DSP_fft16x16t
/*−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−*/
/* | Set up | offsets to access ”N/4”, ”N/2”, ”3N/4” complex point or | */ |
/* | ”N/2”, | ”N”, ”3N/2” half word | */ |
/*−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−*/
h2 = stride>>1;
l1 = stride;
l2 = stride + (stride >> 1);
/*−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−*/
/* | Reset ”x” to point to the start of the input data array. | */ |
/* ”tw_offset” starts off at 0, and increments by ”6 * stride” | */ | |
/* | The stride quarters with every iteration of the outer loop | */ |
/*−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−*/ x = ptr_x;
w = ptr_w + tw_offset; tw_offset += fft_jmp; stride >>= 2;
/*−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−*/ /* The following loop iterates through the different butterflies, */
/* within a given stage. Recall that there are logN to base 4 | */ |
/* stages. Certain butterflies share the twiddle factors. These | */ |
/* are grouped together. On the very first stage there are no | */ |
/* butterflies that share the twiddle factor, all N/4 butter− | */ |
/* flies have different factors. On the next stage two sets of | */ |
/* N/8 butterflies share the same twiddle factor. Hence, after | */ |
/* half the butterflies are performed, j the index into the | */ |
/* factor array resets to 0, and the twiddle factors are reused. */
/* When this happens, the data pointer ’x’ is incremented by the | */ |
/* fft_jmp amount. In addition, the following code is unrolled to | */ |
/* perform ”2” radix4 butterflies in parallel. | */ |
/*−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−*/ for (i = 0; i < npoints; i += 8)
{
/*−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−*/
/* Read the first 12 twiddle factors, six of which are used | */ |
/* for one radix 4 butterfly and six of which are used for | */ |
/* next one. | */ |
