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doc/TCH-bit-access: update for newly resurrected version
author Mychaela Falconia <falcon@freecalypso.org>
date Thu, 29 Dec 2022 09:32:31 +0000
parents 2748f257312b
children 8f7c50e1fa3b
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1 It has been discovered that the DSP ROM in the Calypso GSM baseband processor
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2 implements one nifty feature which is not used at all in standard phone or modem
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3 operation, but which can be used for all kinds of interesting hacks: the traffic
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4 channel (TCH) bits coming out of the GSM 05.03 channel decoder in the downlink
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5 direction (to be fed to the channel mode-appropriate speech decoder) can be read
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6 out of the DSP's API RAM in real time, and in the uplink direction the user can
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7 feed her own bits to the input of the GSM 05.03 channel encoder, effectively
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8 suppressing the output of the internal vocoder.
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10 The DSP mechanism in question is known to work in TCH/FS and TCH/EFS channel
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11 modes, corresponding to FR1 and EFR codecs; it also appears to work for TCH/HS
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12 (HR1 codec), but we (FreeCalypso) haven't tested it because almost no one uses
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13 that infamous HR1 codec - the commercial GSM network in our part of the world
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14 gives you a full-rate channel if your phone does not support AMR. It would be
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15 possible to implement HR1 in our own test GSM network, but the effort that would
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16 be required is difficult to justify. Exploring TCH tap modes with AMR or CSD
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17 traffic channels is likewise a subject for further study.
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19 In order to make use of this TCH bit access feature, one needs 3 things:
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21 1) Firmware on the Calypso (the ARM part) that reads downlink bits from the DSP
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22 and writes uplink bits into it while doing everything else that a GSM fw must
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23 do in order to operate the MS;
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24
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25 2) Some protocol for passing these TCH bits into and out of the Calypso device;
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27 3) A source for TCH UL bits and a sink for TCH DL bits on the external host.
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29 In the case of FreeCalypso, we have defined our own protocol for passing TCH
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30 bits into and out of Calypso GSM devices running one of our firmwares in the
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31 form of an extension to TI's RVTMUX interface, i.e., we have defined a new
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32 RVTMUX channel for this TCH interface and defined the packet types and formats
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33 to be sent over the wire. On the Calypso side the special functionality in
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34 question was originally implemented in FC Citrine firmware in 2016 and then set
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35 aside for some years; when the right time came to resurrect this feature in late
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36 2022, it turned out that the original implementation from 2016 was slightly
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37 incorrect, and the new implementation in FC Tourmaline fw is slightly different.
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38 On the host tools side the RVTMUX-based TCH interface is supported in rvinterf
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39 and fc-shell; the new version as of fc-host-tools-r18 supports both 2016 and
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40 2022 versions of this over-the-wire interface.
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41
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42 The TCH bit access mechanism in FreeCalypso has been designed with an objective
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43 of presenting to the user exactly what TI's DSP presents to us, i.e., standing
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44 out of the way as much as possible. TI's DSP presents TCH downlink bits and
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45 accepts TCH uplink bits in the form of an array of 16-bit words; the bit order
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46 within these words corresponds to the GSM 05.03 channel encoder bit order (with
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47 a couple of TI-specific quirks documented below) and NOT that of the GSM 06.10
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48 or EFR codecs. On the RVTMUX serial interface between the Calypso device and
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49 the external host we transfer each TCH frame as a block of 33 bytes; our Calypso
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50 firmwares translate between these bytes and the DSP's 16-bit words, but do not
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51 reorder or change the bits in any way.
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52
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53 On the host tools side our fc-shell utility provides user commands to save TCH
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54 DL bits into a file and to play TCH UL bits from a file; in the present version
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55 these files are written and read in an ASCII-based hex format. In these ASCII
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56 files each TCH frame is represented as a string of 66 hexadecimal digits, and
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57 these hex digits correspond directly to the 33 bytes being read out of or
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58 written into DSP API words. Therefore, in order to generate and/or interpret
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59 these hexadecimal strings correctly, you (the user) need to understand the bit
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60 order and mapping used by TI's implementation of the GSM 05.03 channel encoder.
