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doc/EFR-library-API article written
author Mychaela Falconia <falcon@freecalypso.org>
date Sat, 10 Dec 2022 22:01:14 +0000
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children 1c529bb31219
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123
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1 The external public interface to Themyscira libgsmefr consists of a single
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2 header file <gsm_efr.h>; it should be installed in the same system include
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3 directory as <gsm.h> from classic libgsm (1990s free software product) for the
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4 original FR codec, and the API of libgsmefr is modeled after that of libgsm.
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5
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6 The dialect of C we chose for libgsmefr is ANSI C (function prototypes), const
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7 qualifier is used where appropriate, and the interface is defined in terms of
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8 <stdint.h> types; <gsm_efr.h> includes <stdint.h>.
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9
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10 State allocation and freeing
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11 ============================
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12
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13 In order to use the EFR encoder, you will need to allocate an encoder state
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14 structure, and to use the EFR decoder, you will need to allocate a decoder state
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15 structure. The necessary state allocation functions are:
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16
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17 extern struct EFR_encoder_state *EFR_encoder_create(int dtx);
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18 extern struct EFR_decoder_state *EFR_decoder_create(void);
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19
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20 struct EFR_encoder_state and struct EFR_decoder_state are opaque structures to
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21 library users: you only get pointers which you remember and pass around, but
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22 <gsm_efr.h> does not give you full definitions of these structs. As a library
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23 user, you don't even get to know the size of these structs, hence the necessary
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24 malloc() operation happens inside EFR_encoder_create() and EFR_decoder_create().
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25 However, each structure is malloc'ed as a single chunk, hence when you are done
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26 with it, simply call free() to relinquish each encoder or decoder state
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27 instance.
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28
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29 EFR_encoder_create() and EFR_decoder_create() functions can fail if the malloc()
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30 call inside fails, in which case the two libgsmefr functions in question return
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31 NULL.
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32
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33 The dtx argument to EFR_encoder_create() is a Boolean flag represented as an
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34 int; it tells the EFR encoder whether it should operate with DTX enabled (run
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35 GSM 06.82 VAD and emit SID frames instead of speech frames per GSM 06.81) or DTX
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36 disabled (skip VAD and always emit speech frames).
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37
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38 Using the EFR encoder
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39 =====================
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40
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41 To encode one 20 ms audio frame per EFR, call EFR_encode_frame():
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42
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43 extern void EFR_encode_frame(struct EFR_encoder_state *st, const int16_t *pcm,
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44 uint8_t *frame, int *sp, int *vad);
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45
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46 You need to provide an encoder state structure allocated earlier with
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47 EFR_encoder_create(), a block of 160 linear PCM samples, and an output buffer of
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48 31 bytes (EFR_RTP_FRAME_LEN constant also defined in <gsm_efr.h>) into which the
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49 encoded EFR frame will be written; the frame format is that defined in ETSI TS
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50 101 318 for EFR in RTP, including the 0xC signature in the upper nibble of the
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51 first byte.
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52
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53 The last two arguments of type (int *) are optional pointers to extra output
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54 flags SP and VAD, defined in GSM 06.81 section 5.1.1; either pointer or both of
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55 them can be NULL if these extra output flags aren't needed. Both of these flags
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56 are needed in order to test our libgsmefr encoder implementation against
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57 official ETSI test sequences (GSM 06.54), but they typically aren't needed
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58 otherwise.
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59
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60 Using the EFR decoder
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61 =====================
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62
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63 The main interface to our EFR decoder is this function:
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64
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65 extern void EFR_decode_frame(struct EFR_decoder_state *st, const uint8_t *frame,
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66 int bfi, int taf, int16_t *pcm);
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67
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68 The inputs consist of 244 bits of frame payload (the 4 upper bits of the first
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69 byte are ignored - there is NO enforcement of 0xC signature in our frame
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70 decoder) and BFI and TAF flags defined in GSM 06.81 section 6.1.1. Note the
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71 absence of a SID flag argument: EFR_decode_frame() calls our own utility
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72 function EFR_sid_classify() to determine SID from the frame itself per the rules
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73 of GSM 06.81 section 6.1.1.
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74
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75 Many EFR decoder applications will also be faced with a situation where they
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76 receive a frame gap (no data at all), and they need to run the EFR decoder with
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77 BFI=1, but don't have any frame-bits input. If you find yourself in this
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78 situation, call the following function:
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79
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80 extern void EFR_decode_bfi_nodata(struct EFR_decoder_state *st, int taf,
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81 int16_t *pcm);
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82
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83 EFR_decode_bfi_nodata() is equivalent to calling EFR_decode_frame() with a frame
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84 buffer of 31 zero bytes (or 0xC signature followed by 244 zero bits) and BFI=1,
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85 but is slightly more efficient in that the internal steps of EFR_frame2params()
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86 and EFR_sid_classify() are skipped, and the made-up "frame" of 244 zero bits is
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87 passed to the decoder core at the params array level.
