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annotate libgsmfrp/prng.c @ 183:452c1d5a6268

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libgsmefr BFI w/o data: emit zero output after decoder reset In real-life usage, each EFR decoder session will most likely begin with lots of BFI frames before the first real frame arrives. However, because the spec-defined home state of the decoder is speech rather than CN, our regular logic for BFI w/o data would have to feed pseudorandom noise to the decoder (in the "fixed codebook excitation pulses" part), which is silly to do at the beginning of the decoder session right out of reset. Therefore, let's check reset_flag_old, and if we are still in the reset state, simply emit zero output.
author Mychaela Falconia <falcon@freecalypso.org>
date Tue, 03 Jan 2023 00:12:18 +0000
parents 3b64f255689a
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 "gsm_fr_preproc.h"
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13 #include "internal.h"
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15 uint16_t gsmfr_preproc_prng(struct gsmfr_preproc_state *st, uint16_t no_bits)
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16 {
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17 uint16_t noise_bits, Sn, i;
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18
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19 noise_bits = 0;
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20 for (i = 0; i < no_bits; i++)
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21 {
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22 /* State n == 31 */
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23 if ((st->cn_random_lfsr & 0x00000001L) != 0)
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24 {
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25 Sn = 1;
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26 }
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27 else
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28 {
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29 Sn = 0;
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30 }
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31
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32 /* State n == 3 */
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33 if ((st->cn_random_lfsr & 0x10000000L) != 0)
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34 {
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35 Sn = Sn ^ 1;
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36 }
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37 else
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38 {
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39 Sn = Sn ^ 0;
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40 }
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42 noise_bits = noise_bits << 1;
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43 noise_bits = noise_bits | st->cn_random_lfsr & 1;
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44
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45 st->cn_random_lfsr >>= 1;
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46 if (Sn & 1)
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47 {
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48 st->cn_random_lfsr |= 0x40000000L;
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49 }
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50 }
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51
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52 return noise_bits;
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53 }