Texas Instruments TMS320C64X manual C64x+ Dsplib Reference 101

DSP_fft

{

#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++)

{

shortw1c, 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	*/

C64x+ DSPLIB Reference 4-101