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annotate dev/s2u-regen.c @ 236:4c7d0dc1eecb

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doc/PCM8-conversions: finish encoding description
author Mychaela Falconia <falcon@freecalypso.org>
date Mon, 08 May 2023 03:28:40 +0000
parents a5200ad12d58
children
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1 /*
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2 * This program generates a G.711 mu-law encoding table of the same form
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3 * as the s2u[] table in the toast_ulaw.c module in libgsm/toast, i.e.,
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4 * an encoding table that takes only the upper 13 bits of the linear PCM
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5 * sample to be encoded, even though canonical mu-law encoding requires
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6 * 14-bit input. The 13-bit table constructed by this program is computed
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7 * as if the lsb of the "proper" 14-bit input is always zero, like it is
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8 * expected to be when the linear PCM samples came from the output of a
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9 * GSM speech decoder.
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10 *
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11 * The "engine" function that does the computation is based on ulaw_compress()
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12 * from ITU-T G.191 STL.
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13 */
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15 #include <stdio.h>
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16 #include <stdlib.h>
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18 static unsigned
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19 ulaw_compress(input)
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20 unsigned input;
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21 {
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22 short i; /* aux.var. */
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23 short absno; /* absolute value of linear (input) sample */
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24 short segno; /* segment (Table 2/G711, column 1) */
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25 short low_nibble; /* low nibble of log companded sample */
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26 short high_nibble; /* high nibble of log companded sample */
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27 unsigned output;
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29 /* -------------------------------------------------------------------- */
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30 /* Input is 14-bit right-justified in this version */
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31 /* Compute absolute value; adjust for easy processing */
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32 /* -------------------------------------------------------------------- */
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33 absno = input >= 0x2000 /* compute 1's complement in case of */
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34 ? (~input & 0x1FFF) + 33 /* negative samples */
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35 : input + 33; /* NB: 33 is the difference value */
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36 /* between the thresholds for */
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37 /* A-law and u-law. */
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38 if (absno > (0x1FFF)) /* limitation to "absno" < 8192 */
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39 absno = (0x1FFF);
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41 /* Determination of sample's segment */
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42 i = absno >> 6;
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43 segno = 1;
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44 while (i != 0) {
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45 segno++;
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46 i >>= 1;
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47 }
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49 /* Mounting the high-nibble of the log-PCM sample */
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50 high_nibble = (0x0008) - segno;
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52 /* Mounting the low-nibble of the log PCM sample */
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53 low_nibble = (absno >> segno) /* right shift of mantissa and */
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54 &(0x000F); /* masking away leading '1' */
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55 low_nibble = (0x000F) - low_nibble;
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57 /* Joining the high-nibble and the low-nibble of the log PCM sample */
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58 output = (high_nibble << 4) | low_nibble;
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59
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60 /* Add sign bit */
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61 if (input < 0x2000)
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62 output = output | (0x0080);
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63
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64 return output;
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65 }
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67 main(argc, argv)
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68 char **argv;
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69 {
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70 unsigned input, output;
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71
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72 for (input = 0; input < 8192; input++) {
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73 output = ulaw_compress(input << 1);
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74 printf("%04o,", output);
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75 if ((input % 15) == 14 || input == 8191)
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76 putchar('\n');
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77 }
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78 exit(0);
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79 }