Texas Instruments TMS320C64X - Manual (Page 129)

DSP_fft
4-101 C64x+ DSPLIB Reference
{
#ifndef NOASSUME
_nassert(i % 4 == 0);
_nassert(s >= 4);
#pragma MUST_ITERATE(2,,2);
#endif
for (j = 0; j < s; j += 2)
{
for (k = 0; k < 2; k++)
{
short w1c, w1s, w2c, w2s, w3c, w3s;
short x0r, x0i, x1r, x1i, x2r, x2i, x3r, x3i;
short y0r, y0i, y1r, y1i, y2r, y2i, y3r, y3i;
/* −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− */
/* Read the four samples that are the input to this */
/* particular butterfly. */
/* −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− */
x0r = x[2*(i+j+k ) + 0]; x0i = x[2*(i+j+k ) + 1];
x1r = x[2*(i+j+k + s) + 0]; x1i = x[2*(i+j+k + s) + 1];
x2r = x[2*(i+j+k + 2*s) + 0]; x2i = x[2*(i+j+k + 2*s) + 1];
x3r = x[2*(i+j+k + 3*s) + 0]; x3i = x[2*(i+j+k + 3*s) + 1];
/* −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− */
/* Read the six twiddle factors that are needed for 3 */
/* of the four outputs. (The first output has no mpys.) */
/* −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−*/
w1s = w[t + 2*k + 6*j + 0]; w1c = w[t + 2*k + 6*j + 1];
w2s = w[t + 2*k + 6*j + 4]; w2c = w[t + 2*k + 6*j + 5];
w3s = w[t + 2*k + 6*j + 8]; w3c = w[t + 2*k + 6*j + 9];
/* −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− */
/* Calculate the four outputs, remembering that radix4 */
/* FFT accepts 4 inputs and produces 4 outputs. If we */
/* imagine the inputs as being complex, and look at the */
/* first stage as an example: */
/* */
/* Four inputs are x(n) x(n+N/4) x(n+N/2) x(n+3N/4) */
/* In general the four inputs can be generalized using */