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beginning of GSM-HR documentation
author Mychaela Falconia <falcon@freecalypso.org>
date Thu, 19 Mar 2026 04:13:45 +0000
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Mychaela Falconia <falcon@freecalypso.org>
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1 Rx DTX handler logic for GSM-HR speech codec
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2 ============================================
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3
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4 With all 3 classic GSM speech codecs (FR, HR and EFR), as TCH UL Rx traffic on
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5 the network side passes from the BTS to the TRAU, the first processing step
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6 performed by the TRAU prior to actual speech decoding is an Rx DTX handler.
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7 (For TCH DL Rx on the mobile side, exactly the same processing steps happen in
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8 total, but because everything is integrated into a single device, interfaces
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9 between steps may be implemented more loosely.)
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10
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11 For GSM-HR codec the 3 controlling specs for different parts of Rx DTX handler
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12 logic are GSM 06.21, GSM 06.22 and GSM 06.41 - however, for the full details
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13 these specs defer to the reference C code in GSM 06.06. This article explains
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14 this logic from all aspects which we find important: what the Rx DTX logic was
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15 in the original reference code from ETSI and how we adapted it in libgsmhr1,
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16 both for the full speech decoder and for our implementation of TFO transform.
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17
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18 Normative vs freely changeable aspects
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19 ======================================
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20
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21 In the case of error-free transmission, such that the receiver never encounters
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22 a frame with BFI or UFI set except during continuation of a DTX pause (after
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23 receiving a valid SID that begins comfort noise insertion) and is never asked
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24 to begin CN insertion with an invalid SID, the full behaviour of the speech
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25 decoder to the final linear PCM output is required to be bit-exact and gets
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26 exercised by test sequences. This bit-exact behaviour includes non-error-
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27 handling aspects of the Rx DTX handler and comfort noise generation, complete
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28 with interpolation for periodic CN updates via subsequent SID frames.
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29
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30 However, the reference C implementation becomes a non-normative example
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31 (allowing changes in logic without violating spec requirements) in the
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32 following aspects:
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33
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34 * Handling of BFI and UFI outside of DTX pauses previously entered via a valid
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35 SID, including most aspects of error concealment;
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36
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37 * Exact manner of comfort noise muting when expected SID updates fail to arrive;
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38
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39 * The exact logic to be applied when a CN insertion period begins with an
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40 invalid SID frame.
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41
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42 Almost-modular nature of GSM-HR Rx DTX handler
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43 ==============================================
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44
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45 An Rx DTX handler can be considered fully modular if its output (which is then
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46 passed as input to the main body of the speech decoder) is a potentially
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47 modified set of speech parameters that can be packed into a new speech frame
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48 and transmitted through a second radio leg with no change in the final output
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49 of the speech decoder. The Rx DTX handler implemented in the reference code
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50 from ETSI (both spec-normative and "example" aspects as broken down above)
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51 _almost_ meets this modularity criterion, but not fully. The following aspects
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52 are non-modular:
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53
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54 * The interpolation of R0 and LPC parameters during comfort noise insertion
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55 (bit-exact implementation considered normative) happens after expansion of
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56 transmitted parameter bits into linear form. In the general case one cannot
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57 produce a new set of encoded parameters (that can be transmitted through a
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58 second radio leg) that will produce the same bit-exact result upon final
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59 decoding.
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60
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61 * Handling of speech frames (not SID, outside of DTX pause state) that are
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62 marked with BFI=0 and UFI=1 (unreliable frames) has both a modular and a
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63 non-modular aspect. If R0 increment is either small enough to not trigger
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64 any mitigation or large enough to where UFI is converted into BFI, the applied
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65 handling is fully modular. However, if R0 increment falls into the narrow
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66 window between the two thresholds, the applied handling (output signal
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67 concealment per GSM 06.21 section 5.1.2) is non-modular: it happens deep in
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68 the guts of the speech decoder and cannot be represented via a modified set
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69 of speech parameters.
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70
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71 TFO transform derived from the reference Rx DTX handler
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72 =======================================================
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73
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74 If one extracts the reference Rx DTX handler from GSM 06.06 code and removes
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75 the two non-modular aspects detailed above, leaving only fully modular logic,
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76 the result can be used as a TFO transform that implements the functions of
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77 TS 28.062 section C.3.2.1.1, specifically Case 1 in which UL may have DTX, but
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78 DL is required to consist of speech frames only.
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79
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80 How does one address the two non-modular aspects of the standard GSM-HR Rx DTX
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81 handler that are not possible in TFO? The simplest implementation is to remove
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82 them altogether:
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83
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84 * Comfort noise parameters are not interpolated, instead an abrupt change in R0
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85 and LPC parameters occurs every 240 ms when a new SID frame arrives.
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86
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87 * UFI is simply dropped in the case when the standard decoder would apply output
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88 signal concealment, i.e., the latter feature is given up.
