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354 lines
27 KiB
354 lines
27 KiB
;* ======================================================================== *;
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;* TEXAS INSTRUMENTS, INC. *;
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;* *;
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;* DSPLIB DSP Signal Processing Library *;
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;* *;
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;* Release: Revision 1.04b *;
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;* CVS Revision: 1.5 Sun Sep 29 03:32:21 2002 (UTC) *;
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;* Snapshot date: 23-Oct-2003 *;
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;* *;
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;* This library contains proprietary intellectual property of Texas *;
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;* Instruments, Inc. The library and its source code are protected by *;
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;* various copyrights, and portions may also be protected by patents or *;
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;* other legal protections. *;
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;* *;
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;* This software is licensed for use with Texas Instruments TMS320 *;
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;* family DSPs. This license was provided to you prior to installing *;
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;* the software. You may review this license by consulting the file *;
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;* TI_license.PDF which accompanies the files in this library. *;
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;* ------------------------------------------------------------------------ *;
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;* Copyright (C) 2003 Texas Instruments, Incorporated. *;
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;* All Rights Reserved. *;
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;* ======================================================================== *;
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;* ======================================================================== *;
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;* Assembler compatibility shim for assembling 4.30 and later code on *;
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;* tools prior to 4.30. *;
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;* ======================================================================== *;
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;* ======================================================================== *;
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;* End of assembler compatibility shim. *;
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;* ======================================================================== *;
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*========================================================================== *
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* TEXAS INSTRUMENTS, INC. *
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* *
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* NAME *
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* DSP_fft16x32 *
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* *
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* USAGE *
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* This routine is C-callable and can be called as: *
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* *
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* void DSP_fft16x32(const short * ptr_w, int npoints, *
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* int * ptr_x, int *ptr_y ) ; *
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* *
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* ptr_w = input twiddle factors *
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* npoints = number of points *
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* ptr_x = transformed data reversed *
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* ptr_y = linear transformed data *
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* *
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* (See the C compiler reference guide.) *
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* *
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* DESCRIPTION *
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* The following code performs a mixed radix FFT for "npoints" which *
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* is either a multiple of 4 or 2. It uses logN4 - 1 stages of radix4 *
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* transform and performs either a radix2 or radix4 transform on the *
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* last stage depending on "npoints". If "npoints" is a multiple of 4, *
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* then this last stage is also a radix4 transform, otherwise it is a *
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* radix2 transform. This program is available as a C compilable file *
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* to automatically generate the twiddle factors "twiddle_split.c" *
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* *
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* int i, j, k, n = N; *
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* double theta1, theta2, theta3, x_t, y_t; *
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* const double M = 32768.0, PI = 3 41592654; *
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* *
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* for (j=1, k=0; j < n>>2; j = j<<2) *
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* { *
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* for (i=0; i < n>>2; i += j<<1) *
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* { *
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* theta1 = 2*PI*i/n; *
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* x_t = M*cos(theta1); *
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* y_t = M*sin(theta1); *
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* w[k+1] = (short) x_t; *
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* if (x_t >= M) w[k+1] = 0x7fff; *
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* w[k+0] = (short) y_t; *
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* if (y_t >= M) w[k+0] = 0x7fff; *
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* *
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* theta1 = 2*PI*(i+j)/n; *
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* x_t = M*cos(theta1); *
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* y_t = M*sin(theta1); *
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* w[k+7] = (short) x_t; *
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* if (x_t >= M) w[k+3] = 0x7fff; *
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* w[k+6] = (short) y_t; *
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* if (y_t >= M) w[k+2] = 0x7fff; *
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* *
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* theta2 = 4*PI*i/n; *
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* x_t = M*cos(theta2); *
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* y_t = M*sin(theta2); *
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* w[k+3] = (short) x_t; *
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* if (x_t >= M) w[k+5] = 0x7fff; *
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* w[k+2] = (short) y_t; *
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* if (y_t >= M) w[k+4] = 0x7fff; *
