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doc/AMR-library-API: describe the decoder
author Mychaela Falconia <falcon@freecalypso.org>
date Sun, 19 May 2024 21:32:31 +0000
parents 4034c2b06ec8
children
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1 At the level of provided functionality and architectural structure, ETSI GSM
22601ae99434 doc/FR1-Rx-DTX article written
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2 specifications for DTX (discontinuous transmission) are very symmetric between
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3 FR and EFR: the same DTX functionality is specified for both codecs, with the
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4 same overall architecture. However, there is one important difference: in the
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5 case of EFR the complete implementation of all DTX functions (for both Tx and
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6 Rx) forms an integral and inseparable part of the reference codec (implemented
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7 in C) from the beginning, whereas in the case of FR1 the addition of DTX is
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8 somewhat of an afterthought. GSM 06.10 defines a "pure" FR codec without any
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9 DTX functions, and this most basic spec can be and has been implemented in this
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10 "pure" form - classic Unix libgsm from 1990s is a proper, fully compliant
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11 implementation of GSM 06.10, but only this spec, without any DTX. In contrast,
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12 there has never existed a "pure" implementation of GSM 06.60 EFR codec without
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13 associated Tx and Rx DTX functions. Furthermore, there is an important
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14 distinction between Tx and Rx DTX handlers for FR1:
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15
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16 * Anyone who seeks to implement Tx DTX for FR1 would have to dig into the guts
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17 of GSM 06.10 encoder and augment it with VAD and SID encoding functions per
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18 GSM 06.32 and 06.12 specs.
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19
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20 * In contrast, the Rx DTX handler for FR1 is modular: the way it is specified
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21 in GSM 06.11, 06.12 and 06.31 is a front-end to unmodified GSM 06.10 decoder.
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22 On the Rx side, the interface from the radio subsystem to the Rx DTX handler
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23 consists of 260 bits of frame plus BFI and TAF flags (the spec also defines a
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24 SID flag, but it is determined from frame payload bits), and then the
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25 interface from the Rx DTX handler to the GSM 06.10 decoder is another FR frame
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26 of 260 bits.
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27
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28 What are the implications of this situation for the GSM published-source
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29 software community? Prior to the present Themyscira offering, there has always
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30 been libgsm, but no Rx DTX handler. If you are working with a GSM uplink RTP
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31 stream from a BTS or a GSM downlink frame stream read out of TI Calypso DSP or
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32 some other GSM MS PHY, feeding that stream directly to libgsm (without passing
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33 through an Rx DTX handler) is NOT acceptable: a "bare" GSM 06.10 decoder won't
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34 recognize SID frames and won't produce the expected comfort noise output, and
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35 what are you going to do in those 20 ms windows in which no good traffic frame
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36 was received? The situation becomes especially bad (unkind on ears) if you are
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37 reading received downlink frames out of TI Calypso DSP: the DSP's buffer will
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38 have *some* bit content in every 20 ms window, but naturally this bit content
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39 will be garbage during those frame windows when no good frame was received;
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40 feeding that garbage to libgsm produces noises that are very unkind on ears.
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41
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42 The correct solution is to implement an Rx DTX handler, pass the stream of
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43 frames and flags from the BTS or the MS PHY to this handler first, and then pass
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44 the output of this handler to the standard GSM 06.10 decoder (classic libgsm or
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45 some updated port thereof). Themyscira libgsmfrp was our first Free Software
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46 implementation of Rx DTX handler for GSM-FR, implementing SID classification,
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47 comfort noise generation and error concealment. Our new libgsmfr2 offering
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48 takes the harmonization effort (between GSM-FR and other GSM codecs) one step
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49 further, eliminating the dependency on old libgsm and putting all GSM-FR codec
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50 functions "under one roof".
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51
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52 libgsmfrp/libgsmfr2 API documentation
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53 =====================================
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54
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55 The Rx DTX component of libgsmfr2 has the same API as our previous libgsmfrp,
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56 except for dropping the use of <gsm.h> and its types and needing to include our
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57 new API header <tw_gsmfr.h>. The present article previously contained the full
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58 description of this API; that description has now been moved to FR1-library-API
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59 article, where the whole of libgsmfr2 is documented.
