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annotate doc/Calypso-TCH-downlink @ 282:9ee8ad3d4d30

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frtest: rm gsmfr-hand-test and gsmfr-max-out utils These hack programs were never properly documented and were written only as part of a debug chase, in pursuit of a bug that ultimately turned out to be in our then-hacky patch to osmo-bts-sysmo, before beginning of proper patches in Osmocom. These hack programs need to be dropped from the present sw package because they depend on old libgsm, and we are eliminating that dependency.
author Mychaela Falconia <falcon@freecalypso.org>
date Sun, 14 Apr 2024 05:44:47 +0000
parents 2254fa16ec5f
children 03b0702f4463
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1 It has been discovered that the implementation of standard signal processing
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2 chains for speech TCH downlink and uplink in the DSP ROM in the Calypso GSM
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3 baseband processor allows these signal processing chains to be tapped at certain
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4 points, as detailed in the TCH-tap-modes article in our freecalypso-docs Hg
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5 repository. There is a mechanism to capture the stream of received traffic
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6 frames on TCH DL, and there is another mechanism by which an externally supplied
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7 stream can be "played" into TCH UL.
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8
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9 I (Mother Mychaela) previously played with this functionality back in 2016, and
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10 it's been mostly shelved since then. This functionality became interesting
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11 once again in late 2022: now that we have a proper set of codec libraries (the
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12 present package) and a proper understanding of Rx DTX handling requirements, we
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13 can take another shot at decoding TCH downlink captures taken from Calypso GSM
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14 MS.
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15
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16 The overall functionality is described in the TCH-tap-modes article in
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17 freecalypso-docs; the mechanism for capturing TCH DL bits from Calypso DSP is
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18 split between FreeCalypso GSM MS firmware (added to FC Tourmaline as of
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19 2022-12-13) and the fc-shell utility in the FC host tools package, updated as
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20 of fc-host-tools-r18 to support the new FreeCalypso fw. There is also a set of
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21 utilities included in the present GSM codec libraries & utilities package for
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22 parsing and decoding these Calypso TCH DL captures; the present document
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23 describes these utilities.
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24
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25 As explained in the TCH-tap-modes article in freecalypso-docs, the mechanism
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26 for capturing TCH DL is currently implemented for TCH/FS, TCH/HS and TCH/EFS,
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27 corresponding to FR1, HR1 and EFR codecs. However, further parsing and decoding
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28 support has only been implemented for FR1 and EFR codecs in the present package,
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29 in the form of the following utilities:
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31 gsmfr-dlcap-parse This program reads a TCH/FS DL capture file and parses
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32 it for human analysis. All input fields are passed
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33 through to the output, but the program also computes
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34 the ternary SID flag of GSM 06.31 section 6.1.1 from
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35 the payload bits (for comparison against what the DSP
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36 wrote in its status word 0) and prints all broken-down
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37 parameter fields of each GSM 06.10 FR1 codec frame.
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38
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39 gsmfr-dlcap-gsmx This program reads a TCH/FS DL capture file and converts
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40 it into an extended-libgsm (gsmx) file containing a mix
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41 of FR1 codec frames and Themyscira BFI markers. The
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42 latter BFI markers will be emitted in those frame
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43 positions where FACCH was received instead of speech,
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44 or where the DSP otherwise indicated BFI=1. The gsmx
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45 output from this utility needs to be fed to gsmfr-decode
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46 from the present package, so that our FR1 Rx DTX
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47 preprocessor will take care of SIDs and BFIs, completing
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48 the required GSM MS processing chain for TCH/FS DL.
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49
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50 gsmefr-dlcap-parse This program reads a TCH/EFS DL capture file and parses
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51 it for human analysis. All input fields are passed
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52 through to the output, but the program also computes
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53 the ternary SID flag of GSM 06.81 section 6.1.1 from
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54 the payload bits (for comparison against what the DSP
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55 wrote in its status word 0) and prints all broken-down
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56 parameter fields of each EFR codec frame. Finally, each
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57 triplicated bit group of GSM 05.03 section 3.1.1.2 is
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58 printed as an octal digit, to aid human analysis of how
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59 the DSP writes these bits in its a_dd_0 buffer.
