音乐频谱显示 FFT of waveIn audio signals. Free source code and programming help

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<ul class="download"><li><a href="http://www.codeproject.com/KB/audio-video/waveInFFT/waveinFFT_demo.zip">Download demo project - 33.3 KB</a> </li></ul>
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<h2>Introduction</h2>
The Fast Fourier Transform (FFT) allows users to view the spectrum content of an audio signal. The FFT code presented here was written by <a href="http://www.intersrv.com/~dcross/fft.html" rel="noopener noreferrer" target="_blank">Don Cross</a>, his homepage appears to have subsequently been taken down. Rather than explain the mathematical theory of the FFT, I will attempt to explain its usefulness as it relates to audio signals.
The FFT allows users to obtain the spectral makeup of an audio signal, obtain the decibels of its various frequencies, or obtain the intensity of its various frequencies. Spectral viewers (shown in the image above), Equalizers, or VU-Meters may all use the FFT in order to display their results. The difference between them then depends upon one of a couple of equations that take the real and imaginary components of the FFT, and return either the intensity or decibel levels to be used in the graphed result. The following code takes both the real and imaginary components of the FFT result, and returns the intensity and decibels.
<pre class="blockcode">inline double GetFrequencyIntensity(double re, double im)
{
return sqrt((re*re)+(im*im));
}
#define mag_sqrd(re,im) (re*re+im*im)
#define Decibels(re,im) ((re == 0 && im == 0) ? (0) :
 10.0 * log10(double(mag_sqrd(re,im))))
#define Amplitude(re,im,len) (GetFrequencyIntensity(re,im)/(len))
#define AmplitudeScaled(re,im,len,scale) ((int)Amplitude(re,im,len)%scale)</pre>
The FFT uses the audio signal as its real component, and uses a <code>NULL</code> pointer for its imaginary component indicating that the imaginary data does not exist. Upon its return, the FFT will return both the real and imaginary data components based upon the data given as the real component. The is mirrored with the return samples so that <code>0-FFT_LEN/2</code> contains the data, and <code>FFT_LEN/2</code> to <code>FFT_LEN</code> contains a reverse of the data. This mistake was corrected in my code. The code that performs the FFT follows:
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<pre id="pre1" style="MARGIN-TOP: 0px"> DWORD nCount = 0;
for (DWORD dw = 0; dw < FFT_LEN; dw++)
{
 {
 //copy audio signal to fft real component for left chann