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annotate dev/a2s-regen.c @ 550:de333989a12b

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document gsm[e]fr-decode-tw5[-r] utilities
author Mychaela Falconia <falcon@freecalypso.org>
date Sat, 05 Oct 2024 02:16:48 +0000
parents ba737a0203e2
children
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1 /*
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2 * This program regenerates a G.711 A-law decoding table equivalent to the
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3 * a2s[] 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_expand() 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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11 static unsigned
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12 alaw_expand (input)
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13 unsigned input;
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14 {
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15 short ix, mant, iexp;
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17 ix = input ^ (0x0055); /* re-toggle toggled bits */
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19 ix &= (0x007F); /* remove sign bit */
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20 iexp = ix >> 4; /* extract exponent */
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21 mant = ix & (0x000F); /* now get mantissa */
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22 if (iexp > 0)
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23 mant = mant + 16; /* add leading '1', if exponent > 0 */
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25 mant = (mant << 4) + (0x0008); /* now mantissa left justified and */
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26 /* 1/2 quantization step added */
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27 if (iexp > 1) /* now left shift according exponent */
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28 mant = mant << (iexp - 1);
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30 if (input & 0x80) /* invert, if negative sample */
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31 return mant;
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32 else
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33 return -mant & 0xFFFF;
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34 }
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36 main(argc, argv)
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37 char **argv;
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38 {
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39 unsigned input, output;
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40
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41 for (input = 0; input < 256; input++) {
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42 output = alaw_expand(input);
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43 printf("%6u,", output);
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44 if ((input & 7) == 7)
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45 putchar('\n');
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46 }
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47 exit(0);
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48 }