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60
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61 Standalone exerciser utility
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62 ============================
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63
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64 The present GSM codec libraries and utilities package includes a standalone
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65 utility that exercises our Rx DTX handler for GSM-FR. This utility is
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66 gsmfr-preproc, to be run as follows:
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67
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68 gsmfr-preproc input.gsmx output.gsm
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69
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70 The input is an extended-libgsm file that can contain SIDs and BFI frame gaps
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71 in addition to regular GSM 06.10 speech frames (see Binary-file-format article);
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72 the output is GSM 06.10 speech frames only.
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73
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74 False SID detection
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75 ===================
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76
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77 The intent of GSM-FR spec authors was that the sets of possible speech frames
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78 and possible SID frames be disjoint. Prior to introduction of DTX, there were
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79 only regular speech frames per GSM 06.10, no SID, and a receiver had to deal
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80 with only two possibilities: either a good speech frame was received, or the
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81 frame was lost to radio errors or FACCH stealing (unusable frame). When SID
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82 frames were introduced for the purpose of intentional DTX as distinct from
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83 radio errors, the intent was that SID was to be a "new animal" not seen before,
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84 distinct from regular speech frames. There is, however, a small blemish in the
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85 actual system as realized: if the SID frame detector and the Rx DTX handler
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86 that follows it in the Rx chain follow the rules of GSM 06.31 sections 6.1.1
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87 and 6.1.2, respectively (like our implementation does), then some speech frames
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88 may be mistaken for invalid SID, or perhaps even for valid SID, producing a
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89 nonzero failure rate in this mechanism.
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90
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91 Official test sequence 02 in the set of 5 provided by ETSI exhibits this effect:
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92 Seq02.inp is a legitimate 13-bit linear PCM input to the speech encoder, and the
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93 corresponding output of GSM 06.10 encoder is contained in Seq02.cod. However,
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94 that output contains some frames that are mistakenly classified as SID=1
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95 (invalid SID) by the rules of GSM 06.31 section 6.1.1! It is true that these
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96 ancient test sequences chronologically predate the invention of DTX and
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97 GSM 06.31, but we still need to bear in mind that this problematic Seq02.cod is
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98 not an artificially constructed sequence of 06.10 codec parameters: it is the
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99 required output of the prescribed bit-exact encoder given a legitimate PCM
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100 input! There does not exist a perfect solution to this problem: as usual,
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101 real-world engineering is all about trade-offs and compromises, and occasionally
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102 a gear will slip. The best we can do is to model the probability of such
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103 gear-slip or wrong detection events, and engineer our systems to reduce this
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104 probability to a level that is deemed acceptable - which is exactly what GSM
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105 spec designers did here.
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106
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107 As of gsm-codec-lib-r3, gsmrec-dump utility shows the SID classification result
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108 (GSM 06.31 section 6.1.1) in addition to parsed 06.10 codec parameters for each
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109 frame, thus one can inspect FR-encoded streams and check for this blemish.
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110
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111 Effect of extra preprocessing
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112 =============================
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114 What will happen if the output of our Rx DTX preprocessor (e.g., the output of
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115 gsmfr-preproc utility) is fed to another utility such as gsmfr-decode that also
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116 applies the same preprocessor to its input? In other words, what is the effect
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117 of a secondary preprocessor application to previous preprocessor output?
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119 Most of the time, the second preprocessor pass will be an identity transform
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120 under these conditions, as the input to that second pass will consist entirely
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121 of good speech frames, no SIDs and no BFIs. Any speech frames in the original
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122 input that were mistakenly classified as SID (valid or invalid) have already
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123 been converted to comfort noise (or to the silence frame in one corner case of
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124 invalid SID), hence they are no longer present in the output to trigger this
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125 effect a second time. However, there is still a small possibility that a
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126 second pass will be a non-identity transform: pseudorandom RPE pulse parameters
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127 in our comfort noise output are uniformly distributed between 1 and 6 (GSM 06.12
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128 section 6.1), and if PRNG dice roll such that at least 80 out of 95 SID codeword
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129 bit positions (all in the xMc part of the frame) are all zeros, the resulting
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130 CN frame will be liable to misinterpretation as SID (invalid SID most of the
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131 time, or even more rarely valid SID if at least 94 out of 95 SID codeword bit
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132 positions are all zeros) if fed to the preprocessor a second time. That second
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133 pass would then further alter those affected frames, but no others.