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doc: new article FR1-Rx-DTX-detail
author Mychaela Falconia <falcon@freecalypso.org>
date Mon, 07 Oct 2024 00:25:50 +0000
parents de333989a12b
children
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1 Standalone command line utilities for FR and EFR codecs
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2 =======================================================
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3
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4 The pre-existing FOSS opencore-amr package includes amrnb-enc and amrnb-dec test
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5 programs: the first reads linear PCM from a WAV file and emits AMR encoder
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6 output in a .amr file (RFC 4867 AMR storage format), the second reads this .amr
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7 format and emits AMR decoder output as WAV. Inspired by these simple test
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8 programs, the present package offers equivalent command line utilities for GSM
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9 FR and EFR codecs. Here they are:
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10
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11 gsmfr-encode This utility reads linear PCM from a WAV file, runs the
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12 bit-exact GSM 06.10 encoder and writes the output in the
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13 classic .gsm format (directly abutted FR codec frames of 33
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14 bytes each). We don't currently have a Tx-side DTX
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15 implementation (VAD etc) for GSM-FR, hence the output from
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16 gsmfr-encode will always consist of good speech frames only.
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17
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18 gsmfr-decode This utility reads our gsmx format (see Binary-file-format
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19 article), which is a superset of the classic libgsm format.
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20 The input to gsmfr-decode may be a pure .gsm recording as
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21 produced by gsmfr-encode or toast from libgsm package, or it
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22 can also contain SID frames and/or BFI markers. The processing
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23 performed by gsmfr-decode begins with our FR1 Rx DTX handler
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24 preprocessor, which will be an identity transform for pure .gsm
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25 input (most of the time) but becomes important for real-world
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26 input containing SIDs and BFIs, and is followed by the bit-exact
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27 GSM 06.10 decoder. The decoded output is written as WAV.
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28
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29 gsmefr-encode This utility reads linear PCM from a WAV file, runs our EFR
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30 encoder (Themyscira libgsmefr) and writes the output in our gsmx
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31 format. There is an option to enable or disable DTX: -d enables
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32 DTX, otherwise it is disabled. (This option mirrors amrnb-enc.)
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33
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34 gsmefr-decode This utility reads our gsmx format (which must be EFR, not FR1)
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35 and feeds all frames and BFIs to our EFR decoder. The decoded
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36 output is written as WAV.
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37
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38 The above are original programs that read WAV input for encoding and write WAV
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39 output from decoding. We now also have raw versions that read and write our
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40 "robe" (raw big-endian) format instead:
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41
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42 gsmfr-encode-r Just like gsm[e]fr-encode, but reading "robe" instead of WAV.
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43 gsmefr-encode-r
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44
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45 gsmfr-decode-r Just like gsm[e]fr-decode, but writing "robe" instead of WAV.
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46 gsmefr-decode-r
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48 Please see PCM-file-formats article for the rationale.
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49
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50 Additions for libgsmfr2
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51 =======================
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52
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53 With the introduction of libgsmfr2, gsmfr-* codec utilities have undergone some
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54 changes:
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55
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56 * gsmfr-decode and gsmfr-decode-r now implement the optional decoder homing
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57 feature, detecting and acting upon GSM 06.10 decoder homing frames.
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58
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59 * gsmfr-encode-r takes an optional -h flag that enables the encoder homing
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60 function; it is disabled by default. The same feature was not replicated in
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61 WAV-reading gsmfr-encode, as WAV format is poorly suited for tinkering-
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62 oriented bit-exact work.
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63
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64 * There is a new utility named gsmfr-decode-rb, where rb stands for "raw basic".
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65 This utility emits "robe" output like gsmfr-decode-r, but it performs only
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66 "basic" GSM 06.10 decoding, without the Rx DTX preprocessor step. BFI frame
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67 gaps in input are not allowed, and there is no SID or DHF detection.
