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TW-TS-005 reader: fix maximum line length bug TW-TS-005 section 4.1 states: The maximum allowed length of each line is 80 characters, not including the OS-specific newline encoding. The implementation of this line length limit in the TW-TS-005 hex file reader function in the present suite was wrong, such that lines of the full maximum length could not be read. Fix it. Note that this bug affects comment lines too, not just actual RTP payloads. Neither Annex A nor Annex B features an RTP payload format that goes to the maximum of 40 bytes, but if a comment line goes to the maximum allowed length of 80 characters not including the terminating newline, the bug will be triggered, necessitating the present fix.
author Mychaela Falconia <falcon@freecalypso.org>
date Tue, 25 Feb 2025 07:49:28 +0000
parents 62943a1ad64e
children
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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
555
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104 In accord with GSM 06.11 section 5.3, when we receive an unusable frame in a
552
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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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Mychaela Falconia <falcon@freecalypso.org>
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144 state after either speech or CN muting has fully decayed), we emit the fixed
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Mychaela Falconia <falcon@freecalypso.org>
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145 silence frame of TS 46.011 Table 1.