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* *
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* theta2 = 4*PI*(i+j)/n; *
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* x_t = M*cos(theta2); *
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* y_t = M*sin(theta2); *
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* w[k+9] = (short) x_t; *
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* if (x_t >= M) w[k+7] = 0x7fff; *
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* w[k+8] = (short) y_t; *
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* if (y_t >= M) w[k+6] = 0x7fff; *
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* *
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* theta3 = 6*PI*i/n; *
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* x_t = M*cos(theta3); *
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* y_t = M*sin(theta3); *
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* w[k+5] = (short) x_t; *
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* if (x_t >= M) w[k+9] = 0x7fff; *
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* w[k+4] = (short) y_t; *
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* if (y_t >= M) w[k+8] = 0x7fff; *
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* *
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* theta3 = 6*PI*(i+j)/n; *
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* x_t = M*cos(theta3); *
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* y_t = M*sin(theta3); *
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* w[k+11] = (short) x_t; *
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* if (x_t >= M) w[k+11] = 0x7fff; *
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* w[k+10] = (short) y_t; *
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* if (y_t >= M) w[k+10] = 0x7fff; *
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* *
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* k += 12; *
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* } *
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* } *
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* w[2*n-1] = w[2*n-3] = w[2*n-5] = 0x7fff; *
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* w[2*n-2] = w[2*n-4] = w[2*n-6] = 0x0000; *
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* *
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* ASSUMPTIONS *
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* This code works for both "npoints" a multiple of 2 or 4. *
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* The arrays 'x[]', 'y[]', and 'w[]' all must be aligned on a *
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* double-word boundary for the "optimized" implementations. *
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* *
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* The input and output data are complex, with the real/imaginary *
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* components stored in adjacent locations in the array. The real *
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* components are stored at even array indices, and the imaginary *
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* components are stored at odd array indices. *
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* *
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* TECHNIQUES *
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* The following C code represents an implementation of the Cooley *
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* Tukey radix 4 DIF FFT. It accepts the inputs in normal order and *
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* produces the outputs in digit reversed order. The natural C code *
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* shown in this file on the other hand, accepts the inputs in nor- *
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* mal order and produces the outputs in normal order. *
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* *
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* Several transformations have been applied to the original Cooley *
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* Tukey code to produce the natural C code description shown here. *
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* In order to understand these it would first be educational to *
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* understand some of the issues involved in the conventional Cooley *
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* Tukey FFT code. *
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* *
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* void radix4(int n, short x[], short wn[]) *
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* { *
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* int n1, n2, ie, ia1, ia2, ia3; *
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* int i0, i1, i2, i3, i, j, k; *
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* short co1, co2, co3, si1, si2, si3; *
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* short xt0, yt0, xt1, yt1, xt2, yt2; *
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* short xh0, xh1, xh20, xh21, xl0, xl1,xl20,xl21; *
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* *
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* n2 = n; *
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* ie = 1; *
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* for (k = n; k > 1; k >>= 2) *
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* { *
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* n1 = n2; *
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* n2 >>= 2; *
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* ia1 = 0; *
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* *
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* for (j = 0; j < n2; j++) *
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* { *
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* ia2 = ia1 + ia1; *
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* ia3 = ia2 + ia1; *
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* *
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* co1 = wn[2 * ia1 ]; *
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* si1 = wn[2 * ia1 + 1]; *
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* co2 = wn[2 * ia2 ]; *
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* si2 = wn[2 * ia2 + 1]; *
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* co3 = wn[2 * ia3 ]; *
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* si3 = wn[2 * ia3 + 1]; *
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* ia1 = ia1 + ie; *
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* *
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* for (i0 = j; i0< n; i0 += n1) *
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* { *
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* i1 = i0 + n2; *
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* i2 = i1 + n2; *
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* i3 = i2 + n2; *
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* *
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* *
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* xh0 = x[2 * i0 ] + x[2 * i2 ]; *
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* xh1 = x[2 * i0 + 1] + x[2 * i2 + 1]; *
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* xl0 = x[2 * i0 ] - x[2 * i2 ]; *
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* xl1 = x[2 * i0 + 1] - x[2 * i2 + 1]; *
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* *
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* xh20 = x[2 * i1 ] + x[2 * i3 ]; *
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* xh21 = x[2 * i1 + 1] + x[2 * i3 + 1]; *
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* xl20 = x[2 * i1 ] - x[2 * i3 ]; *
