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annotate dev/u2s-regen.c @ 301:019eed8b1948

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doc/FR1-library-history: new article
author Mychaela Falconia <falcon@freecalypso.org>
date Mon, 15 Apr 2024 18:36:21 +0000
parents 20750ffb1c3e
children
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1 /*
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2 * This program generates a G.711 mu-law decoding table of the same form
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3 * as the u2s[] table in the toast_ulaw.c module in libgsm/toast; the intent
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4 * is to replace that incorrect table with a corrected one. The "engine"
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5 * function that does the computation is based on ulaw_expand() from ITU-T
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6 * G.191 STL.
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7 */
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8
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9 #include <stdio.h>
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10 #include <stdlib.h>
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12 static unsigned
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13 ulaw_expand (input)
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14 unsigned input;
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15 {
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16 short segment; /* segment (Table 2/G711, column 1) */
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17 short mantissa; /* low nibble of log companded sample */
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18 short exponent; /* high nibble of log companded sample */
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19 short sign; /* sign of output sample */
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20 short step;
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21 short output;
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23 sign = input < (0x0080) /* sign-bit = 1 for positiv values */
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24 ? -1 : 1;
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25 mantissa = ~input; /* 1's complement of input value */
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26 exponent = (mantissa >> 4) & (0x0007); /* extract exponent */
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27 segment = exponent + 1; /* compute segment number */
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28 mantissa = mantissa & (0x000F); /* extract mantissa */
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29
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30 /* Compute Quantized Sample (14 bit left justified!) */
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31 step = (4) << segment; /* position of the LSB */
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32 /* = 1 quantization step) */
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33 output = sign * /* sign */
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34 (((0x0080) << exponent) /* '1', preceding the mantissa */
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35 +step * mantissa /* left shift of mantissa */
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36 + step / 2 /* 1/2 quantization step */
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37 - 4 * 33);
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38 return output & 0xFFFF;
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39 }
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41 main(argc, argv)
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42 char **argv;
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43 {
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44 unsigned input, output;
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45
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46 for (input = 0; input < 256; input++) {
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47 output = ulaw_expand(input);
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48 printf("%6u,", output);
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49 if ((input & 7) == 7)
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50 putchar('\n');
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51 }
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52 exit(0);
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53 }