FreeCalypso > hg > gsm-codec-lib
annotate doc/FR1-Rx-DTX @ 181:3a0ee08a3b9d
doc/EFR-rationale: typo fix
author | Mychaela Falconia <falcon@freecalypso.org> |
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date | Mon, 02 Jan 2023 20:09:07 +0000 |
parents | aa4cdab30dc8 |
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1 At the level of provided functionality and architectural structure, ETSI GSM |
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2 specifications for DTX (discontinuous transmission) are very symmetric between |
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3 FR and EFR: the same DTX functionality is specified for both codecs, with the |
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4 same overall architecture. However, there is one important difference: in the |
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5 case of EFR the complete implementation of all DTX functions (for both Tx and |
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6 Rx) forms an integral and inseparable part of the reference codec (implemented |
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7 in C) from the beginning, whereas in the case of FR1 the addition of DTX is |
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8 somewhat of an afterthought. GSM 06.10 defines a "pure" FR codec without any |
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9 DTX functions, and this most basic spec can be and has been implemented in this |
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10 "pure" form - classic Unix libgsm from 1990s is a proper, fully compliant |
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11 implementation of GSM 06.10, but only this spec, without any DTX. In contrast, |
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12 there has never existed a "pure" implementation of GSM 06.60 EFR codec without |
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13 associated Tx and Rx DTX functions. Furthermore, there is an important |
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14 distinction between Tx and Rx DTX handlers for FR1: |
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15 |
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16 * Anyone who seeks to implement Tx DTX for FR1 would have to dig into the guts |
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17 of GSM 06.10 encoder and augment it with VAD and SID encoding functions per |
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18 GSM 06.32 and 06.12 specs. |
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19 |
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20 * In contrast, the Rx DTX handler for FR1 is modular: the way it is specified |
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21 in GSM 06.11, 06.12 and 06.31 is a front-end to unmodified GSM 06.10 decoder. |
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22 On the Rx side, the interface from the radio subsystem to the Rx DTX handler |
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23 consists of 260 bits of frame plus BFI and TAF flags (the spec also defines a |
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24 SID flag, but it is determined from frame payload bits), and then the |
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25 interface from the Rx DTX handler to the GSM 06.10 decoder is another FR frame |
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26 of 260 bits. |
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27 |
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28 What are the implications of this situation for the GSM published-source |
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29 software community? Prior to the present libgsmfrp offering, there has always |
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30 been libgsm, but no Rx DTX handler. If you are working with a GSM uplink RTP |
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31 stream from a BTS or a GSM downlink frame stream read out of TI Calypso DSP or |
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32 some other GSM MS PHY, feeding that stream directly to libgsm (without passing |
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33 through an Rx DTX handler) is NOT acceptable: a "bare" GSM 06.10 decoder won't |
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34 recognize SID frames and won't produce the expected comfort noise output, and |
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35 what are you going to do in those 20 ms windows in which no good traffic frame |
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36 was received? The situation becomes especially bad (unkind on ears) if you are |
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37 reading received downlink frames out of TI Calypso DSP: the DSP's buffer will |
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38 have *some* bit content in every 20 ms window, but naturally this bit content |
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39 will be garbage during those frame windows when no good frame was received; |
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40 feeding that garbage to libgsm produces noises that are very unkind on ears. |
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41 |
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42 The correct solution is to implement an Rx DTX handler, pass the stream of |
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43 frames and flags from the BTS or the MS PHY to this handler first, and then pass |
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44 the output of this handler to libgsm 06.10 decoder. Themyscira libgsmfrp is a |
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45 Free Software implementation of Rx DTX handler for GSM FR, implementing SID |
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46 classification, comfort noise generation and error concealment. |
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47 |
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48 Effect of extra preprocessing |
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49 ============================= |
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50 |
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51 One key detail deserves extra emphasis before going into library API details: |
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52 if the input to libgsmfrp consists entirely of good speech frames (no SID frames |
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53 and no BFIs), then the preprocessor becomes an identity transform. Therefore, |
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54 if the output of our libgsmfrp preprocessor were to be fed to an additional |
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55 instance of the same further down the processing chain, no extra transformation |
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56 of any kind will happen. |
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57 |
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58 Using libgsmfrp |
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59 =============== |
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60 |
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61 The external public interface to Themyscira libgsmfrp consists of a single |
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62 header file <gsm_fr_preproc.h>; it should be installed in the same system |
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63 include directory as <gsm.h> from libgsm. Please note that <gsm_fr_preproc.h> |
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64 includes <gsm.h>, as needed for gsm_byte and gsm_frame defined types. |
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65 |
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66 The dialect of C we chose for libgsmfrp is ANSI C (function prototypes), const |
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67 qualifier is used where appropriate; however, unlike libgsmefr, the interface |
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68 to libgsmfrp is defined in terms of gsm_byte type defined in <gsm.h>, included |
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69 from <gsm_fr_preproc.h>. |
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70 |
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71 State allocation and freeing |
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72 ============================ |
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73 |
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74 The Rx DTX handler is stateful, hence you will need to allocate a preprocessor |
