Texas Instruments TMS320C64X - Manual (Page 137)

DSP_fft16x16t
4-109 C64x+ DSPLIB Reference
/*−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−*/
/* Determine the magnitude od the number of points to be transformed. */
/* Check whether we can use a radix4 decomposition or a mixed radix */
/* transformation, by determining modulo 2. */
/*−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−*/
for (i = 31, m = 1; (npoints & (1 << i)) == 0; i−−, m++) ;
radix = m & 1 ? 2 : 4;
norm = m − 2;
/*−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−*/
/* The stride is quartered with every iteration of the outer loop. It */
/* denotes the seperation between any two adjacent inputs to the butter */
/* −fly. This should start out at N/4, hence stride is initially set to */
/* N. For every stride, 6*stride twiddle factors are accessed. The */
/* ”tw_offset” is the offset within the current twiddle factor sub− */
/* table. This is set to zero, at the start of the code and is used to */
/* obtain the appropriate sub−table twiddle pointer by offsetting it */
/* with the base pointer ”ptr_w”. */
/*−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−*/
stride = npoints;
tw_offset = 0;
fft_jmp = 6 * stride;
while (stride > radix)
{
/*−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−*/
/* At the start of every iteration of the outer loop, ”j” is set */
/* to zero, as ”w” is pointing to the correct location within the */
/* twiddle factor array. For every iteration of the inner loop */
/* 6 * stride twiddle factors are accessed. For eg, */
/* */
/* #Iteration of outer loop # twiddle factors #times cycled */
/* 1 6 N/4 1 */
/* 2 6 N/16 4 */
/* ... */
/*−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−*/
j = 0;
fft_jmp >>= 2;