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89
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90 Obviously this approach constitutes functional regression relative to the
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91 standard speech decoder - thus we were initially hesitant to adopt it. However,
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92 experiments with a real historical TRAU that supports TFO (Nokia TCSM2) reveal
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93 that Nokia implemented exactly the same approach (minimal complexity at the
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94 price of slight functional degradation) in their TRAU DSP firmware. Seeing
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95 that a major classic vendor of GSM infrastructure implemented this simplistic
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96 approach, we are now comfortable with doing the same - especially considering
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97 the work scope limits explained in HR-codec-limits article.
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98
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99 In Themyscira libgsmhr1 implementation, a component has been factored out which
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100 we call the Rx front end (RxFE). This RxFE is our cleaned-up reimplementation
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101 of those parts of the original Rx DTX handler that are fully modular (including
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102 the speech ECU and all CN parameters that aren't interpolated), plus some
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103 additional internal flag inputs and outputs. Out of the latter internal flags,
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104 some are used only by the full speech decoder, while others are used only by
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105 the TFO transform. RxFE state, which also serves as the API-visible TFO
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106 transform state, is a subset of full speech decoder state. However, the core
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107 RxFE function is not exported directly as API; instead the TFO transform API
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108 function is a TFO-specific wrapper around the RxFE.
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109
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110 Detailed RxFE logic and its evolution
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111 =====================================
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112
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113 Now that we have covered the background of the previous sections, we can
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114 properly examine the actual logic of our RxFE, the follow-up logic for CN
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115 interpolation that exists only in the full decoder, and their origins in the
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116 reference GSM 06.06 code.
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117
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118 Unless noted otherwise, all logic described in the following sections is the
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119 same between ETSI original and the present Themyscira implementation. The
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120 internal representation and code structure may be different, but the behavioral
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121 logic remains the same unless explicitly called out otherwise.
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122
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123 Input frame classification
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124 --------------------------
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125
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126 As the very first processing step for every incoming frame, BFI, UFI and SID
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127 flags are combined per GSM 06.41 Table 1 to classify the frame as good speech,
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128 valid SID, invalid SID or unusable for DTX purposes. Note that UFI turns valid
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129 SID into invalid just like BFI, and for DTX purposes all non-SID frames marked
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130 with UFI are considered "unusable". But as we shall see shortly, this
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131 "unusable" classification matters only for DTX and not for speech ECU logic,
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132 which is separate.
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133
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134 Speech vs CNI state
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135 -------------------
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
136
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
137 RxFE state that carries from one frame to the next includes one very important
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
138 two-state flag: either speech or CNI (comfort noise insertion) mode. By
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
139 combining the 4 possible frame classifications from GSM 06.41 Table 1 (see
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
140 above) with these two possible carry-over states, we get 4 possible ways in
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
141 which the current frame may be handled:
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
142
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
143 Input frame class Previously speech Previously CNI
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
144 --------------------------------------------------------------
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
145 SID (valid or invalid) CNIFIRSTSID CNICONT
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
146 Good speech SPEECH SPEECH
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
147 Unusable SPEECH CNIBFI
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
148
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
149 Here we can see that unless we enter DTX/CNI state, neither BFI nor UFI moves
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
150 RxFE logic out of SPEECH handling. This SPEECH handling mode includes the ECU
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
151 and handles both good and bad speech frames. However, once DTX/CNI state has
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
152 been entered, then only a (BFI==0 && UFI==0 && SID==0) good speech frame can
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
153 effect exit from this state!
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
154
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
155 Speech ECU logic
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
156 ================
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
157
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
158 The frame-to-frame persistent state for the ECU consists of the state counter
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
159 variable (range [0,7]) described in GSM 06.21 section 6.3 and a saved copy of
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
160 the last good speech frame. The just-referenced spec section describes the
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
161 logic quite well, but a few additional notes are in order:
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
162
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
163 * The last good speech frame that gets regurgitated in substitution/muting
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
164 states of the ECU is not exactly the same as the actual last good speech frame
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
165 that went through:
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
166
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
167 + GSP0 parameters for the first 3 subframes are replaced with GSP0 parameter
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
168 for the last subframe;
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
169
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
170 + If the frame is voiced, LTP lag parameters are modified - read the code for
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
171 the details.
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
172
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
173 In the original ETSI implementation, these modifications are applied at the
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
174 time of substitution/muting output; in our implementation, they are applied
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
175 at the time when a good speech frame is saved. Our implementation approach
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
176 makes it clearer what state is actually retained, but the functional behaviour
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
177 is exactly the same.
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
178
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
179 * When that last good speech frame gets regurgitated during bad frame handling,
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
180 codevector parameters may be taken either from that saved last good speech
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
181 frame or from the current bad frame. Use of codevector parameters from the
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
182 current bad frame is possible only when the current bad frame and the saved
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
183 last good speech frame have the same voiced vs unvoiced mode. If this mode
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
184 matches for one frame and bad-frame codevector parameters get passed on, but
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
185 the next bad frame has incompatible mode, the saved last good speech frame
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
186 gets used in its entirety once again, subject only to the modifications
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
187 described above.
