如何验证Core Audio中音频幅度是否正确？

Question

我使用以下代码来录制音频并生成幅度浮点数据。但我不确定数据是否正确。有人可以验证吗？

用于录制音频的主程序类似于示例AudioQueueRecorder。

在我创建队列时，我将AudioQueueProcessingTapNew与callback一起添加为：

guard let queue = queue else { return }        

var maxFrames : UInt32 = 0;
var tapFormat = AudioStreamBasicDescription()
var tap : AudioQueueProcessingTapRef? = nil
var player = MyInfo()

utils.checkError(AudioQueueProcessingTapNew(queue, tapCallback, &player, AudioQueueProcessingTapFlags.preEffects, &maxFrames, &tapFormat, &tap),
                 "Failed to create audio queue tap")

在点击回调（上面链接）中，我使用code来获得幅度：

let info = userData.assumingMemoryBound(to: MyInfo.self)
var sourceFlags: AudioQueueProcessingTapFlags = AudioQueueProcessingTapFlags(rawValue: 0)
var sourceNumFrames: UInt32 = 0
AudioQueueProcessingTapGetSourceAudio(tap, inNumFrames, ts, &sourceFlags, &sourceNumFrames, ioData)

print("ts:", ts.pointee)
print("num frames:", inNumFrames)

let numBuffers = ioData.pointee.mNumberBuffers
print("num buffers:", numBuffers)

if numBuffers > 0 {
    let numChans = ioData.pointee.mBuffers.mNumberChannels
    let size = ioData.pointee.mBuffers.mDataByteSize / 4 //Size of a float is 4
    let data: [Float] = Array(UnsafeMutableBufferPointer(start: ioData.pointee.mBuffers.mData!.assumingMemoryBound(to: Float.self), count: Int(size)))

    print("num chans:", numChans)
    print("size:", size)

    let fft = TempiFFT(withSize: Int(size), sampleRate: 44100.0)
    fft.windowType = TempiFFTWindowType.hamming
    fft.fftForward(data)
    fft.calculateLinearBands(minFrequency: 0, maxFrequency: fft.nyquistFrequency, numberOfBands: 20)

    var magnitudeArr = [Float](repeating: Float(0), count: 20)
    var magnitudeDBArr = [Float](repeating: Float(0), count: 20)
    for i in 0..<20 {
        magnitudeArr[i] = fft.magnitudeAtBand(i)
        //var magnitudeDB = TempiFFT.toDB(fft.magnitudeAtBand(i))
        log.debug("fft.magnitudeAtBand(\(i)) = \(fft.magnitudeAtBand(i))")
    }
}

我得到以下输出。我想绘制幅度图。

ts: AudioTimeStamp(mSampleTime: 359433.0, mHostTime: 7385045310414, mRateScalar: 0.9999903367259503, mWordClockTime: 4424786032, mSMPTETime: __C.SMPTETime(mSubframes: 0, mSubframeDivisor: 16384, mCounter: 0, mType: __C.SMPTETimeType, mFlags: __C.SMPTETimeFlags(rawValue: 1), mHours: 4800, mMinutes: 179, mSeconds: 1, mFrames: 0), mFlags: __C.AudioTimeStampFlags(rawValue: 7), mReserved: 28672)
num frames: 2048
num buffers: 2
num chans: 1
size: 2048
data: [-0.00013435386, 4.471163e-05, 2.5250512e-07, -0.00013112863, -4.2305917e-05, -6.0883203e-06, -6.9609705e-05, 1.3280656e-05, 2.6565194e-06, -7.048986e-05, -4.371278e-05, -0.00012498278, -0.00015581526, 2.6477476e-06, 0.00014767727, 0.00015129645, 0.0002481592, 0.00020998582, 7.0496775e-05, 0.00023169718, 0.00032023923, 9.4876516e-05, 3.0334933e-05, -0.00010365831, -0.00012052287, -3.4284778e-06, -8.221614e-05, -0.00013551829, -8.154745e-05, -8.845353e-06, -5.211319e-05, 6.356722e-05, 0.00014827582] ...

fft.magnitudeAtBand(0) = 0.0019885404
fft.magnitudeAtBand(1) = 0.0002433162
fft.magnitudeAtBand(2) = 0.00011211308
fft.magnitudeAtBand(3) = 0.00019586945
fft.magnitudeAtBand(4) = 0.00015595475
fft.magnitudeAtBand(5) = 1.6589744e-05
fft.magnitudeAtBand(6) = 0.0019934585
fft.magnitudeAtBand(7) = 0.00035403066
fft.magnitudeAtBand(8) = 0.0020195032
fft.magnitudeAtBand(9) = 0.0019526586
fft.magnitudeAtBand(10) = 0.0010482509
fft.magnitudeAtBand(11) = 0.0015899779
fft.magnitudeAtBand(12) = 0.00021513038
fft.magnitudeAtBand(13) = 8.8768e-05
fft.magnitudeAtBand(14) = 0.00088070636
fft.magnitudeAtBand(15) = 3.6925994e-05
fft.magnitudeAtBand(16) = 0.0013688633
fft.magnitudeAtBand(17) = 0.0020811968
fft.magnitudeAtBand(18) = 0.00039211413
fft.magnitudeAtBand(19) = 1.50605065e-05

stop record did tap

但magnitudeAtBand值似乎是非常小的价值。它是否正确？如何绘制振幅图？

---

在数据值中，存在负面影响。它为什么是否定的？

Answer 1

OS X上32位浮点样本的范围是-1.0f到1.0f。负数是存在的，因为（行）样本描述了模拟波形信号。

为了验证数据的有效性，我建议将其播放，以便您可以听到数据是否正确。

要验证的更复杂的解决方案是提供测试信号并在示波器或频谱分析仪上查看。