kopia lustrzana https://github.com/xdsopl/robot36
85 wiersze
2.5 KiB
C
85 wiersze
2.5 KiB
C
/*
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robot36 - encode and decode images using SSTV in Robot 36 mode
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Written in 2011 by <Ahmet Inan> <xdsopl@googlemail.com>
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To the extent possible under law, the author(s) have dedicated all copyright and related and neighboring rights to this software to the public domain worldwide. This software is distributed without any warranty.
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You should have received a copy of the CC0 Public Domain Dedication along with this software. If not, see <http://creativecommons.org/publicdomain/zero/1.0/>.
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*/
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#include <math.h>
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#include <stdlib.h>
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#include <complex.h>
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#include "window.h"
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#include "ddc.h"
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void do_ddc(ddc_t *ddc, float *input, float complex *output)
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{
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int in = 0;
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ddc->s[ddc->last] = input[in++];
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ddc->last = (ddc->last + 1) < ddc->samples ? ddc->last + 1 : 0;
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ddc->skip += ddc->L;
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// this works only for L <= M
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for (int k = 0; k < ddc->L; k++) {
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float complex sum = 0.0;
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for (int i = ddc->offset, j = ddc->last; i < ddc->taps; i += ddc->L) {
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sum += ddc->b[i] * ddc->s[j];
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j += j ? - 1 : ddc->samples - 1;
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}
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ddc->offset = (ddc->offset + ddc->M) % ddc->L;
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while (ddc->skip < ddc->M) {
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ddc->s[ddc->last] = input[in++];
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ddc->last = (ddc->last + 1) < ddc->samples ? ddc->last + 1 : 0;
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ddc->skip += ddc->L;
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}
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ddc->skip %= ddc->M;
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output[k] = ddc->osc * sum;
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ddc->osc *= ddc->d;
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// ddc->osc /= cabsf(ddc->osc); // not really needed
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}
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}
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ddc_t *alloc_ddc(float freq, float bw, float step, int taps, int L, int M, float (*window)(float, float, float), float a)
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{
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float lstep = step / (float)L;
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float ostep = step * (float)M / (float)L;
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ddc_t *ddc = malloc(sizeof(ddc_t));
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ddc->taps = taps;
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ddc->samples = (taps + L - 1) / L;
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ddc->b = malloc(sizeof(float complex) * ddc->taps);
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ddc->s = malloc(sizeof(float) * ddc->samples);
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ddc->osc = I;
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ddc->d = cexpf(-I * 2.0 * M_PI * freq * ostep);
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ddc->offset = 0;
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ddc->last = 0;
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ddc->skip = 0;
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ddc->L = L;
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ddc->M = M;
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for (int i = 0; i < ddc->samples; i++)
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ddc->s[i] = 0.0;
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float sum = 0.0;
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for (int i = 0; i < ddc->taps; i++) {
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float N = (float)ddc->taps;
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float n = (float)i;
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float x = n - (N - 1.0) / 2.0;
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float l = 2.0 * M_PI * bw * lstep;
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float w = window(n, ddc->taps, a);
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float h = 0.0 == x ? l / M_PI : sinf(l * x) / (x * M_PI);
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float b = w * h;
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sum += b;
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complex float o = cexpf(I * 2.0 * M_PI * freq * lstep * n);
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ddc->b[i] = b * o * (float)L;
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}
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for (int i = 0; i < ddc->taps; i++)
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ddc->b[i] /= sum;
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return ddc;
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}
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void free_ddc(ddc_t *ddc)
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{
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free(ddc->b);
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free(ddc->s);
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free(ddc);
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}
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