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annotate Voice-memo-feature @ 103:c5f1f936dec5

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author Mychaela Falconia <falcon@freecalypso.org>
date Mon, 20 Nov 2023 08:28:16 +0000
parents 80f0996bfd16
children
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1 The full Calypso hw+fw solution as delivered by TI (the relevant components here
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2 are the DSP, the official L1 code and RiViera Audio Service) implements an
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3 interesting feature called voice memos. It is actually two paired features:
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4
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5 * Voice memo recording: in almost all states of the MS (no GSM network at all,
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6 or idle mode, or in an active call) it is possible to activate an extra
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7 instance of GSM 06.10 encoder that takes input from the microphone (and also
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8 from the active call downlink if invoked during a speech call) and writes its
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9 output into an otherwise-unused DSP buffer. The combination of L1 and RiViera
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10 Audio Service then writes this speech recording into a file in FFS.
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11
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12 * Voice memo playback: voice memo files recorded with the just-described VM
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13 record feature can be played into the phone's speaker output. The operation
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14 of playing a previously recorded voice memo is conceptually no different from
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15 playing tones or melodies, and can likewise be done in any state: with no GSM
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16 network at all, in idle mode, or in an active call.
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17
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18 VM recording and VM playback cannot be active at the same time: they use the
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19 same DSP buffer, and likely other mutually exclusive DSP resources too.
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20 Furthermore, the same DSP buffer that is used for these VM features is also
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21 used for TCH UL substitution debug/test feature described in the TCH-tap-modes
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22 article - therefore, all 3 features (VM record, VM play and TCH UL play) need
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23 to be treated as mutually exclusive in time. However, aside from this mutual
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24 exclusion, it is very remarkable that VM recording or VM playback can be invoked
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25 during an active speech call (which can use any codec!), and the extra instance
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Mychaela Falconia <falcon@freecalypso.org>
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26 of FR1 encoder or decoder (always FR1) invoked by VM features is essentially
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27 independent from the main TCH encoder and the main TCH decoder, all of which
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28 run simultaneously. It is worth noting that all newer GSM speech codecs (HR1,
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29 EFR and AMR) are much more computationally intensive than FR1, thus given that
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30 the DSP has the necessary horsepower to run any one of those "heavy" codecs, it
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31 probably isn't too much extra work to also run a simultaneous instance of
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32 unidirectional (encoder only or decoder only) FR1.
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33
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34 The entire voice memo facility was already fully implemented in the TCS211 code
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35 delivery from TI, but prior to FreeCalypso there was no way to exercise it. In
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Mychaela Falconia <falcon@freecalypso.org>
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36 order to exercise VM functionality in TCS211, one needs to invoke these RiViera
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37 Audio Service API functions:
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38
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39 audio_vm_record_start()
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40 audio_vm_record_stop()
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41 audio_vm_play_start()
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42 audio_vm_play_stop()
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43
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44 In FreeCalypso we've added some simple AT commands that call the just-listed API
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45 functions, and the facility that has been there all along is now accessible to
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46 play - it is the same situation as with Melody E1.
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47
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48 FreeCalypso AT commands for voice memo testing
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49 ==============================================
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50
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51 AT@VMR="/pathname",dur,dtx
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52
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53 This command initiates VM recording. The FFS pathname into which the recording
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54 should be written must be given as a quoted string (and as a reminder, all FFS
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55 pathnames must be absolute - there are no current directories in the firmware
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56 architecture), and there is a second required argument that sets the maximum
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57 size of the recording. The duration argument is a decimal integer, and it is
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58 reckoned in 1000-word units: if you specify duration as 1, the maximum recording
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59 size is 1000 words (2000 bytes), if you specify duration as 2, the maximum
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60 recording size is 2000 words (4000 bytes), and so forth. If you record with DTX
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61 disabled, each block of 1000 words corresponds to 1 second in time (every 20 ms
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62 frame turns into a block of 20 words), thus with DTX disabled the duration
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63 argument becomes the actual duration in seconds. However, if you record with
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64 DTX enabled, then periods of silence will be written in a compressed format
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65 described later in this article, and the time duration of the recording will
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66 depend on how much silence there is.
