        }
 }