FreeCalypso > hg > gsm-codec-lib
annotate libgsmfr2/prng.c @ 269:bd2271cb95d4
libgsmfr2: integrate preprocess.c from libgsm
author | Mychaela Falconia <falcon@freecalypso.org> |
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date | Sun, 14 Apr 2024 01:58:35 +0000 |
parents | 573afa985df6 |
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libgsmfrp: implement comfort noise generation
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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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10 |
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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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15 |
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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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19 |
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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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42 |
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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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52 |
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53 return noise_bits; |
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54 } |