问IOS和FFT (vDSP_fft_zrip)：频率低于appr。100赫兹被切断-为什么？

EN

- (NSMutableArray*)performFFT: (float*) data  withFrames: (int) numSamples {
    // 1. init
    float bufferSize = numSamples;
    uint32_t maxFrames = numSamples;
    displayData = (float*)malloc(maxFrames*sizeof(float));
    bzero(displayData, maxFrames*sizeof(float));
    int log2n = log2f(maxFrames);
    int n = 1 << log2n;
    assert(n == maxFrames);
    float nOver2 = maxFrames/2;
    
    
    A.realp = (float*)malloc(nOver2 * sizeof(float));
    A.imagp = (float*)malloc(nOver2 * sizeof(float));
    fftSetup = vDSP_create_fftsetup(log2n, FFT_RADIX2);
    
    // 2. calcuate
    bufferSize = numSamples;
    float ln = log2f(numSamples);
    vDSP_ctoz((COMPLEX*)data, 2, &A, 1, numSamples/2);
    
    //fft
    vDSP_fft_zrip(fftSetup, &A, 1, ln, FFT_FORWARD);
    
    // Absolute square (equivalent to mag^2)
    vDSP_zvmags(&A, 1, A.realp, 1, numSamples/2);
    // make imaginary part to zero in order to filter them out in the following loop
    bzero(A.imagp, (numSamples/2) * sizeof(float));
    
    //convert complex split to real
    vDSP_ztoc(&A, 1, (COMPLEX*)displayData, 2, numSamples/2);
    
    // Normalize
    float scale = 1.f/displayData[0];
    vDSP_vsmul(displayData, 1, &scale, displayData, 1, numSamples);
    
    
    //scale fft
    Float32 mFFTNormFactor = 1.0/(2*numSamples);
    vDSP_vsmul(A.realp, 1, &mFFTNormFactor, A.realp, 1, numSamples/2);
    vDSP_vsmul(A.imagp, 1, &mFFTNormFactor, A.imagp, 1, numSamples/2);

...
}

[audioManager setInputBlock:^(float *data, UInt32 numFrames, UInt32 numChannels) {
            
            // frequency analysis
            vDSP_rmsqv(data, 1, &magnitude, numFrames*numChannels);
            self->ringBuffer->AddNewInterleavedFloatData(data, numFrames, numChannels);
...
}

[audioManager setOutputBlock:^(float *data, UInt32 numFrames, UInt32 numChannels)
     { 
   self->ringBuffer->FetchInterleavedData(data, numFrames, numChannels);
   ....
   [fft performFFT:data withFrames:numFrames];
   ...
   }