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75 state structure in addition to the usual libgsm state structure for your GSM FR |
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76 Rx session. The necessary function is: |
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77 |
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78 extern struct gsmfr_preproc_state *gsmfr_preproc_create(void); |
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79 |
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80 struct gsmfr_preproc_state is an opaque structure to library users: you only get |
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81 a pointer which you remember and pass around, but <gsm_fr_preproc.h> does not |
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82 give you a full definition of this struct. As a library user, you don't even |
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83 get to know the size of this struct, hence the necessary malloc() operation |
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84 happens inside gsmfr_preproc_create(). However, the structure is malloc'ed as |
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85 a single chunk, hence when you are done with it, simply call free() on the |
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86 pointer you got from gsmfr_preproc_create(). |
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87 |
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88 gsmfr_preproc_create() can fail if the malloc() call inside fails, in which case |
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89 it returns NULL. |
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90 |
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91 Preprocessing good frames |
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92 ========================= |
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93 |
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94 For every good traffic frame (BFI=0) you receive from the radio subsystem, you |
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95 need to call this preprocessor function: |
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96 |
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97 extern void gsmfr_preproc_good_frame(struct gsmfr_preproc_state *state, |
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98 gsm_byte *frame); |
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99 |
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100 The second argument is both input and output, i.e., the frame is modified in |
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101 place. If the received frame is not SID (specifically, if the SID field |
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102 deviates from the SID codeword by 16 or more bits, per GSM 06.31 section 6.1.1), |
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103 then the frame (considered a good speech frame) will be left unmodified (i.e., |
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104 it is to be passed unchanged to the GSM 06.10 decoder), but preprocessor state |
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105 will be updated. OTOH, if the received frame is classified as either valid or |
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106 invalid SID per GSM 06.31, then the output frame will contain comfort noise |
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107 generated by the preprocessor using a PRNG, or a silence frame in one particular |
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108 corner case. |
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109 |
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110 GSM-FR RTP (or libgsm) 0xD magic: the upper nibble of the first byte can be |
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111 anything on input to gsmfr_preproc_good_frame(), but the output frame will |
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112 always have the correct magic in it. |
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113 |
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114 Handling BFI conditions |
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115 ======================= |
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116 |
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117 If you received a lost/missing frame indication instead of a good traffic frame, |
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118 call this preprocessor function: |
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119 |
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120 extern void gsmfr_preproc_bfi(struct gsmfr_preproc_state *state, int taf, |
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121 gsm_byte *frame_out); |
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122 |
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123 TAF is a flag defined in GSM 06.31 section 6.1.1; if you don't have this flag, |
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124 pass 0 - you will lose the function of comfort noise muting in the event of |
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125 prolonged SID loss, but all other Rx DTX functions will still work the same. |
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126 |
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127 With this function the 33-byte frame buffer is only an output, i.e., prior |
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128 buffer content is a don't-care and there is no provision for making any use of |
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129 erroneous frames like in EFR. The frame generated by the preprocessor may be |
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130 substitution/muting, comfort noise or silence depending on the state. |
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131 |
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132 Other miscellaneous functions |
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133 ============================= |
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134 |
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135 extern void gsmfr_preproc_reset(struct gsmfr_preproc_state *state); |
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136 |
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137 This function resets the preprocessor state to what it is right out of |
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138 gsmfr_preproc_create(), which is naturally just a combination of malloc() and |
159
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139 gsmfr_preproc_reset(). Given that our Rx DTX handler state is much simpler |
135
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140 than, for example, EFR codec state, there does not seem to be any need for |
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141 explicit resets, but the reset function is made public for the sake of |
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142 completeness. |
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143 |
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144 extern int gsmfr_preproc_sid_classify(const gsm_byte *frame); |
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145 |
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146 This function analyzes an RTP-encoded FR frame (the upper nibble of the first |
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147 byte is NOT checked for 0xD signature) for the SID codeword of GSM 06.12 and |
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148 classifies the frame as SID=0, SID=1 or SID=2 per the rules of GSM 06.31 |
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149 section 6.1.1. |
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150 |
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151 Silence frame datum |
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152 =================== |
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153 |
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154 extern const gsm_frame gsmfr_preproc_silence_frame; |
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155 |
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156 Many implementors make the mistake of thinking that a GSM FR silence frame is a |
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157 frame of 260 zero bits, but the official specs disagree: the silence frame given |
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158 in GSM 06.11 (3GPP TS 46.011, at the very end of the spec) is quite different. |
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159 Themyscira libgsmfrp implements the correct silence frame per the spec, and that |
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160 datum is also made public. |