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doc/FR1-Rx-DTX: document changes from 1.0.1 to 1.0.2
author Mychaela Falconia <falcon@freecalypso.org>
date Fri, 12 May 2023 04:49:09 +0000
parents fcc0887ff0d0
children 4034c2b06ec8
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22601ae99434 doc/FR1-Rx-DTX article written
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1 At the level of provided functionality and architectural structure, ETSI GSM
22601ae99434 doc/FR1-Rx-DTX article written
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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.
244
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161
250
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162 libgsmfrp change history: version 1.0.1 to version 1.0.2
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163 ========================================================
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164
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165 There are only two changes, both involving corner cases with invalid SID frames
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166 being received:
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167
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168 1) An invalid SID frame was received immediately following a good speech frame.
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169 In this case we start CN generation, but we take the needed LARc and Xmaxc
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170 parameters from the last speech frame, instead of the usual procedure of
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171 extracting them from a valid SID frame. The change from 1.0.1 to 1.0.2
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172 concerns the Xmaxc parameter in this corner case: in 1.0.1 we took Xmaxc
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173 from the last subframe and used it for ensuing CN generation, but in 1.0.2
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174 we compute a more proper mean Xmaxc from all 4 subframes, by dequantizing,
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175 summing and requantizing.
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176
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177 2) An invalid SID frame was received in the speech muting state. The sequence
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178 of inputs would have to be:
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179
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180 - a good speech frame;
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181 - one or more BFIs, but not too many, so that the cached speech frame
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182 does not decay fully by Xmaxc reduction;
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183 - an invalid SID frame.
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184
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185 In version 1.0.1 we handled this even more obscure corner case by entering
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186 the CN muting state, i.e., the state that is normally entered upon the
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187 second lost SID. In version 1.0.2 we ignore invalid SID in the speech
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188 muting state and act as if we got BFI, i.e., continue speech muting rather
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189 than switch to CN muting.
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190
244
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191 libgsmfrp change history: version 1.0.0 to version 1.0.1
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192 ========================================================
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193
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194 Version 1.0.0 exhibited the following defects, which are fixed in 1.0.1:
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195
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196 1) The last received valid SID was cached forever for the purpose of
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197 handling future invalid SIDs - we could have received some valid
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198 SID ages ago, then lots of speech or NO_DATA, and if we then get
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199 an invalid SID, we would resurrect the last valid SID from ancient
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200 history - a bad design. In our new design, we handle invalid SID
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201 based on the current state, much like BFI.
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202
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203 2) GSM 06.11 spec says clearly that after the second lost SID
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204 (received BFI=1 && TAF=1 in CN state) we need to gradually decrease
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205 the output level, rather than jump directly to emitting silence
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206 frames - we previously failed to implement such logic.
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207
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208 3) Per GSM 06.12 section 5.2, Xmaxc should be the same in all 4 subframes
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209 in a SID frame. What should we do if we receive an otherwise valid
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210 SID frame with different Xmaxc? Our previous approach would
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211 replicate this Xmaxc oddity in every subsequent generated CN frame,
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212 which is rather bad. In our new design, the very first CN frame
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213 (which can be seen as a transformation of the SID frame itself)
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214 retains the original 4 distinct Xmaxc, but all subsequent CN frames
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215 are based on the Xmaxc from the last subframe of the most recent SID.