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67
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68 The dtx argument is 1 to enable DTX or 0 to disable it; the default is DTX
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69 disabled. The employed FR1 DTX algorithm appears to be the same as would be
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70 used for TCH/FS uplink, except that an "artificial" (there is no SACCH with
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71 independent-of-GSM voice memos) TAF position is generated on every 16th audio
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72 frame, i.e., every 320 ms. (Note the shortening of this SID interval compared
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73 to official TCH, where it is 24 frames or 480 ms.)
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74
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75 AT@VMRS
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76
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77 This command stops any VM recording in progress, but it is rarely needed - the
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78 recording will stop automatically when the size limit is reached.
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79
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80 AT@VMP="/pathname"
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81
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82 This command initiates playback of the VM recording contained in the named file
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83 in FFS. The FFS pathname is the only argument.
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84
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85 AT@VMPS
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86
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87 This command stops any VM playback in progress, but it is rarely needed - the
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88 playback will stop automatically when the end-marker is read from the file.
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89
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90 Voice memo file format
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91 ======================
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92
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93 Using fc-fsio, you can read out voice memo files written by the VM record
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94 facility, and you can likewise construct your own memo files externally, upload
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95 them into FC device FFS and then play them via the VM play facility. The format
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96 of these files is determined by TI's firmware stack (RV Audio Service on top of
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97 L1 on top of the DSP), but is fundamentally based on a DSP buffer that is just
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98 like those used for TCH. The companion TCH-tap-modes article describes the
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99 format of the DSP buffer from which TCH DL bits can be read out; in the present
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100 article we are going to cover the differences specific to the voice memo
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101 facility.
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102
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103 When VM recording is done with DTX disabled, every 20 ms speech frame turns into
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104 a block of 40 bytes in the memo file. This block of 40 bytes is produced from
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105 20 16-bit words in the DSP buffer, each word turned into two bytes in LE order
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106 by the ARM part of Calypso. The DSP buffer used for the VM facility has the
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107 same overall format as the one used for TCH DL, described in the TCH-tap-modes
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108 article - 3 status or header words followed by 17 words of payload, with the
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109 latter words carrying a 260-bit FR1 codec frame in the bit order of GSM 05.03
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110 interface 1. As explained in the TCH-tap-modes article, speech codec payload
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111 words are filled in the msb-to-lsb direction by the DSP, thus the natural byte-
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112 oriented representation would be big-endian - but because the little-endian ARM
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113 core sits in between the DSP and the on-media file format, the byte order in
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114 voice memo files comes out "wrong". Oh well - it is what it is.
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115
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116 Of the 3 header words that precede every 20 ms speech frame, words 1 and 2
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117 appear to be dummies - they have meaning related to the channel decoder block
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118 in the case of TCH DL, but in the case of isolated-from-GSM voice memos, there
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119 does not seem to be any meaning. However, header or status word 0, consisting
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120 of bit flags, is still important, but the bit flags for the VM facility are
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121 different from those of TCH DL.
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122
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123 When VM recording is done with DTX disabled, status word 0 is observed to always
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124 equal 0xC400 on every frame. However, when DTX is enabled, the following bits
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125 are seen in status word 0:
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126
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127 * Bit 15 will be set if this frame needs to be saved in its entirety, or cleared
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128 if it is to be skipped. When VM recording code in L1S sees that the DSP has
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129 delivered a frame with this status bit cleared, it will save only this status
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130 word 0, i.e., 2 bytes will be written into the memo file instead of 40 bytes
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131 for this 20 ms frame. On VM playback, the code likewise checks this bit to
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132 see how many words need to be read from the file, so synchronization is
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133 maintained.
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134
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135 * Bit 14 appears to be the SP flag of GSM 06.31 section 5.1: set when a speech
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136 frame has been generated, or cleared when a SID frame has been generated
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137 instead.
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138
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139 * Bit 11 is a TAF-like flag: when DTX is enabled, this bit is set in every 16th
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140 frame generated by the DSP in the VM recording session, otherwise it is
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141 cleared.
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142
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143 * Bit 10 will always be set in every status word 0 that gets written to voice
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144 memo files: this bit is set by the DSP when it has finished encoding a 20 ms
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145 audio frame and is checked by L1S on every TDMA frame, serving as a
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146 synchronization mechanism telling L1S when it needs to copy a speech frame
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147 from the DSP to the memo file.