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* xl21 = x[2 * i1 + 1] - x[2 * i3 + 1]; *
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* *
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* x[2 * i0 ] = xh0 + xh20; *
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* x[2 * i0 + 1] = xh1 + xh21; *
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* *
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* xt0 = xh0 - xh20; *
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* yt0 = xh1 - xh21; *
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* xt1 = xl0 + xl21; *
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* yt2 = xl1 + xl20; *
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* xt2 = xl0 - xl21; *
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* yt1 = xl1 - xl20; *
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* *
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* x[2 * i1 ] = (xt1 * co1 + yt1 * si1) >> 15; *
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* x[2 * i1 + 1] = (yt1 * co1 - xt1 * si1) >> 15; *
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* x[2 * i2 ] = (xt0 * co2 + yt0 * si2) >> 15; *
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* x[2 * i2 + 1] = (yt0 * co2 - xt0 * si2) >> 15; *
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* x[2 * i3 ] = (xt2 * co3 + yt2 * si3) >> 15; *
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* x[2 * i3 + 1] = (yt2 * co3 - xt2 * si3) >> 15; *
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* } *
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* } *
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* *
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* ie <<= 2; *
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* } *
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* } *
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* *
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* The conventional Cooley Tukey FFT, is written using three loops. *
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* The outermost loop "k" cycles through the stages. There are log *
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* N to the base 4 stages in all. The loop "j" cycles through the *
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* groups of butterflies with different twiddle factors, loop "i" *
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* reuses the twiddle factors for the different butterflies within *
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* a stage. It is interesting to note the following: *
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* *
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*-------------------------------------------------------------------------- *
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* Stage# #Groups # Butterflies with common #Groups*Bflys *
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* twiddle factors *
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*-------------------------------------------------------------------------- *
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* 1 N/4 1 N/4 *
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* 2 N/16 4 N/4 *
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* .. *
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* logN 1 N/4 N/4 *
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*-------------------------------------------------------------------------- *
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* *
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* The following statements can be made based on above observations: *
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* *
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* a) Inner loop "i0" iterates a veriable number of times. In *
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* particular the number of iterations quadruples every time from *
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* 1..N/4. Hence software pipelining a loop that iterates a vraiable *
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* number of times is not profitable. *
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* *
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* b) Outer loop "j" iterates a variable number of times as well. *
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* However the number of iterations is quartered every time from *
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* N/4 . . Hence the behaviour in (a) and (b) are exactly opposite *
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* to each other. *
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* *
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* c) If the two loops "i" and "j" are colaesced together then they *
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* will iterate for a fixed number of times namely N/4. This allows *
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* us to combine the "i" and "j" loops into 1 loop. Optimized impl- *
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* ementations will make use of this fact. *
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* *
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* In addition the Cooley Tukey FFT accesses three twiddle factors *
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* per iteration of the inner loop, as the butterflies that re-use *
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* twiddle factors are lumped together. This leads to accessing the *
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* twiddle factor array at three points each sepearted by "ie". Note *
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* that "ie" is initially 1, and is quadrupled with every iteration. *
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* Therfore these three twiddle factors are not even contiguous in *
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* the array. *
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* *
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* In order to vectorize the FFT, it is desirable to access twiddle *
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* factor array using double word wide loads and fetch the twiddle *
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* factors needed. In order to do this a modified twiddle factor *
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* array is created, in which the factors WN/4, WN/2, W3N/4 are *
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* arranged to be contiguous. This eliminates the seperation between *
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* twiddle factors within a butterfly. However this implies that as *
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* the loop is traversed from one stage to another, that we maintain *
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* a redundant version of the twiddle factor array. Hence the size *
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* of the twiddle factor array increases as compared to the normal *
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* Cooley Tukey FFT. The modified twiddle factor array is of size *
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* "2 * N" where the conventional Cooley Tukey FFT is of size"3N/4" *
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* where N is the number of complex points to be transformed. The *
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* routine that generates the modified twiddle factor array was *
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* presented earlier. With the above transformation of the FFT, *
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* both the input data and the twiddle factor array can be accessed *