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69 Additions for TW-TS-005
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70 =======================
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71
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72 All of the speech decoder utilities described above read FR or EFR compressed
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73 speech in gsmx binary file format. However, we now also have a newer
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74 hexadecimal file format defined in Themyscira Wireless Technical Specification
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75 TW-TS-005 - see TW-TS-005 article. TW-TS-005 Annex A is the application of
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76 this hex format to FR and EFR codecs. Here are our updated speech decoder
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77 utilities that read TW-TS-005 Annex A file format:
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78
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79 Original utility, reads gsmx New utility, reads TW-TS-005 Annex A
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80 --------------------------------------------------------------------
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81 gsmfr-decode gsmfr-decode-tw5
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82 gsmfr-decode-r gsmfr-decode-tw5-r
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83 gsmefr-decode gsmefr-decode-tw5
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84 gsmefr-decode-r gsmefr-decode-tw5-r
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85
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86 There are no changes to speech encoder utilities: generating gsmx and then
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87 converting it to TW-TS-005 with gsmx-to-tw5a is a perfectly good workflow.
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88
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89 Standalone command line utilities for AMR codec
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90 ===============================================
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91
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92 As described above, gsm[e]fr-encode and gsm[e]fr-decode were modeled after
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93 amrnb-enc and amrnb-dec from opencore-amr, a piece of pre-existing FOSS.
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94 However, now that we have libtwamr, a Themyscira library for AMR codec that is
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95 designed to serve as a replacement for libopencore-amrnb in our workflows
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96 involving AMR, we also have our own twamr-encode and twamr-decode utilities
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97 that directly replace amrnb-enc and amrnb-dec.
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98
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99 twamr-encode is a functional replacement for amrnb-enc: it reads 16-bit linear
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100 PCM speech input from a WAV file and writes the AMR encoder output in a .amr
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101 file (RFC 4867 storage format). However, there is a difference in the command
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102 line structure and a small difference in operation. The command line structure
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103 of twamr-encode is as follows:
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104
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105 twamr-encode [-d] [-2] input.wav mode output.amr
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106
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107 The middle argument specifies the codec mode to be used; there is no default.
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108 Ordinarily the mode argument is one of these 8 keywords:
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109
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110 MR475
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111 MR515
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112 MR59
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113 MR67
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114 MR74
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115 MR795
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116 MR102
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117 MR122
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119 However, this mode argument can also take the form of "file:$modefile", where
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120 $modefile is an ASCII text file giving one of the above mode keywords per line.
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121 This form is not likely to be useful in casual twamr-encode usage, but it exists
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122 for the sake of symmetry with twamr-tseq-enc program used for verification
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123 testing with the official test sequences from 3GPP.
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124
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125 Aside from this difference in the command line structure, the small functional
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126 difference between amrnb-enc and twamr-encode is that libopencore-amrnb (the
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127 engine underlying amrnb-enc) omits the codec homing feature, whereas libtwamr
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128 (the engine underlying twamr-encode) implements the homing feature as a
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129 mandatory part of the codec definition per 3GPP specs.
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130
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131 twamr-encode flag options: -d enables DTX, -2 switches the VAD algorithm from
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132 VAD1 default to VAD2 alternative. The two options can be combined as -d2.
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133
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134 twamr-decode is a more straightforward replacement for amrnb-dec, with this
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135 simple command line structure:
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136
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137 twamr-decode input.amr output.wav
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138
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139 The functional difference from amrnb-dec is once again in the codec homing
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140 feature: present in libtwamr and hence twamr-decode, but absent in
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141 libopencore-amrnb and hence amrnb-dec.
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142
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143 Finally, for the sake of completeness and symmetry with the other supported
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144 codecs, the present suite includes twamr-encode-r and twamr-decode-r utilities.
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145 They function just like twamr-encode and twamr-decode, with the same command
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146 line structure, but the file format for 16-bit linear PCM speech is "robe"
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147 instead of WAV.