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doc: new article FR1-Rx-DTX-detail
author Mychaela Falconia <falcon@freecalypso.org>
date Mon, 07 Oct 2024 00:25:50 +0000
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children 62943a1ad64e
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6ab066180ec2 doc: new article FR1-Rx-DTX-detail
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1 Rx DTX handler implementation details
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2 =====================================
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3
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4 As explained in the basic FR1-Rx-DTX article, an Rx DTX handler has to be
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5 inserted between the output of the Rx radio subsystem and the input to the
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6 basic GSM 06.10 speech decoder. In ThemWi codec library architecture, we
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7 normally run a full decoder for GSM-FR that combines the Rx DTX handler and
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8 the basic 06.10 decoder, and the Rx DTX handler block by itself also serves
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9 as a TFO transform.
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10
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11 This Rx DTX handler is based on several GSM specs: 06.11 for the error
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12 concealment function, 06.12 for the comfort noise insertion function, and 06.31
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13 for overall Rx DTX handling. However, these specs give a lot of leeway to
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14 implementors, hence it is prudent to document the specific choices made in the
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15 present ThemWi implementation.
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16
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17 Error concealment implementation
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18 ================================
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19
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20 Error concealment is also called substitution and muting of lost frames. The
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21 implementation of this function in Themyscira libgsmfr2 is based on the Example
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22 solution presented in chapter 6 of 3GPP TS 46.011 (formerly GSM 06.11), applying
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23 the most literal reading to this spec section.
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24
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25 When unusable frames (as defined in GSM 06.31) occur during speech state (i.e.,
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26 not following a SID), the present logic kicks in. For the first BFI following
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27 good speech, the last speech frame is repeated verbatim. On the second BFI the
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28 muting logic of Xmaxc reduction kicks in, decrementing each of the 4 Xmaxc
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29 parameters by 4 with each emitted frame. RPE grid position parameters are
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30 randomized at the same time. The frame in which all 4 Xmaxc parameters equal 0
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31 (either because they were already 0 or because they got reduced to 0 by the
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32 muting sequence) is the last frame emitted in this state; all subsequent BFIs
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33 will be turned into fixed-bit-pattern silence frames as given in TS 46.011
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34 Table 1.
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35
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36 If a BFI comes in when the Rx DTX handler is in its reset (or homed) state, the
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37 output proceeds directly to silence frames.
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38
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39 Comfort noise insertion
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40 =======================
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41
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42 Comfort noise generation and updating is specified in GSM 06.12 section 6.1.
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43 Most of this section is very straightforward, and is implemented in ThemWi
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44 libgsmfr2 exactly as specified, except for the very last sentence in that
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45 section:
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46
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47 "When updating the comfort noise, the parameters above should preferably be
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48 interpolated over a few frames to obtain smooth transitions."
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49
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50 ThemWi implementation of Rx DTX handler in libgsmfr2 does not do this "should
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51 preferably" part: no interpolation is done on CN parameters; as soon as each
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52 SID update comes in, the new parameters are used immediately for all generated
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53 CN frames.
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54
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55 Because the spec says "should preferably" rather than "shall", we can "get away"
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56 with not implementing CN interpolation. But there is an even more profound
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57 issue: we have yet to find anyone else's implementation, which we could use as
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58 guidance, that does CN parameter interpolation for FRv1. (Such interpolation
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59 is mandatory and defined in bit-exact terms for HRv1 and EFR, but FRv1 is a
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60 different story.)
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61
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62 We had a hope that Nokia TCSM2 (a historical hw implementation of GSM TRAU
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63 network element) might implement CN interpolation for FRv1 - but our
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64 experimental findings on that platform are inconclusive:
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65
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66 * When acting as a TFO transform for FRv1, this TRAU does not interpolate CN
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67 parameters, it makes abrupt changes in CN output just like our implementation
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68 - but it effects a strange delay of 24 frames, suggesting that they have some
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69 code paths that assume CN interpolation would be applied.
