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doc/Codec-utils: clarify lack of DHF in gsmfr-decode-rb
author Mychaela Falconia <falcon@freecalypso.org>
date Mon, 20 May 2024 01:47:22 +0000
parents 2d46abdfbe91
children bd32bb1e8dab
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1 Libgsmefr general usage
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2 =======================
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3
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4 The external public interface to Themyscira libgsmefr consists of a single
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5 header file <gsm_efr.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 we chose for libgsmefr is ANSI C (function prototypes), const
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9 qualifier is used where appropriate, and the interface is defined in terms of
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10 <stdint.h> types; <gsm_efr.h> includes <stdint.h>.
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11
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12 State allocation and freeing
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13 ============================
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14
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15 In order to use the EFR encoder, you will need to allocate an encoder state
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16 structure, and to use the EFR decoder, you will need to allocate a decoder state
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17 structure. The necessary state allocation functions are:
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18
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19 extern struct EFR_encoder_state *EFR_encoder_create(int dtx);
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20 extern struct EFR_decoder_state *EFR_decoder_create(void);
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21
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22 struct EFR_encoder_state and struct EFR_decoder_state are opaque structures to
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23 library users: you only get pointers which you remember and pass around, but
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24 <gsm_efr.h> does not give you full definitions of these structs. As a library
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25 user, you don't even get to know the size of these structs, hence the necessary
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26 malloc() operation happens inside EFR_encoder_create() and EFR_decoder_create().
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27 However, each structure is malloc'ed as a single chunk, hence when you are done
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28 with it, simply call free() to relinquish each encoder or decoder state
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29 instance.
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30
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31 EFR_encoder_create() and EFR_decoder_create() functions can fail if the malloc()
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32 call inside fails, in which case the two libgsmefr functions in question return
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33 NULL.
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34
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35 The dtx argument to EFR_encoder_create() is a Boolean flag represented as an
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36 int; it tells the EFR encoder whether it should operate with DTX enabled (run
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37 GSM 06.82 VAD and emit SID frames instead of speech frames per GSM 06.81) or DTX
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38 disabled (skip VAD and always emit speech frames).
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39
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40 Using the EFR encoder
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41 =====================
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42
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43 To encode one 20 ms audio frame per EFR, call EFR_encode_frame():
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44
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45 extern void EFR_encode_frame(struct EFR_encoder_state *st, const int16_t *pcm,
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46 uint8_t *frame, int *sp, int *vad);
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47
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48 You need to provide an encoder state structure allocated earlier with
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49 EFR_encoder_create(), a block of 160 linear PCM samples, and an output buffer of
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50 31 bytes (EFR_RTP_FRAME_LEN constant also defined in <gsm_efr.h>) into which the
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51 encoded EFR frame will be written; the frame format is that defined in ETSI TS
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52 101 318 for EFR in RTP, including the 0xC signature in the upper nibble of the
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53 first byte.
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54
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55 The last two arguments of type (int *) are optional pointers to extra output
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56 flags SP and VAD, defined in GSM 06.81 section 5.1.1; either pointer or both of
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57 them can be NULL if these extra output flags aren't needed. Both of these flags
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58 are needed in order to test our libgsmefr encoder implementation against
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59 official ETSI test sequences (GSM 06.54), but they typically aren't needed
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60 otherwise.
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61
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62 Using the EFR decoder
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63 =====================
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64
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65 The main interface to our EFR decoder is this function:
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66
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67 extern void EFR_decode_frame(struct EFR_decoder_state *st, const uint8_t *frame,
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68 int bfi, int taf, int16_t *pcm);
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69
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70 The inputs consist of 244 bits of frame payload (the 4 upper bits of the first
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71 byte are ignored - there is NO enforcement of 0xC signature in our frame
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72 decoder) and BFI and TAF flags defined in GSM 06.81 section 6.1.1. Note the
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73 absence of a SID flag argument: EFR_decode_frame() calls our own utility
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74 function EFR_sid_classify() to determine SID from the frame itself per the rules
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75 of GSM 06.81 section 6.1.1.
