Question

我似乎迷失了这种傅里叶变换功能。我有一个示例程序，但我不明白。 ggFFTworksp包含数据，fftFrameSize只是数据的帧大小。我不明白如果代码中没有实际编辑或操作fftBuffer的部分，函数应该如何将FFT版本的数据放入fftBuffer。提前谢谢！

函数调用是：

static float gFFTworksp[2*MAX_FRAME_LENGTH];
long fftFrameSize;    
smbFft(gFFTworksp, fftFrameSize, -1);

有问题的功能是：

void smbFft(float *fftBuffer, long fftFrameSize, long sign)
/* 
    FFT routine, (C)1996 S.M.Bernsee. Sign = -1 is FFT, 1 is iFFT (inverse)
    Fills fftBuffer[0...2*fftFrameSize-1] with the Fourier transform of the
    time domain data in fftBuffer[0...2*fftFrameSize-1]. The FFT array takes
    and returns the cosine and sine parts in an interleaved manner, ie.
    fftBuffer[0] = cosPart[0], fftBuffer[1] = sinPart[0], asf. fftFrameSize
    must be a power of 2. It expects a complex input signal (see footnote 2),
    ie. when working with 'common' audio signals our input signal has to be
    passed as {in[0],0.,in[1],0.,in[2],0.,...} asf. In that case, the transform
    of the frequencies of interest is in fftBuffer[0...fftFrameSize].
*/
{
    float wr, wi, arg, *p1, *p2, temp;
    float tr, ti, ur, ui, *p1r, *p1i, *p2r, *p2i;
    long i, bitm, j, le, le2, k;

    for (i = 2; i < 2*fftFrameSize-2; i += 2) {
        for (bitm = 2, j = 0; bitm < 2*fftFrameSize; bitm <<= 1) {
            if (i & bitm) j++;
            j <<= 1;
        }
        if (i < j) {
            p1 = fftBuffer+i; p2 = fftBuffer+j;
            temp = *p1; *(p1++) = *p2;
            *(p2++) = temp; temp = *p1;
            *p1 = *p2; *p2 = temp;
        }
    }
    for (k = 0, le = 2; k < (long)(log(fftFrameSize)/log(2.)+.5); k++) {
        le <<= 1;
        le2 = le>>1;
        ur = 1.0;
        ui = 0.0;
        arg = M_PI / (le2>>1);
        wr = cos(arg);
        wi = sign*sin(arg);
        for (j = 0; j < le2; j += 2) {
            p1r = fftBuffer+j; p1i = p1r+1;
            p2r = p1r+le2; p2i = p2r+1;
            for (i = j; i < 2*fftFrameSize; i += le) {
                tr = *p2r * ur - *p2i * ui;
                ti = *p2r * ui + *p2i * ur;
                *p2r = *p1r - tr; *p2i = *p1i - ti;
                *p1r += tr; *p1i += ti;
                p1r += le; p1i += le;
                p2r += le; p2i += le;
            }
            tr = ur*wr - ui*wi;
            ui = ur*wi + ui*wr;
            ur = tr;
        }
    }
}

Answer 1

在以下行中：

p1 = fftBuffer+i; p2 = fftBuffer+j;

p1和p2成为指向fftBuffer数组的内存位置的指针。在这些方面：

*(p2++) = temp; temp = *p1;
*p1 = *p2; *p2 = temp;

正在更改这些内存位置中的值。

该函数如何更改传递给它的参数？

1 个答案: