FreeCalypso > hg > gsm-codec-lib
annotate doc/FR1-library-API @ 373:128ec87489b6
libtwamr: integrate q_plsf_5.c
author | Mychaela Falconia <falcon@freecalypso.org> |
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date | Mon, 06 May 2024 04:03:21 +0000 |
parents | 59751c8fc773 |
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rev | line source |
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1 Libgsmfr2 general usage |
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2 ======================= |
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3 |
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4 The external public interface to Themyscira libgsmfr2 consists of a single |
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5 header file <tw_gsmfr.h>; it should be installed in some system include |
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6 directory. |
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7 |
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8 The dialect of C used by all Themyscira GSM codec libraries is ANSI C (function |
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9 prototypes), const qualifier is used where appropriate, and the interface is |
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10 defined in terms of <stdint.h> types; <tw_gsmfr.h> includes <stdint.h>. The |
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11 use of old libgsm defined types (gsm_byte, gsm_frame and gsm_signal) has been |
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12 abolished in the migration from libgsm+libgsmfrp to libgsmfr2. |
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13 |
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14 GSM 06.10 encoder and decoder |
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15 ============================= |
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16 |
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17 Both the encoder and the decoder are stateful; each running instance of either |
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18 element needs its own state structure. However, this GSM 06.10 component of |
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19 libgsmfr2 shares a peculiar property with old libgsm from which it was derived: |
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20 the same state structure (struct gsmfr_0610_state) is used by both entities. |
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21 Needless to say, each given instance of struct gsmfr_0610_state must be used |
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22 for only one purpose, either for the encoder or for the decoder; mixing calls |
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23 to encoder and decoder functions with the same state structure is an invalid |
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24 operation with undefined results. |
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25 |
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26 State structures for the basic encoder or decoder are allocated with this |
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27 function: |
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28 |
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29 struct gsmfr_0610_state *gsmfr_0610_create(void); |
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30 |
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31 This function allocates dynamic memory for the state structure with malloc() |
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32 (the size of the struct is internal to the library and not exposed) and returns |
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33 a pointer to the allocated and initialized struct if successful, or NULL if |
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34 malloc() fails. The state structure is malloc'ed as a single chunk, hence when |
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35 you are done with it, simply free() it. |
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36 |
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37 The initialization or reset portion of gsmfr_0610_create() operation can always |
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38 be repeated with this function: |
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39 |
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40 void gsmfr_0610_reset(struct gsmfr_0610_state *state); |
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41 |
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42 Immediately after gsmfr_0610_create() or gsmfr_0610_reset(), the "virgin" state |
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43 structure can be used either for the encoder or for the decoder; however, once |
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44 that state struct has been passed to functions of either group, it can only be |
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45 used for that functional group. |
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46 |
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47 Encoder specifics |
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48 ----------------- |
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49 |
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50 The most elementary single-frame processing function of libgsmfr2 GSM 06.10 |
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51 encoder is: |
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52 |
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53 void gsmfr_0610_encode_params(struct gsmfr_0610_state *st, const int16_t *pcm, |
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54 struct gsmfr_param_frame *param); |
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55 |
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56 The input is an array of 160 linear PCM samples (left-justified in int16_t), |
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57 and the output is this structure: |
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58 |
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59 struct gsmfr_param_frame { |
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60 int16_t LARc[8]; |
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61 int16_t Nc[4]; |
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62 int16_t bc[4]; |
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63 int16_t Mc[4]; |
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64 int16_t xmaxc[4]; |
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65 int16_t xMc[4][13]; |
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66 }; |
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67 |
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68 Most of the time the following wrapper function is more useful: |
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69 |
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70 void gsmfr_0610_encode_frame(struct gsmfr_0610_state *st, const int16_t *pcm, |
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71 uint8_t *frame); |
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72 |
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73 The output is a 33-byte buffer, filled with the encoded GSM-FR speech frame in |
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74 the RTP format specified in ETSI TS 101 318 and IETF RFC 3551. |
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75 |
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76 If the optional encoder homing feature is desired, call this function right |
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77 after the call to gsmfr_0610_encode_frame() or gsmfr_0610_encode_params(): |
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78 |
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79 void gsmfr_0610_encoder_homing(struct gsmfr_0610_state *st, const int16_t *pcm); |
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80 |
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81 This function checks to see if the PCM frame (160 linear PCM samples) is an EHF; |
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82 if the input frame is indeed EHF, the function calls gsmfr_0610_reset(). |
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83 |
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84 Decoder specifics |
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85 ----------------- |
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86 |
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87 The internal native form of the 06.10 decoder once again uses |
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88 struct gsmfr_param_frame: |
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89 |
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90 void gsmfr_0610_decode_params(struct gsmfr_0610_state *st, |
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91 const struct gsmfr_param_frame *param, |
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92 int16_t *pcm); |
