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annotate dev/s2a-regen.c @ 478:936a08cc73ce

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doc/AMR-library-API: describe the decoder
author Mychaela Falconia <falcon@freecalypso.org>
date Sun, 19 May 2024 21:32:31 +0000
parents 3afbc6c64172
children
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1 /*
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2 * This program regenerates a G.711 A-law encoding table equivalent to the
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3 * s2a[] table in the toast_alaw.c module in libgsm/toast; the intent is
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4 * to check that table for correctness. The "engine" function that does
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5 * the computation is based on alaw_compress() from ITU-T G.191 STL.
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6 */
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7
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8 #include <stdio.h>
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9 #include <stdlib.h>
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10
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11 static unsigned
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12 alaw_compress (input)
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13 unsigned input;
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14 {
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15 short ix, iexp;
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16
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17 ix = input >= 2048 /* 0 <= ix < 2048 */
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18 ? ~input & 2047 /* 1's complement for negative values */
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19 : input;
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20
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21 /* Do more, if exponent > 0 */
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22 if (ix > 15) { /* exponent=0 for ix <= 15 */
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23 iexp = 1; /* first step: */
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24 while (ix > 16 + 15) { /* find mantissa and exponent */
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25 ix >>= 1;
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26 iexp++;
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27 }
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28 ix -= 16; /* second step: remove leading '1' */
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29
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30 ix += iexp << 4; /* now compute encoded value */
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31 }
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32 if (input < 2048)
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33 ix |= (0x0080); /* add sign bit */
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34
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35 return ix ^ (0x0055); /* toggle even bits */
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36 }
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37
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38 main(argc, argv)
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39 char **argv;
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40 {
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41 unsigned input, output;
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42
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43 for (input = 0; input < 4096; input++) {
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44 if ((input & 15) == 0)
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45 putchar('\t');
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46 output = alaw_compress(input);
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47 printf("%3u,", output);
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48 if ((input & 15) == 15)
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49 putchar('\n');
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50 }
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51 exit(0);
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52 }