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doc/AMR-library-API: document AMR-EFR hybrid decoder
author Mychaela Falconia <falcon@freecalypso.org>
date Sun, 19 May 2024 22:22:40 +0000
parents ad032051166a
children
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1 Emulation of other people's AMR-EFR hybrid implementations
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2 ==========================================================
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3
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4 [Please see AMR-EFR-philosophy article for background information on the
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5 differences between classic GSM-EFR and the 12k2 mode of AMR, and how ETSI/3GPP
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6 loosened their regulation on bit-exactness of EFR, then continue here.]
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8 Experiments reveal that the extant commercial GSM networks of T-Mobile USA and
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9 Telcel Mexico (and likely other countries' GSM networks too) use a GSM speech
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10 transcoder implementation that performs EFR encoding and decoding (for times
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11 when the MS declares no support for AMR and the network falls back to EFR) per
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12 the alternative which we call AMR-EFR hybrid. The needed experiments are done
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13 by using a FreeCalypso phone or devboard as the MS (declaring yourself to the
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14 network as non-AMR-capable via AT%SPVER), capturing TCH DL and feeding TCH UL
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15 with FreeCalypso tools, and using a SIP-to-PSTN connectivity provider (BulkVS
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16 or Anveo) on the other end of the test call that allows the experimenter to
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17 receive the PCMU or PCMA sample stream coming out of the GSM network's speech
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18 transcoder and feed a crafted PCMU/PCMA sample stream in the other direction.
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20 In this experimental setup, bit-exact details of how the GSM network under study
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21 implements EFR decoding can be tested by feeding a controlled sequence of EFR
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22 codec frames (beginning with at least two DHFs) to GSM Um uplink and observing
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23 the PCMU or PCMA sample stream received on the IP-PSTN end of the call.
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24 Similarly, bit-exact details of how the NUS implements EFR encoding can be
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25 tested by feeding controlled PCMU/PCMA sample streams into the call from IP-PSTN
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26 and observing what the network emits on GSM Um downlink. In the latter case,
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27 frame synchronization finding tricks described in ETSI/3GPP test sequence specs
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28 need to included as part of the experiment.
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30 When these experiments were performed on the GSM networks of T-Mobile USA and
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31 Telcel Mexico, it was immediately apparent that they do not implement EFR
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32 following the original bit-exact code of GSM 06.53: feeding any of the original
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33 EFR test sequences from GSM 06.54 to the NUS does not produce matching results.
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34 However, when I tried feeding EFR codec frame sequences from amr122_efr.zip
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35 (the late addendum to GSM 06.54 for the AMR-EFR hybrid option) to GSM UL, the
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36 PCMU (T-Mobile USA) or PCMA (Telcel Mexico) output from the GSM network's EFR
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37 decoder matched _those_ test sequences, indicating that these networks use the
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38 AMR-EFR alternative implementation.
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39
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40 Creating tinkerer-oriented FOSS tools that can emulate or replicate the poorly
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41 defined "EFR alternative 2" implemented by these extant commercial networks has
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42 been a sportive challenge ever since. The present development in Themyscira
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43 GSM codec libraries and utilities suite is a step toward conquering that
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44 challenge: we are now able to replicate the mystery commercial transcoder in
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45 non-DTX operation, specifically:
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46
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47 a) We can feed a SID-free stream of EFR codec frames to GSM UL, beginning with
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48 DHF, and get the expected result on PCMU or PCMA;
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49
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50 b) In the encoder direction, for the first 7 frames after EHF, before DTX is
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51 allowed to kick in, we can get GSM DL output from the network that matches
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52 our expectations.
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53
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54 Encoder 5 ms delay and DHF transformation
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55 =========================================
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56
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57 One of the diffs between classic EFR and MR122 in the encoder direction is the
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58 artificial delay of 5 ms introduced in the AMR version. In true multirate
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59 operation this delay is needed to support seamless switching between codec
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60 modes, but when the only allowed codec rate is 12k2 (which is the case with EFR
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61 by definition), this delay is pure waste. (Needless to say, an extra delay of
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62 5 ms is nothing compared to the egregious latencies introduced by today's ugly
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63 and horrible world of IP-based transport everywhere, but still...) This
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64 artificial 5 ms delay in the encoder is the reason for the DHF difference
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65 between EFR and MR122 - but here is the wild part: instead of recognizing this
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66 artificial delay as unnecessary and wasteful for 12k2-only EFR and removing it
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67 from the AMR-EFR hybrid contraption, those commercial transcoder vendors and
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68 the people who prepared amr122_efr.zip for ETSI/3GPP (were they the same
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69 people?) kept this 5 ms encoder delay, keeping the whole encoder unchanged AMR
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70 except for whatever insane trickery they did to fit EFR DTX logic and EFR SID
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71 generation into it, but added special DHF transformation logic on the output of
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72 this AMR encoder to produce compliant EFR DHF when the input is EHF.