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88
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89 Note that the official EFR decoder from ETSI, which we've replicated in our
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90 librified form in libgsmefr, does make use of some presumed-invalid frame data
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91 bits under BFI=1 conditions: see the description in GSM 06.61 section 6.1, where
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92 the last sentence reads "The received fixed codebook excitation pulses from the
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93 erroneous frame are always used as such." With our current implementation, the
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94 "erroneous frame" in the case of completely lost or missing frames is a made-up
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95 frame of 244 zero bits; the question of whether this approach is good enough or
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96 if we need to do something more complex remains for further study.
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97
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98 Stateless utility functions
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99 ===========================
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100
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101 All functions in this section are stateless (no encoder state or decoder state
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102 structure is needed); they merely manipulate bit fields.
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103
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104 extern void EFR_frame2params(const uint8_t *frame, int16_t *params);
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105
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106 This function unpacks an EFR codec frame in ETSI TS 101 318 RTP encoding (the
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107 upper nibble of the first byte is NOT checked, i.e., there is NO enforcement of
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108 0xC signature) into an array of 57 (EFR_NUM_PARAMS) parameter words for the
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109 codec. int16_t signed type is used for the params array (even though all
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110 parameters are actually unsigned) in order to match the guts of ETSI-based EFR
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111 codec, and EFR_frame2params() is called internally by EFR_decode_frame().
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112
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113 extern void EFR_params2frame(const int16_t *params, uint8_t *frame);
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114
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115 This function takes an array of 57 (EFR_NUM_PARAMS) EFR codec parameter words
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116 and packs them into a 31-byte (EFR_RTP_FRAME_LEN) frame in ETSI TS 101 318
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117 format. The 0xC signature is generated by this function, and every byte of the
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118 output buffer is fully written without regard to any previous content. This
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119 function is called internally by EFR_encode_frame().
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120
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121 extern int EFR_sid_classify(const uint8_t *frame);
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122
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123 This function analyzes an RTP-encoded EFR frame (the upper nibble of the first
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124 byte is NOT checked for 0xC signature) for the SID codeword of GSM 06.62 and
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125 classifies the frame as SID=0, SID=1 or SID=2 per the rules of GSM 06.81
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126 section 6.1.1.
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127
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128 extern void EFR_insert_sid_codeword(uint8_t *frame);
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129
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130 This function inserts the SID codeword of GSM 06.62 into the frame in the
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131 pointed-to buffer; specifically, the 95 bits that make up the SID field are all
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132 set to 1s, but all other bits remain unchanged. This function is arguably least
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133 useful to external users of libgsmefr, but it exists because of how the original
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134 code from ETSI generates SID frames produced by the encoder in DTX mode.
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135
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136 Parameter-based encoder and decoder functions
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137 =============================================
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138
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139 The EFR_encode_frame() and EFR_decode_frame() functions described earlier in
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140 this document constitute the most practically useful (intended for actual use)
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141 interfaces to our EFR encoder and decoder, but they are actually wrappers around
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142 these parameter-based functions:
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143
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144 extern void EFR_encode_params(struct EFR_encoder_state *st, const int16_t *pcm,
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145 int16_t *params, int *sp, int *vad);
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146
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147 This function is similar to EFR_encode_frame(), but the output is an array of
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148 57 (EFR_NUM_PARAMS) codec parameter words rather than a finished frame. The two
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149 extra output flags are optional (pointers may be NULL) just like with
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150 EFR_encode_frame(), but there is a catch: if the output frame is a SID (which
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151 can only happen if DTX is enabled), the bits inside parameter words that would
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152 correspond to SID codeword bits are NOT set, instead one MUST call
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153 EFR_insert_sid_codeword() after packing the frame with EFR_params2frame(). The
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154 wrapper in EFR_encode_frame() does exactly as described, and the overall logic
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155 follows the original code structure from ETSI.
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156
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157 extern void EFR_decode_params(struct EFR_decoder_state *st,
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158 const int16_t *params, int bfi, int sid, int taf,
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159 int16_t *pcm);
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160
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161 This function is similar to EFR_decode_frame() with the frame input replaced
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162 with params array input, but the SID classification per the rules of GSM 06.81
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163 section 6.1.1 needs to be provided by the caller. The wrapper in
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164 EFR_decode_frame() calls both EFR_frame2params() and EFR_sid_classify() before
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165 passing the work to EFR_decode_params().
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166
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167 State reset functions
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168 =====================
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169
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170 extern void EFR_encoder_reset(struct EFR_encoder_state *st, int dtx);
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171 extern void EFR_decoder_reset(struct EFR_decoder_state *st);
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172
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173 These functions reset the state of the encoder or the decoder, respectively;
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174 the entire state structure is fully initialized to the respective home state
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175 defined in GSM 06.60 section 8.5 for the encoder or section 8.6 for the decoder.
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176
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177 EFR_encoder_reset() is called internally by EFR_encoder_create() and by the
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178 encoder itself when it encounters the ETSI-prescribed encoder homing frame;
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179 EFR_decoder_reset() is called internally by EFR_decoder_create() and by the
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180 decoder itself when it encounters the ETSI-prescribed decoder homing frame.
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181 Therefore, there is generally no need for libgsmefr users to call these
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182 functions directly - but they are made public for the sake of completeness.