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70
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71 * When the TRAU acts as a regular speech decoder (not TFO), it is not clear how
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72 it performs any of Rx DTX functions: Nokia chose to not implement the optional
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73 in-band homing feature for FRv1, thus we have no way to explore bit-exact
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74 behaviour of their speech decoder via test sequences.
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75
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76 Another enticing idea would be to statically reverse-engineer the DSP ROM of TI
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77 Calypso chip and thus recover its complete speech Rx chain - but of course the
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78 effort would be extremely massive, and is not likely to happen any time soon.
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79
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80 Until we either get around to the far-future task of Calypso DSP static
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81 reversing or find some other implementation of GSM-FR Rx DTX handler that does
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82 CN interpolation and whose operation we can replicate, we shall stick to the
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83 simple approach of not doing CN interpolation.
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84
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85 Handling of SID frames with Xmaxc discrepancy
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86 =============================================
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87
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88 Per GSM 06.12 section 5.2, all 4 subframe Xmaxc parameters in a SID frame are
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89 supposed to be equal, encoding the quantized form of mean(Xmax). However, what
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90 should Rx DTX implementations do when they receive an otherwise-valid SID frame
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91 in which these 4 parameters are not all equal? In our implementation, we handle
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92 such discrepancy as follows:
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93
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94 * In those frame positions in which we receive a fresh SID (initial or update),
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95 the CN frame we emit is a direct transformation of the received SID, and all
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96 4 Xmaxc parameters are passed through intact.
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97
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98 * When we emit CN frames based on remembered LARc and Xmaxc parameters, we use
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99 the last-subframe Xmaxc from the most recently received SID frame.
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100
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101 Lost SID handling and CN muting
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102 ===============================
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103
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104 In accord with GSM 06.11 sections 5.3, when we receive an unusable frame in a
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105 TAF position during CN insertion state, we set a flag that remembers this
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106 condition, but don't switch to CN muting right away. Per section 5.4 of the
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107 same spec, we initiate CN muting when a second lost SID event occurs (unusable
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108 frame received in a TAF position) without intervening good speech frames or
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109 accepted SID frames.
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110
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111 When we do enter CN muting state, we decrement CN Xmaxc (always the same for
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112 all 4 subframes) by 4 on each output frame, following the Example solution of
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113 3GPP TS 46.011 (formerly GSM 06.11) chapter 6. Once this CN Xmaxc reaches 0,
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114 we switch to emitting fixed-bit-pattern silence frames of TS 46.011 Table 1.
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115
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116 Handling of invalid SID frames
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117 ==============================
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118
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119 In agreement with GSM 06.31 spec, we recognize invalid SID and invoke the
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120 appropriate handler in all 3 combinations: BFI=0 SID=1, BFI=1 SID=1, and
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121 BFI=1 SID=2. The real complexity, however, lies in what that invalid SID
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122 handler actually does:
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123
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124 * If invalid SID arrives when we are already in CN insertion state, we treat it
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125 the same as an unusable frame (continue CN output with current parameters),
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126 but the flag of lost SID is reset, as required by our interpretation of the
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127 specs.
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128
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129 * If invalid SID arrives in CN muting state, i.e., after two consecutive lost
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130 SID events, the muting continues unaffected, i.e., we don't "rejuvenate"
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131 already-started-muting comfort noise upon receiving invalid SID.
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132
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133 * If invalid SID arrives in good speech state, meaning that we are supposed to
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134 begin a CN insertion period but we didn't get usable parameters for it, we
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135 obtain LARc and mean(Xmax) parameters from the last good speech frame,
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136 following the second option permitted by the "NOTE" at the end of GSM 06.31
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137 section 6.1.2. To get Xmaxc for CN, we dequantize all 4 Xmaxc parameters of
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138 the last good speech frame, average them, then requantize.
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139
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140 * If invalid SID arrives in speech muting state, the invalid SID is ignored and
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141 speech muting continues unaffected.
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142
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143 * If invalid SID arrives in NO_DATA state (initial state out of reset, or the
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144 state after either speech or CN muting has fully decayed), we emit the fixed
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145 silence frame of TS 46.011 Table 1.