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76
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77 The canonical EFR decoder always expects frame bits input to be present, even
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78 during BFI condtions! More specifically, if a BFI=1 decoding call comes in when
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79 the decoder is in comfort noise generation state (after a SID), then all frame
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80 bits passed along with BFI=1 are ignored as one would naturally expect for
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81 frames that typically aren't transmitted at all - but if a BFI=1 decoding call
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82 comes in when the decoder is in regular speech mode, the canonical decoder will
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83 use the "fixed codebook excitation pulses" part of the erroneous frame (one
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84 declared to be garbage) as part of its decoding operation! (This part
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85 constitutes 35 bits per subframe or 140 bits out of 244 per frame.)
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86
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87 BFI with no data
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88 ================
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89
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90 Many EFR decoder applications will be faced with a situation where they receive
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91 a frame gap (no data at all), and they need to run the EFR decoder with BFI=1 -
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92 but the application doesn't have any frame-bits input. Yet the canonical EFR
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93 decoder requires *some* erroneous frame bits to be fed to it - so what gives?
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94 Our initial approach was to feed the decoder all zeros in the place of codec
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95 parameters - but further analysis reveals that approach to be bad. (To see for
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96 yourself, study the code in d1035pf.c and think what it will do when the input
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97 is fixed at all zeros.) Our new approach is to generate pseudorandom bits for
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98 these pulse parameters, as detailed below.
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99
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100 If you find yourself in the situation of needing to feed BFI=1 with no frame
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101 data bits to the decoder, call the following function in the place of
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102 EFR_decode_frame():
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103
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104 extern void EFR_decode_bfi_nodata(struct EFR_decoder_state *st, int taf,
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105 int16_t *pcm);
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106
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107 This function begins by checking the internal state flag RX_SP_FLAG, indicating
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108 whether the decoder is in speech or comfort noise generation mode. If
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109 RX_SP_FLAG is set, indicating speech state, then the main body of the decoder
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110 will be making use of fixed codebook pulse parameters even for erroneous frames,
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111 and EFR_decode_bfi_nodata() will invoke a PRNG to fill in pseudorandom bits.
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112 If RX_SP_FLAG is clear, then the decoder is generating comfort noise following
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113 reception of a SID, and BFI conditions are fully expected because the
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114 transmitter is expected to be off. In this case EFR_decode_bfi_nodata() feeds
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115 all-zeros parameters to the main body of the decoder, as none of them will be
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116 used.
123
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117
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118 Stateless utility functions
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119 ===========================
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120
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121 All functions in this section are stateless (no encoder state or decoder state
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122 structure is needed); they merely manipulate bit fields.
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123
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124 extern void EFR_frame2params(const uint8_t *frame, int16_t *params);
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125
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126 This function unpacks an EFR codec frame in ETSI TS 101 318 RTP encoding (the
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127 upper nibble of the first byte is NOT checked, i.e., there is NO enforcement of
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128 0xC signature) into an array of 57 (EFR_NUM_PARAMS) parameter words for the
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129 codec. int16_t signed type is used for the params array (even though all
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130 parameters are actually unsigned) in order to match the guts of ETSI-based EFR
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131 codec, and EFR_frame2params() is called internally by EFR_decode_frame().
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132
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133 extern void EFR_params2frame(const int16_t *params, uint8_t *frame);
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134
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135 This function takes an array of 57 (EFR_NUM_PARAMS) EFR codec parameter words
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136 and packs them into a 31-byte (EFR_RTP_FRAME_LEN) frame in ETSI TS 101 318
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137 format. The 0xC signature is generated by this function, and every byte of the
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138 output buffer is fully written without regard to any previous content. This
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139 function is called internally by EFR_encode_frame().
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140
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141 extern int EFR_sid_classify(const uint8_t *frame);
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142
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143 This function analyzes an RTP-encoded EFR frame (the upper nibble of the first
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144 byte is NOT checked for 0xC signature) for the SID codeword of GSM 06.62 and
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145 classifies the frame as SID=0, SID=1 or SID=2 per the rules of GSM 06.81
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146 section 6.1.1.
