如何验证 Core Audio 中的音频幅度是否正确？

Question

我正在使用以下代码录制音频并生成幅度浮点数据。但是我不确定数据是否正确。有人可以验证一下吗？

用于录音的主程序与示例类似AudioQueueRecorder。

在创建队列后，我将 AudioQueueProcessingTapNew 和 callback 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，我使用来获取幅度：

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。我想绘制振幅图。

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

但是magnitudeAtBand的值好像很小。这个对吗？如何绘制振幅图？

在数据值中，有负数。为什么是负数？

Answer 1

32 位浮点样本在 OS X 上的范围是 -1.0f 到 1.0f。负数在那里是因为（行）样本描述了一个模拟波形信号。

为了验证数据的有效性，我建议播放它，这样您就可以听到数据是否正确。

更复杂的验证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它。

如何验证 Core Audio 中的音频幅度是否正确？

How to verify if audio magnitude is correct in Core Audio?

plot

core-audio

ios

swift