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61
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62 As of late 2022, there is a new TCH-tap-modes article in our freecalypso-docs
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63 repository that covers in detail the format of TI's DSP buffers for TCH DL and
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64 UL bits, as well as all known information about TCH DL status words and bit
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65 flags. But here is our original description from 2016:
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66
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67 Recall from the GSM specs that the 260 bits which comprise one speech frame are
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68 not all treated equally, instead they are divided into 182 class 1 bits which
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69 are protected by a convolutional encoder and 78 class 2 bits which are
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70 transmitted without any forward error correction. Furthermore, the first 50 of
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71 the class 1 bits are also protected by a CRC. The order in which the bits
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72 appear on TI's DSP interface corresponds to this division, known as the order of
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73 subjective importance.
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74
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75 Now let's look at the actual bit order and mapping which you need to understand
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76 in order to make sense of the hex strings in tch record and tch play files.
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77 The bit numbering is from the most significant bit to the least significant bit,
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78 i.e., in our string of 66 hex digits the most significant bit of the leftmost
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79 digit is bit 0 and the least significant bit of the rightmost digit is bit 263.
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80 TI's DSP assigns these bits as follows:
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81
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82 * Bits 0 through 181 correspond to the 182 protected (class 1) bits in the
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83 standard GSM 05.03 order;
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84
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85 * Bits 182 through 185 are unused - put zeros there if you are generating them
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86 yourself;
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87
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88 * Bits 186 through 263 correspond to the 78 unprotected (class 2) bits in the
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89 standard GSM 05.03 order.
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90
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91 TCH DL recording
8
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92 ================
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93
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94 When you are in an established voice call in fc-shell, you can record the
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95 downlink TCH bits as follows:
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96
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97 tch record <name of file to put the recording into>
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98
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99 If you would like to record an entire call from the beginning, issue the
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100 tch record command as above before the ATD or ATA command that dials or answers
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101 the call. Either way, whether you are recording a call from the beginning or
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102 from the middle, you need to eventually stop your recording with this command:
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103
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104 tch record stop
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105
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106 You can issue this stop command before or after the call is terminated, but
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107 until you issue this tch record stop command, the output file is not closed and
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108 thus may not be written to the file system.
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109
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110 The recording is written in an ASCII line-based format with one line for every
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111 received TCH DL frame, but the exact format of each written line will depend on
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112 which firmware version is in use. If you are running ancient Citrine firmware
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113 that emits its TCH DL output in the old format from 2016 (now known to be
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114 incomplete and thus unusable for proper decoding), fc-shell will likewise write
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115 its ASCII output in the old format, which won't be covered further as it is
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116 deprecated and not practically useful. However, if you are running current
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117 FreeCalypso firmware with the resurrected (late 2022) version of the TCH tap
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118 feature, each TCH DL frame will be sent by the fw and received by fc-shell in
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119 the new over-the-wire format, and fc-shell will write the recording file in the
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120 new ASCII format documented in the TCH-tap-modes article in freecalypso-docs.
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121
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122 Once you have captured a TCH DL recording, what can you do with it? If the
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123 recording came from an FR1 call, you will need to pass it through an Rx DTX
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124 handler for FR1 (see GSM 06.11, 06.12 and 06.31 specs) before you can pass it
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125 to a naive GSM 06.10 decoder such as classic Unix libgsm, and if the recording
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126 came from an EFR call, you will need to pass it to a proper EFR (not AMR!)
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127 decoder that includes the necessary EFR Rx DTX handler. Neither of the two
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128 just-mentioned library pieces (neither the Rx DTX handler for FR1 nor a proper,
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129 not-same-as-AMR implementation of GSM EFR) could be found among the existing
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130 body of FOSS as of 2022, thus we (FreeCalypso and Themyscira Wireless)
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131 implemented our own. Please look for our GSM codec libraries & utilities
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132 package, which is expected to reach its first official release some time in
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133 early 2023.
