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annotate dev/s2u-regen-plus4.c @ 485:751f06541fbb

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doc/Codec-utils: clarify lack of DHF in gsmfr-decode-rb
author Mychaela Falconia <falcon@freecalypso.org>
date Mon, 20 May 2024 01:47:22 +0000
parents 67d60915fbbe
children
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1 /*
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2 * This program is a companion to s2u-regen.c: it computes a mu-law encoding
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3 * table with 13-bit input just like s2u-regen, but computes it as if the
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4 * lsb following the 13-bit part is always 1 rather than always 0. The
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5 * purpose of this program is to illustrate the effect of that lsb on the
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6 * mu-law output.
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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_compress(input)
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14 unsigned input;
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15 {
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16 short i; /* aux.var. */
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17 short absno; /* absolute value of linear (input) sample */
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18 short segno; /* segment (Table 2/G711, column 1) */
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19 short low_nibble; /* low nibble of log companded sample */
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20 short high_nibble; /* high nibble of log companded sample */
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21 unsigned output;
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23 /* -------------------------------------------------------------------- */
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24 /* Input is 14-bit right-justified in this version */
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25 /* Compute absolute value; adjust for easy processing */
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26 /* -------------------------------------------------------------------- */
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27 absno = input >= 0x2000 /* compute 1's complement in case of */
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28 ? (~input & 0x1FFF) + 33 /* negative samples */
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29 : input + 33; /* NB: 33 is the difference value */
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30 /* between the thresholds for */
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31 /* A-law and u-law. */
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32 if (absno > (0x1FFF)) /* limitation to "absno" < 8192 */
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33 absno = (0x1FFF);
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34
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35 /* Determination of sample's segment */
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36 i = absno >> 6;
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37 segno = 1;
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38 while (i != 0) {
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39 segno++;
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40 i >>= 1;
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41 }
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42
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43 /* Mounting the high-nibble of the log-PCM sample */
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44 high_nibble = (0x0008) - segno;
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46 /* Mounting the low-nibble of the log PCM sample */
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47 low_nibble = (absno >> segno) /* right shift of mantissa and */
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48 &(0x000F); /* masking away leading '1' */
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49 low_nibble = (0x000F) - low_nibble;
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51 /* Joining the high-nibble and the low-nibble of the log PCM sample */
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52 output = (high_nibble << 4) | low_nibble;
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53
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54 /* Add sign bit */
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55 if (input < 0x2000)
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56 output = output | (0x0080);
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57
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58 return output;
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59 }
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61 main(argc, argv)
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62 char **argv;
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63 {
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64 unsigned input, output;
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65
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66 for (input = 0; input < 8192; input++) {
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67 output = ulaw_compress((input << 1) + 1);
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68 printf("%04o,", output);
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69 if ((input % 15) == 14 || input == 8191)
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70 putchar('\n');
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71 }
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72 exit(0);
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73 }