Texas Instruments TMS320C67 DSP Series Programmer's Reference Manual page 36

{
n1 = n2;
n2 >>= 2;
ia1 = 0;
for(j=0; j<n2; j++)
{
ia2 = ia1 + ia1;
ia3 = ia1 + ia2;
co1 = w[ia1*2];
si1 = w[ia1*2 + 1];
co2 = w[ia2*2];
si2 = w[ia2*2 + 1];
co3 = w[ia3*2];
si3 = w[ia3*2 + 1];
ia1 += ie;
for(i0=j; i0<n; i0+=n1)
{
i1 = i0 + n2;
i2 = i1 + n2;
i3 = i2 + n2;
r1 = x[i0*2]
r3 = x[i0*2]
s1 = x[i0*2+1] + x[i2*2+1];
s3 = x[i0*2+1] - x[i2*2+1];
r2 = x[i1*2]
r4 = x[i1*2]
s2 = x[i1*2+1] + x[i3*2+1];
s4 = x[i1*2+1] - x[i3*2+1];
x[i0*2]
r2
r1
r3
x[i0*2+1] = s1 + s2;
s2
s1
s3
+ x[i2*2];
- x[i2*2];
+ x[i3*2];
- x[i3*2];
= r1 + r2;
= r1 - r2;
= r3 - s4;
= r3 + s4;
= s1 - s2;
= s3 + r4;
= s3 - r4;
DSPLIB Reference
DSPF_sp_cfftr4_dif
4-11