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* using double-word wide loads to enable packed data processing. *
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* *
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* The final stage is optimised to remove the multiplication as *
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* w0 = 1. This stage also performs digit reversal on the data, *
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* so the final output is in natural order. *
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* *
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* The fft() code shown here performs the bulk of the computation *
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* in place. However, because digit-reversal cannot be performed *
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* in-place, the final result is written to a separate array, y[]. *
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* *
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* There is one slight break in the flow of packed processing that *
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* needs to be comprehended. The real part of the complex number is *
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* in the lower half, and the imaginary part is in the upper half. *
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* The flow breaks in case of "xl0" and "xl1" because in this case *
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* the real part needs to be combined with the imaginary part because *
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* of the multiplication by "j". This requires a packed quantity like *
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* "xl21xl20" to be rotated as "xl20xl21" so that it can be combined *
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* using add2's and sub2's. Hence the natural version of C code *
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* shown below is transformed using packed data processing as shown: *
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* *
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* xl0 = x[2 * i0 ] - x[2 * i2 ]; *
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* xl1 = x[2 * i0 + 1] - x[2 * i2 + 1]; *
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* xl20 = x[2 * i1 ] - x[2 * i3 ]; *
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* xl21 = x[2 * i1 + 1] - x[2 * i3 + 1]; *
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* *
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* xt1 = xl0 + xl21; *
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* yt2 = xl1 + xl20; *
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* xt2 = xl0 - xl21; *
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* yt1 = xl1 - xl20; *
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* *
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* xl1_xl0 = _sub2(x21_x20, x21_x20) *
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* xl21_xl20 = _sub2(x32_x22, x23_x22) *
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* xl20_xl21 = _rotl(xl21_xl20, 16) *
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* *
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* yt2_xt1 = _add2(xl1_xl0, xl20_xl21) *
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* yt1_xt2 = _sub2(xl1_xl0, xl20_xl21) *
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* *
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* Also notice that xt1, yt1 endup on seperate words, these need to *
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* be packed together to take advantage of the packed twiddle fact *
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* ors that have been loaded. In order for this to be achieved they *
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* are re-aligned as follows: *
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* *
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* yt1_xt1 = _packhl2(yt1_xt2, yt2_xt1) *
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* yt2_xt2 = _packhl2(yt2_xt1, yt1_xt2) *
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* *
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* The packed words "yt1_xt1" allows the loaded"sc" twiddle factor *
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* to be used for the complex multiplies. The real part os the *
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* complex multiply is implemented using _dotp2. The imaginary *
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* part of the complex multiply is implemented using the 16x32 *
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* multiply instruction "mpylir" or "mpyhir". *
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* *
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* (X + jY) ( C + j S) = (XC + YS) + j (YC - XS). *
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* *
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* The actual twiddle factors for the FFT are cosine, - sine. The *
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* twiddle factors stored in the table are csine and sine, hence *
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* the sign of the "sine" term is comprehended during multipli- *
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* cation as shown above. *
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* *
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* MEMORY NOTE *
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* The optimized implementations are written for LITTLE ENDIAN. *
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* *
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* INTERRUPTS *
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* This code is interrupt tolerant but not interruptible. It masks out *
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* interrupts for the entire duration of the code. *
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* *
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* CYCLES *
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* (13 * N/8 + 24) * ceil(log4(N) - 1) + (N + 8) * 1.5 + 27 *
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* *
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* N = 512, (13 * 64 + 24) * 4 + 520 * 1.5 + 27 = 4231 cycles *
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* *
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* CODESIZE *
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* 1068 bytes *
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* ------------------------------------------------------------------------- *
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* Copyright (c) 2003 Texas Instruments, Incorporated. *
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* All Rights Reserved. *
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* ========================================================================= *
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*============================================================================*
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.global _DSP_fft16x32
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* ========================================================================= *
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* End of file: dsp_fft16x32.h64 *
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* ------------------------------------------------------------------------- *
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* Copyright (c) 2003 Texas Instruments, Incorporated. *
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* All Rights Reserved. *
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* ========================================================================= *
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