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comparison fir/freqresp.c @ 376:9b3e5be96bab

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fir2freq: a tool for analyzing captured FIR coefficient sets
author Mychaela Falconia <falcon@freecalypso.org>
date Mon, 02 Aug 2021 04:59:46 +0000
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375:6057c98d11a9 376:9b3e5be96bab
1 /*
2 * This program computes the frequency response of a Calypso DSP FIR filter
3 * whose coefficients have been captured somewhere out in the wild.
4 * The math has been taken from section 2.2.2 of this article:
5 *
6 * https://dspguru.com/dsp/faqs/fir/properties/
7 */
8
9 #include <math.h>
10 #include <stdio.h>
11 #include <stdlib.h>
12
13 #define NCOEFF 31
14
15 int coeff_int[NCOEFF];
16 float coeff_float[NCOEFF];
17 unsigned nsteps;
18 float freq_step, omega_step;
19 float freq, omega;
20
21 static void
22 coeff_to_float()
23 {
24 unsigned n;
25
26 for (n = 0; n < NCOEFF; n++)
27 coeff_float[n] = coeff_int[n] / 16384.0f;
28 }
29
30 static void
31 do_one_freq()
32 {
33 unsigned n;
34 float angle_arg, rsum, isum;
35 float gain, db;
36
37 angle_arg = 0;
38 rsum = 0;
39 isum = 0;
40 for (n = 0; n < NCOEFF; n++) {
41 rsum += coeff_float[n] * cosf(angle_arg);
42 isum -= coeff_float[n] * sinf(angle_arg);
43 angle_arg += omega;
44 }
45 gain = hypotf(rsum, isum);
46 db = log10f(gain) * 20.0f;
47 printf("%.2f\t%f\t%.2f\n", freq, gain, db);
48 }
49
50 main(argc, argv)
51 char **argv;
52 {
53 unsigned n;
54
55 if (argc != 3) {
56 fprintf(stderr, "usage: %s coeff-file nsteps\n", argv[0]);
57 exit(1);
58 }
59 if (read_fir_coeff_table_int(argv[1], coeff_int) < 0)
60 exit(1); /* error msg already printed */
61 coeff_to_float();
62 nsteps = strtoul(argv[2], 0, 0);
63 if (nsteps < 1) {
64 fprintf(stderr, "error: nsteps argument must be >= 1\n");
65 exit(1);
66 }
67 freq_step = 4000.0f / nsteps;
68 omega_step = M_PI / nsteps;
69 n = 0;
70 freq = 0;
71 omega = 0;
72 for (;;) {
73 do_one_freq();
74 if (n >= nsteps)
75 break;
76 n++;
77 freq += freq_step;
78 omega += omega_step;
79 }
80 exit(0);
81 }