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annotate libgsmfr2/tfo_dtxd.c @ 556:18aca50d68df default tip

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doc/Calypso-TCH-downlink: update for FR1 BFI-with-data
author Mychaela Falconia <falcon@freecalypso.org>
date Fri, 11 Oct 2024 01:54:00 +0000
parents 5a3535470c95
children
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5a3535470c95 libgsmfr2 TFO: implement DTXd function
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1 /*
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2 * The function implemented in this module is a post-processor to be invoked
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3 * after Rx DTX preproc functions if the Rx DTX handler block is used as a
5a3535470c95 libgsmfr2 TFO: implement DTXd function
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4 * TFO transform with DTXd enabled. It selectively transforms certain
5a3535470c95 libgsmfr2 TFO: implement DTXd function
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5 * preproc output frames (CN and silence frames specifically) into SID,
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6 * thereby allowing the network operator to reap the benefits of DTX on
5a3535470c95 libgsmfr2 TFO: implement DTXd function
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7 * the radio downlink leg.
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8 */
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9
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10 #include <stdint.h>
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11 #include "tw_gsmfr.h"
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12 #include "pp_state.h"
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13
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14 int gsmfr_tfo_xfrm_dtxd(struct gsmfr_preproc_state *st, uint8_t *frame)
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15 {
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16 uint8_t *p, sub;
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17
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18 if (!st->dtxd_sid_flag)
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19 return 1; /* SP=1: output is a speech frame */
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20 /* turn the output frame into SID */
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21 p = frame + 5; /* skip magic+LARc */
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22 for (sub = 0; sub < 4; sub++) {
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23 *p++ = 0;
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24 *p++ &= 0x1F; /* upper 5 bits of Xmaxc field */
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25 *p++ &= 0x80; /* and the lsb spilling into the next byte */
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26 *p++ = 0;
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27 *p++ = 0;
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28 *p++ = 0;
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29 *p++ = 0;
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30 }
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31 return 0; /* SP=0: output is SID */
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32 }