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73
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74 Exactly how this DHF transformation is done in those actually-deployed AMR-EFR
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75 hybrid encoders is a bit of a mystery. My first thought was to compare the
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76 speech parameters emitted by the AMR encoder against MR122 DHF, and if the
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77 result is a match, replace that MR122-DHF parameter set with EFR DHF. This
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78 approach is implemented in the simple amr_dhf_subst_efr() function in libtwamr.
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79 One distinctive signature of this approach is that the output of a hybrid
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80 encoder following this method can never equal MR122 DHF: this one particular
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81 bit pattern is precluded from the set of possible outputs under all conditions.
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82
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83 However, subsequent experiments quickly revealed that the logic implemented by
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84 the transcoder in the network of T-Mobile USA must be different. One of the
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85 counter-intuitive effects of the 5 ms artificial delay in the MR122 encoder is
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86 what happens when the encoder is in its homed state and you feed it an input
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87 frame whose first 120 samples are all 0x0008, but some (as few as one or as many
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88 as all) of the last 40 samples are different. This frame does not meet the
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89 definition of EHF and won't be recognized as such - the encoder won't get
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90 rehomed once again after processing this frame - yet the output will be
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91 bit-exact MR122 DHF. How do those AMR-EFR hybrid encoders handle *this* case?
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92
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93 Experiments on T-Mobile reveal that in the case in question, the encoded frame
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94 is emitted with the bit pattern of MR122 DHF, *not* transformed into EFR DHF.
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95 Because MR122-DHF output is impossible with an encoder that implements logic
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96 like our amr_dhf_subst_efr() first cut, we know (by modus tollens) that
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97 T-Mobile's implementation uses some different logic.
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98
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99 Our new (current) working model is implemented in amr_dhf_subst_efr2(): we
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100 replace the output of the AMR encoder with EFR DHF if the raw encoder output
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101 was MR122 DHF *and* the input frame was EHF. This version appears to match
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102 the observed behavior of T-Mobile USA so far.
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103
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104 EFR DHF in the decoder direction
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105 ================================
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106
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107 The way decoder homing works in all ETSI/3GPP-defined speech codecs, there is
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108 an explicit check against known DHF bit pattern (up to first subframe only) at
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109 the beginning of the decoder (if the decoder is homed and the input is DHF per
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110 this reduced check, artificially emit EHF, stay homed and do nothing more), and
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111 a second similar check against the known DHF bit pattern (full frame comparison
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112 this time) at the end of the decoder, triggering the state reset function on
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113 match. These checks are (and can only be) implemented by explicit comparison
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114 against a known hard-coded DHF pattern - hence it doesn't matter in the decoder
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115 case whether the DHF is natural (as in all properly ETSI-defined codecs) or
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116 artificial as in AMR-EFR hybrid. Thus the "correct" handling of DHF in the
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117 AMR-EFR hybrid decoder is a matter of replacing the check against MR122 DHF bit
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118 pattern with a check against the different bit pattern of EFR DHF.
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119
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120 The decoder engine in libtwamr supports this different-DHF option for MR122
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121 decoding by way of a bit set in the mode field in struct amr_param_frame - see
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122 the detailed description in AMR-library-API article.
311
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123
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124 Command line utilities for AMR-EFR hybrid
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125 =========================================
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126
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127 The present package includes a small set of command line utilities that work
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128 with the AMR-EFR hybrid described above:
311
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129
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130 amrefr-encode-r
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131 amrefr-decode-r
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132
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133 These two utilities function just like gsmefr-encode-r and
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134 gsmefr-decode-r described in Codec-utils article, but implement the
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135 AMR-EFR hybrid version of the codec instead of original EFR. The
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136 no-DTX limitation applies: amrefr-encode-r lacks -d option, and the
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137 input to amrefr-decode-r must not contain any SID frames.
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138
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139 amrefr-tseq-enc
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140 amrefr-tseq-dec
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141
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142 These two utilities are AMR-EFR counterparts to gsmefr-etsi-enc and
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143 gsmefr-etsi-dec test programs described in EFR-testing article. They
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144 pass all tests on the non-DTX t??_efr.* sequences in ETSI's
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145 amr122_efr.zip, but not on any of the DTX sequences included in the
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146 same ZIP. Just like amrefr-encode-r, amrefr-tseq-enc lacks -d option,
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147 and amrefr-tseq-dec rejects input containing SID frames.