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93 |
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94 The more commonly used RTP-format version is: |
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95 |
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96 void gsmfr_0610_decode_frame(struct gsmfr_0610_state *st, const uint8_t *frame, |
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97 int16_t *pcm); |
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98 |
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99 Please note: |
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100 |
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101 1) The basic GSM 06.10 decoder is just that: there is no SID recognition or DTX |
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102 handling, every possible input bit pattern will be interpreted and decoded |
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103 as a GSM 06.10 speech frame. |
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104 |
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105 2) There is no decoder homing function at this layer, and no check for DHF. |
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106 |
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107 3) The RTP signature nibble 0xD is ignored (not checked) by |
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108 gsmfr_0610_decode_frame(). |
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109 |
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110 Rx DTX preprocessor block |
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111 ========================= |
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112 |
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113 The Rx DTX preprocessor is its own stateful element, independent from the 06.10 |
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114 decoder to which it is usually coupled. Libgsmfr2 provides a "fulldec" wrapper |
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115 that incorporates both elements, but the ability to use the Rx DTX preprocessor |
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116 by itself still remains, unchanged from our previous libgsmfrp offering. One |
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117 potential application for this preprocessor by itself, without immediately |
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118 following it with the GSM 06.10 decode step, is the possibility of implementing |
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119 the TFO/TrFO transform of 3GPP TS 28.062 section C.3.2.1.1 for GSM-FR: our Rx |
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120 DTX preprocessor does exactly what that section calls for, specifically in |
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121 "case 1" where the input UL frame stream may contain SIDs and BFI frame gaps, |
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122 but the output must be 100% valid frames and SID-free. |
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123 |
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124 The state structure for this block is struct gsmfr_preproc_state, and it is |
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125 allocated with this function: |
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126 |
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127 struct gsmfr_preproc_state *gsmfr_preproc_create(void); |
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128 |
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129 Like other state structures in Themyscira GSM codec libraries, this opaque |
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130 state is malloc'ed as a single chunk and can be simply freed afterward. A |
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131 reset function is also provided: |
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132 |
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133 void gsmfr_preproc_reset(struct gsmfr_preproc_state *state); |
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134 |
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135 Preprocessing good frames |
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136 ------------------------- |
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137 |
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138 For every good traffic frame (BFI=0) you receive from the radio subsystem, you |
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139 need to call this preprocessor function: |
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140 |
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141 void gsmfr_preproc_good_frame(struct gsmfr_preproc_state *state, |
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142 uint8_t *frame); |
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143 |
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144 The second argument is both input and output, i.e., the frame is modified in |
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145 place. If the received frame is not SID (specifically, if the SID field |
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146 deviates from the SID codeword by 16 or more bits, per GSM 06.31 section 6.1.1), |
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147 then the frame (considered a good speech frame) will be left unmodified (i.e., |
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148 it is to be passed unchanged to the GSM 06.10 decoder), but preprocessor state |
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149 will be updated. OTOH, if the received frame is classified as either valid or |
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150 invalid SID per GSM 06.31, then the output frame will contain comfort noise |
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151 generated by the preprocessor using a PRNG, or a silence frame in one particular |
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152 corner case. |
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153 |
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154 GSM-FR RTP (originally libgsm) 0xD magic: the upper nibble of the first byte |
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155 can be anything on input to gsmfr_preproc_good_frame(), but the output frame |
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156 will always have the correct magic in it. |
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157 |
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158 Handling BFI conditions |
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159 ----------------------- |
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160 |
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161 If you received a lost/missing frame indication instead of a good traffic frame, |
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162 call this preprocessor function: |
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163 |
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164 void gsmfr_preproc_bfi(struct gsmfr_preproc_state *state, int taf, |
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165 uint8_t *frame_out); |
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166 |
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167 TAF is a flag defined in GSM 06.31 section 6.1.1; if you don't have this flag, |
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168 pass 0 - you will lose the function of comfort noise muting in the event of |
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169 prolonged SID loss, but all other Rx DTX functions will still work the same. |
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170 |
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171 With this function the 33-byte frame buffer is only an output, i.e., prior |
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172 buffer content is a don't-care and there is no provision for making any use of |
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173 erroneous frames like in EFR. The frame generated by the preprocessor may be |
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174 substitution/muting, comfort noise or silence depending on the state. |
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175 |
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176 GSM-FR full decoder |
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177 =================== |
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178 |
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179 The full decoder is a high-level feature of libgsmfr2, incorporating both the |
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180 Rx DTX preprocessor block and the GSM 06.10 decoder block. The state structure |
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181 for the full decoder (struct gsmfr_fulldec_state) internally incorporates both |
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182 struct gsmfr_0610_state and gsmfr_preproc_state, but because it is implemented |
