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annotate libgsmfr2/prng.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 573afa985df6
children
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1 /*
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2 * We use a pseudorandom sequence generator function in two places:
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3 * when generating comfort noise (GSM 06.12 section 6.1), and when
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4 * randomizing grid position parameters as part of speech muting
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5 * during error handling (GSM 06.11 section 6).
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6 *
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7 * Our PRNG is a copy of the pseudonoise() function from ETSI EFR code,
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8 * where it is used for similar purposes.
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9 */
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11 #include <stdint.h>
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12 #include "tw_gsmfr.h"
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13 #include "pp_state.h"
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14 #include "pp_internal.h"
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16 uint16_t gsmfr_preproc_prng(struct gsmfr_preproc_state *st, uint16_t no_bits)
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17 {
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18 uint16_t noise_bits, Sn, i;
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20 noise_bits = 0;
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21 for (i = 0; i < no_bits; i++)
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22 {
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23 /* State n == 31 */
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24 if ((st->cn_random_lfsr & 0x00000001L) != 0)
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25 {
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26 Sn = 1;
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27 }
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28 else
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29 {
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30 Sn = 0;
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31 }
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32
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33 /* State n == 3 */
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34 if ((st->cn_random_lfsr & 0x10000000L) != 0)
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35 {
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36 Sn = Sn ^ 1;
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37 }
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38 else
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39 {
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40 Sn = Sn ^ 0;
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41 }
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43 noise_bits = noise_bits << 1;
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44 noise_bits = noise_bits | st->cn_random_lfsr & 1;
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45
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46 st->cn_random_lfsr >>= 1;
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47 if (Sn & 1)
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48 {
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49 st->cn_random_lfsr |= 0x40000000L;
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50 }
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51 }
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53 return noise_bits;
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54 }