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148
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149 When VM recording is done with DTX enabled, the recorded memo file will consist
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150 of speech frames (header word 0xC400 or 0xCC00), SID frames (header word 0x8400
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151 or 0x8C00) and skipped frames consisting of only the header word 0x0400, with
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152 the remaining words omitted. There will always be a present (not skipped) frame
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153 in every 16th position (0xCC00 or 0x8C00), thus no 0x0C00 frames are ever seen.
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154
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155 Every voice memo binary file ends with a 0xFBFF end-marker word; this end-marker
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156 is needed because TCS211 fw architecture exhibits a separation between the
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157 actual data reading and writing processes in L1S and the FFS read/write
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158 interface provided by RiViera Audio Service, and because of this separation the
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159 operational code in L1S can't "see" an EOF condition at the file system level.
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160
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161 FreeCalypso tools for decoding voice memo files
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162 ===============================================
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163
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164 If you have recorded a voice memo with AT@VMR and then read it out with fc-fsio,
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165 you can use additional FC tools to analyze it. The following tools are
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166 available, split between FC host tools and GSM codec libs & utilities packages:
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167
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168 * fc-vm2hex converts a binary VM recording into ASCII hex format, similar to
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169 the old (2016) TCH DL recording format before it was extended in late 2022.
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170 Every fully-written frame is emitted in the hex output as 3 space-separated
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171 hex status words followed by a block of 66 hex digits giving the FR1 codec
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172 frame in the unchanged bit order of TI's DSP, and every skipped frame (one
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173 for which only status word 0 was written into the memo file) is emitted in
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174 the hex output as just that one word.
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175
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176 * gsmfr-dlcap-parse utility, originally written for parsing TCH DL capture
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177 files, accepts TCH DL recording files in both old and new formats, and it also
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178 accepts the output from fc-vm2hex as its input. The combination of fc-vm2hex
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179 and gsmfr-dlcap-parse allows a developer or tinkerer to do thorough human
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180 analysis of TCS211 VM recordings in both DTX-disabled and DTX-enabled modes.
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181
97
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182 * As of fc-host-tools-r18 there is a new fc-vm2gsmx utility that reads binary VM
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183 recording files (as you would read out with fc-fsio) and converts them into
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184 extended-libgsm (gsmx) format defined in our GSM codec libraries & utilities
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185 package. This gsmx format is an extension of the classic libgsm (GSM 06.10)
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186 format, adding the possibility of SID frames and BFI markers (frame gaps) in
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187 addition to regular speech frames, thus it can represent the content of a
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188 voice memo recording made in DTX mode. These gsmx files can then be decoded
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189 into playable WAV with our gsmfr-decode utility.
96
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190
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191 FreeCalypso tools for external generation of voice memo files
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192 =============================================================
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193
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194 Using FreeCalypso tools, you can produce an external speech recording in GSM
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195 06.10 FR1 codec format, convert it into TCS211 VM format, upload it into FC
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196 device FFS with fc-fsio, and then play these externally-produced voice memos
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197 with AT@VMP. The steps are as follows:
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198
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199 1) You can use gsmfr-encode to FR1-encode a speech sample from WAV into classic
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200 .gsm format, or gsmfr-encode-r if the source is raw BE instead of WAV.
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201 Alternatively, you can use any other off-the-shelf software that can encode
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202 FR1 and write libgsm format; SoX shipped with Slackware includes the
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203 necessary support.
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204
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205 2) fc-gsm2vm converts a .gsm recording into non-DTX TCS211 VM format.
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206
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207 At the present time we don't have any tools for producing external DTX-enabled
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208 VM recordings: the main limitation is that at least to this Mother's knowledge,
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209 the published source software community does not currently possess a GSM 06.10
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210 encoding library that has been extended with VAD and DTX functions. There is
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211 classic libgsm from 1990s, used by everyone in the FOSS community who needs a
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212 GSM 06.10 encoder or decoder, but it doesn't do DTX; we (FreeCalypso and
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213 Themyscira Wireless) have produced our own libgsmfrp front-end that implements
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214 Rx DTX handler functions (that's how we can properly decode FR1 streams that
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215 contain SIDs and/or missing frames), but it doesn't help with DTX encoding.
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216 Therefore, our ability to produce TCS211-compatible VM recordings externally is
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217 currently limited to non-DTX mode.