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183 inside libgsmfr2, it is still malloc'ed as a single chunk and can thus be |
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184 released with a single free() call. The functions for allocating and |
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185 initializing this state follow the established pattern: |
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186 |
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187 struct gsmfr_fulldec_state *gsmfr_fulldec_create(void); |
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188 |
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189 void gsmfr_fulldec_reset(struct gsmfr_fulldec_state *state); |
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190 |
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191 The reset function internally calls gsmfr_0610_reset() and |
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192 gsmfr_preproc_reset(), initializing both processing blocks. |
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193 |
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194 Frame processing functions are also straightforward: |
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195 |
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196 void gsmfr_fulldec_good_frame(struct gsmfr_fulldec_state *state, |
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197 const uint8_t *frame, int16_t *pcm); |
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198 |
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199 void gsmfr_fulldec_bfi(struct gsmfr_fulldec_state *state, int taf, |
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200 int16_t *pcm); |
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201 |
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202 These functions follow the same pattern as gsmfr_preproc_good_frame() and |
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203 gsmfr_preproc_bfi(), but the output is a 160-sample linear PCM buffer. Also |
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204 note that the frame input to gsmfr_fulldec_good_frame() is const, unlike the |
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205 situation with gsmfr_preproc_good_frame() - the copying into a scratchpad |
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206 buffer (on the stack) happens inside this "fulldec" wrapper. |
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207 |
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208 The "fulldec" layer also adds the decoder homing feature: |
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209 gsmfr_fulldec_good_frame() detects decoder homing frames and invokes |
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210 gsmfr_fulldec_reset() when required, and also implements EHF output per the |
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211 spec. |
298
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212 |
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213 Stateless utility functions |
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214 =========================== |
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215 |
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216 Conversions between RTP packed format and broken-down codec parameters are |
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217 stateless and implemented with highly efficient code. There are two versions; |
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218 this version converts between packed frames and struct gsmfr_param_frame used |
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219 by 06.10 encoder and decoder functions: |
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220 |
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221 void gsmfr_pack_frame(const struct gsmfr_param_frame *param, uint8_t *frame); |
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222 void gsmfr_unpack_frame(const uint8_t *frame, struct gsmfr_param_frame *param); |
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223 |
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224 and this version converts between packed frames and a straight linear array of |
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225 76 parameters: |
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226 |
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227 void gsmfr_pack_from_array(const int16_t *params, uint8_t *frame); |
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228 void gsmfr_unpack_to_array(const uint8_t *frame, int16_t *params); |
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229 |
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230 The latter functions gsmfr_pack_from_array() and gsmfr_unpack_to_array() are |
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231 drop-in replacements for gsm_implode() and gsm_explode() from old libgsm. The |
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232 order of parameters in this array is the canonical one: first all LARc, then |
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233 all params for the first subframe, then the second subframe, then the third and |
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234 the fourth. OTOH, struct gsmfr_param_frame uses functional grouping, chosen |
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235 for ease of porting of original libgsm code. |
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236 |
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237 Both unpacking functions (gsmfr_unpack_frame() and gsmfr_unpack_to_array()) |
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238 ignore the upper nibble of the first byte, i.e., the 0xD signature is not |
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239 enforced. However, this signature is always set correctly by gsmfr_pack_frame() |
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240 and gsmfr_pack_from_array(), and also by gsmfr_0610_encode_frame() function |
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241 which calls gsmfr_pack_frame() as its finishing step. |
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242 |
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243 The last remaining stateless utility function performs SID classification of |
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244 received GSM-FR frames: |
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245 |
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246 int gsmfr_preproc_sid_classify(const uint8_t *frame); |
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247 |
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248 This function analyzes an RTP-encoded FR frame (the upper nibble of the first |
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249 byte is NOT checked for 0xD signature) for the SID codeword of GSM 06.12 and |
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250 classifies the frame as SID=0, SID=1 or SID=2 per the rules of GSM 06.31 |
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251 section 6.1.1. This classification is the first processing step performed by |
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252 gsmfr_preproc_good_frame(). |
299
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253 |
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254 Public constant definitions |
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255 =========================== |
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256 |
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257 Our public header file <tw_gsmfr.h> provides these constant definitions, which |
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258 should be self-explanatory: |
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259 |
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260 #define GSMFR_RTP_FRAME_LEN 33 |
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261 #define GSMFR_NUM_PARAMS 76 |
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262 |
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263 Public const data items |
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264 ======================= |
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265 |
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266 There are two special GSM-FR frame bit patterns defined in the specs: there is |
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267 the silence frame of GSM 06.11, and there is the decoder homing frame specified |
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268 in later versions of GSM 06.10. RTP-packed representations of both frames are |
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269 included in libgsmfr2, and are made public: |
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270 |
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271 extern const uint8_t gsmfr_preproc_silence_frame[GSMFR_RTP_FRAME_LEN]; |
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272 extern const uint8_t gsmfr_decoder_homing_frame[GSMFR_RTP_FRAME_LEN]; |