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61 gsmefr-dlcap-gsmx This program reads a TCH/EFS DL capture file and
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62 converts it into a gsmx binary file, containing a mix
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63 of EFR codec frames and Themyscira BFI markers. The
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64 latter BFI markers will be emitted in those frame
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65 positions where FACCH was received instead of speech,
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66 or where the DSP otherwise indicated BFI=1. The gsmx
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67 output from this utility needs to be fed to
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68 gsmefr-decode (or gsmefr-decode-r) from the present
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69 package.
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71 gsmefr-dlcap-dec This program reads a TCH/EFS DL capture file and feeds
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72 it directly to the EFR reference decoder implemented in
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73 libgsmefr, without going through a gsmx intermediary.
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74
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75 Additional notes:
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76
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77 * The new gsmfr-dlcap-gsmx utility described above replaces the old fc-tch2fr
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78 utility from FC host tools - the latter should now be considered a bogon.
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79 The required GSM MS processing chain for TCH/FS DL includes the step of Rx
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80 DTX handler between the output of GSM 05.03 channel decoder and the input of
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81 GSM 06.10 speech decoder; the old chain of fc-tch2fr followed by libgsm
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82 decoding omitted this critical step and thus produced very unkind-on-ears
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83 sounds.
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85 * gsmefr-dlcap-dec has been written as a bold attempt to replicate the complete
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86 Rx DTX handler and speech decoder (the part of TCH DL processing chain that
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87 sits past the a_dd_0 buffer) as they are implemented inside TI's DSP. Such a
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88 feat won't be possible for FR1 codec (other than by a Herculean effort of full
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89 static reversing of the DSP ROM) because there is no bit-exact definition of
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90 FR1 Rx DTX functions in GSM specs, but for EFR there is a bit-exact reference
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91 implementation from ETSI. *If* TI's DSP matches this bit-exact reference
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92 (there are some aspects of Rx DTX handling where this bit-exact reference is
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93 considered to be an example rather than normative, see GSM 06.61), then there
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94 is a chance we could replicate TI's DSP chain externally - but only if we can
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95 figure out exactly how the bits of a_dd_0[0] drive the logic of their Rx DTX
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96 handler. The Mother's plan is to capture the DSP's decoded speech output from
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97 MCSI on an FCDEV3B using a small FPGA board with a PCM-to-UART logic function,
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98 while simultaneously capturing TCH DL bits in the a_dd_0 buffer, then run
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99 gsmefr-dlcap-dec on the captured TCH DL booty and see if we can replicate the
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100 DSP's end output - but until then, this gsmefr-dlcap-dec program should be
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101 treated as an unfinished experiment in progress.
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102
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103 * In the case of FR1 codec, there is no prescribed bit-exact definition for the
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104 Rx DTX handler (GSM 06.11, 06.12 and 06.31 specs define general requirements,
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105 but aren't bit-exact in most aspects), and the way in which we (Themyscira
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106 Wireless) have implemented our FR1 Rx DTX handler (libgsmfrp in the present
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107 package) perfectly matches our gsmx binary file format for good vs bad frames.
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108 Therefore, in the case of FR1 codec there is nothing to be gained by skipping
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109 gsmx and calling library functions directly, and thus there is no FR1
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110 counterpart to gsmefr-dlcap-dec - just use gsmfr-dlcap-gsmx followed by
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111 gsmfr-decode or gsmfr-decode-r.
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113 * In addition to TCH DL capture files, gsmfr-dlcap-parse also accepts the hex
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Mychaela Falconia <falcon@freecalypso.org>
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114 output from fc-vm2hex, originating from TCS211 voice memo recordings,
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Mychaela Falconia <falcon@freecalypso.org>
parents: 168
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115 including fc-vm2hex output in the case of VM recordings made in DTX mode.
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Mychaela Falconia <falcon@freecalypso.org>
parents: 168
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116 However, if the objective is to play that VM recording and not just look at
62c0359ff2a8 doc/Calypso-TCH-downlink: document use of tools on voice memos
Mychaela Falconia <falcon@freecalypso.org>
parents: 168
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117 parsed bits, the correct approach is to convert the VM file to gsmx with
62c0359ff2a8 doc/Calypso-TCH-downlink: document use of tools on voice memos
Mychaela Falconia <falcon@freecalypso.org>
parents: 168
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118 fc-vm2gsmx, and then decode with gsmfr-decode. Using fc-vm2hex followed by
62c0359ff2a8 doc/Calypso-TCH-downlink: document use of tools on voice memos
Mychaela Falconia <falcon@freecalypso.org>
parents: 168
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119 gsmfr-dlcap-gsmx instead of fc-vm2gsmx won't work!