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147
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148 extern void EFR_insert_sid_codeword(uint8_t *frame);
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149
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150 This function inserts the SID codeword of GSM 06.62 into the frame in the
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151 pointed-to buffer; specifically, the 95 bits that make up the SID field are all
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152 set to 1s, but all other bits remain unchanged. This function is arguably least
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153 useful to external users of libgsmefr, but it exists because of how the original
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154 code from ETSI generates SID frames produced by the encoder in DTX mode.
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155
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156 Parameter-based encoder and decoder functions
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157 =============================================
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158
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159 The EFR_encode_frame() and EFR_decode_frame() functions described earlier in
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160 this document constitute the most practically useful (intended for actual use)
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161 interfaces to our EFR encoder and decoder, but they are actually wrappers around
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162 these parameter-based functions:
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163
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164 extern void EFR_encode_params(struct EFR_encoder_state *st, const int16_t *pcm,
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165 int16_t *params, int *sp, int *vad);
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166
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167 This function is similar to EFR_encode_frame(), but the output is an array of
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168 57 (EFR_NUM_PARAMS) codec parameter words rather than a finished frame. The two
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169 extra output flags are optional (pointers may be NULL) just like with
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170 EFR_encode_frame(), but there is a catch: if the output frame is a SID (which
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171 can only happen if DTX is enabled), the bits inside parameter words that would
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172 correspond to SID codeword bits are NOT set, instead one MUST call
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173 EFR_insert_sid_codeword() after packing the frame with EFR_params2frame(). The
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174 wrapper in EFR_encode_frame() does exactly as described, and the overall logic
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175 follows the original code structure from ETSI.
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176
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177 extern void EFR_decode_params(struct EFR_decoder_state *st,
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178 const int16_t *params, int bfi, int sid, int taf,
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179 int16_t *pcm);
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180
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181 This function is similar to EFR_decode_frame() with the frame input replaced
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182 with params array input, but the SID classification per the rules of GSM 06.81
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183 section 6.1.1 needs to be provided by the caller. The wrapper in
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184 EFR_decode_frame() calls both EFR_frame2params() and EFR_sid_classify() before
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185 passing the work to EFR_decode_params().
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186
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187 State reset functions
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188 =====================
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189
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190 extern void EFR_encoder_reset(struct EFR_encoder_state *st, int dtx);
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191 extern void EFR_decoder_reset(struct EFR_decoder_state *st);
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192
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193 These functions reset the state of the encoder or the decoder, respectively;
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194 the entire state structure is fully initialized to the respective home state
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195 defined in GSM 06.60 section 8.5 for the encoder or section 8.6 for the decoder.
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196
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197 EFR_encoder_reset() is called internally by EFR_encoder_create() and by the
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198 encoder itself when it encounters the ETSI-prescribed encoder homing frame;
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199 EFR_decoder_reset() is called internally by EFR_decoder_create() and by the
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200 decoder itself when it encounters the ETSI-prescribed decoder homing frame.
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201 Therefore, there is generally no need for libgsmefr users to call these
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202 functions directly - but they are made public for the sake of completeness.
130
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203
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204 If you call EFR_encoder_reset() manually, you can change the DTX enable/disable
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205 flag from its initial value given to EFR_encoder_create() - the new value of
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206 this flag passed to EFR_encoder_reset() always takes effect. There is no
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207 provision for changing this mode within an encoder session without a full reset.
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208
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209 Public const datum: decoder homing frame
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210 ========================================
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211
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212 While the encoder homing frame is the same for all codecs defined by ETSI and
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213 then later 3GPP, each codec has its own unique decoder homing frame (DHF). A
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214 public const datum introduced in libgsmefr version 1.1.0 provides the
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215 RTP-encoded form of the ETSI-prescribed DHF for EFR:
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216
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217 extern const uint8_t EFR_decoder_homing_frame[EFR_RTP_FRAME_LEN];