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134
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135 Inside our gsm-codec-lib package you will find gsmfr-dlcap-* and gsmefr-dlcap-*
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136 utilities that read TCH downlink capture files written by fc-shell tch record
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137 and perform various decoding operations - please refer to further documentation
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138 within that package.
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139
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140 Please don't use the old fc-tch2fr utility - the function it performs is now
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141 known to be a bogo-transform, and it can't grok the new TCH DL recording format
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142 which you will get with current FreeCalypso fw.
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143
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144 TCH UL play
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145 ===========
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146
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147 The uplink sending mechanism can be exercised as follows:
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148
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149 1. If you are going to be in an FR1 call, prepare a speech sample in the GSM
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150 06.10 codec format using any Unix/Linux audio tool that can write the de
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151 facto standard libgsm format. For example, using SoX:
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152
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153 rec -c1 recording.gsm
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154
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155 SoX will write the recording in the GSM 06.10 libgsm format based on the
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156 .gsm suffix at the end of the recording file name; the -c1 option is needed
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157 to disable stereo, otherwise the recording will be slowed down 2x.
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158 Alternatively, you can use our new gsmfr-encode utility (gsm-codec-lib
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159 package) to encode from WAV into GSM 06.10, or gsmfr-encode-r for raw BE
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160 input instead of WAV.
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161
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162 OTOH, if you are going to be in an EFR call rather than FR1, you will need to
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163 prepare a speech sample in the EFR codec format instead. You will need to
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164 use Themyscira gsmefr-encode or gsmefr-encode-r utilities, or convert from
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165 AMR (MR122 mode only, no DTX) with our gsm-amr2efr utility.
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166
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167 2. Convert your speech sample from libgsm standard format (FR1) or Themyscira
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168 gsmx format (EFR) into our ad hoc hex strings for playing into a TCH uplink:
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169
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170 fc-fr2tch recording.gsm recording.tch-ul
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171
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172 or
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173
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174 fc-efr2tch recording.gsmx recording.tch-ul
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175
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176 3. In fc-shell, when you are in an established voice call, issue this command:
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177
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178 tch play recording.tch-ul
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179
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180 You should now hear the speech sample you recorded in step 1 above on the other
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181 end of the GSM call. Needless to say, the TCH mode of the call (TCH/FS or
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182 TCH/EFS) needs to match the codec in which your to-be-played recording was
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183 prepared, otherwise the other end of the call will receive garbage!
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184
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185 Controlling the selection of speech codec for calls
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186 ===================================================
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187
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188 One very obvious shortcoming of the present facilities for voice TCH redirection
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189 is that we only support FR1 and EFR codecs, but not AMR. However, most GSM
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190 networks prefer to use AMR when the MS supports it - and in regular operation
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191 with a speaker & mic user connection (as opposed to TCH tap modes), our current
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192 FreeCalypso firmwares do support AMR when running on Calypso C035 silicon with
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193 DSP ROM version 3606. (DSP ROM version 3416 together with the respective patch
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194 version also appears to have working AMR, at least in light testing, although
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195 of course we do NOT recommend it for production use.) Therefore, if you wish
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196 to play with EFR, you need to tell the network (via the Bearer Capabilities
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197 information element in CC messages) that your MS does not support AMR, and if
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198 you wish to play with FR1, you need to tell the network that your MS only
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199 supports FR1 and no others.
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200
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201 The outstanding issue here is that we need to implement some mechanism in our
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202 FreeCalypso firmwares, probably a custom AT command, that will modify the
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203 Bearer Capabilities IE (artificially restrict the set of supported codecs) on a
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204 per-call basis. Until we implement the necessary support, the only current way
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205 to create such codec-restricted operation is by writing a /pcm/MSCAP file into
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206 FFS with the desired bit mask of supported codecs - but this method is hugely
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207 inconvenient because this file is read only on fw boot, thus every modification
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208 requires a full reboot cycle.