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
188
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
189 * Our Themyscira version features an extension: if BFI equals 2 instead of 1,
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
190 indicating BFI without payload bits, then there are no bad-frame codevector
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
191 parameters and the saved last good speech frame is used in its entirety,
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
192 just as if BFI frames always have the wrong voiced vs unvoiced mode.
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
193
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
194 BFI out of reset
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
195 ================
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
196
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
197 What happens if the very first input frame in reset state (after external reset
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
198 or after a decoder homing frame) is a bad frame per BFI, or per UFI treated as
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
199 BFI - what is the default "last" good speech frame? In ETSI original code it
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
200 is a frame of all zero parameters, but this oddity is not readily visible - the
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
201 final output of linear PCM is also all zeros, and all is well. In Themyscira
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
202 implementation, the output of our RxFE may be visible externally if it is used
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
203 as a TFO transform - hence more attention was given to this issue.
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
204
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
205 If we feed all zeros as PCM input to a homed standard GSM-HR speech encoder, we
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
206 get this frame, repeating endlessly as long as all-zeros PCM input continues:
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
207
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
208 R0=00 LPC=164,171,cb Int=0 Mode=0
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
209 s1=00,00,00 s2=00,00,00 s3=00,00,00 s4=00,00,00
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
210
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
211 This frame differs from all-zero params only in the LPC set, and this sane-LPC
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
212 silence frame is the one we have adopted as our reset-default fallback frame.
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
213
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
214 When libgsmhr1 full speech decoder engine is used, as opposed to TFO transform,
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
215 there is an additional check. If the current state is the special home state
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
216 (logic required for spec-mandated EHF output with repeated DHF input) and the
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
217 input frame has BFI flag set (no other flags are considered in this case), the
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
218 PCM output is set to all zero samples without leaving the home state. However,
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
219 the regular speech ECU and its last good frame default can still be reached if
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
220 BFI is clear, UFI is set and R0 is high.
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
221
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
222 Comfort noise logic in RxFE
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
223 ===========================
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
224
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
225 GSM 06.22 spec treats the required bit-exact CN generator as a single entity -
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
226 however, in our implementation it is split between the RxFE and the main body
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
227 of the full speech decoder. The bit-exact result in the case of full speech
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
228 decoding remains the same, but our arrangement allows non-interpolated CN
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
229 generation in the TFO transform as well.
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
230
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
231 When our RxFE is used as a TFO transform with DTXd=0 (the mode that includes CN
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
232 generation), CN output from the transform matches GSM 06.22 Table 2, with the
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
233 exception of R0 and LPC parameters. These R0 and LPC parameters will be filled
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
234 as follows:
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
235
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
236 * If CN insertion period begins with a valid SID, R0 and LPC are taken from
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
237 that SID.
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
238
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
239 * If CN insertion period begins with an invalid SID, R0 and LPC are taken from
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
240 the last good speech frame, the one used by the speech ECU. Directly out of
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
241 reset (or after a DHF), these parameters are as shown above:
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
242
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
243 R0=00 LPC=164,171,cb
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
244
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
245 * Any time a new valid SID frame arrives during a CN insertion period, R0 and
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
246 LPC parameters change to this new SID.
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
247
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
248 * Any time the input during CN insertion is either an unusable frame or an
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
249 invalid SID, R0 and LPC parameters remain unchanged from the most recently
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
250 received valid SID, or from the last good speech frame if only invalid SID
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
251 frames have been received in the entire CN insertion period so far.
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
252
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
253 Comfort noise muting
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
254 ====================
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
255
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
256 Per GSM 06.21 sections 5.2.3 and 5.2.4, when SID frames fail to arrive for 3
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
257 consecutive TAF positions, generated comfort noise needs to be muted. We
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
258 implement this logic in our RxFE, and the actual logic is unchanged from ETSI
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
259 reference code - it is described in GSM 06.21 section 6.4.
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
260
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
261 This SID aging and CN muting logic works by counting unusable frames received
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
262 in between SID updates. In the original GSM 06.06 code the criterion to start
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
263 CN muting is:
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
264
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
265 TAF == 1 && CNIBFI_count >= 25
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
266
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
267 In our version we changed it to:
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
268
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
269 CNIBFI_count >= (TAF ? 25 : 36)
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
270
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
271 When TAF is indicated correctly, once every 12 frames and with the flag always
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
272 present at least in BFI frames (consider GSM 08.61 TRAU-8k format), our extended
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
273 criterion is equivalent to the original; however, our version will also produce
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
274 eventual CN muting if TAF is missing.
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
275
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
276 For the purpose of this logic, invalid SID is as good as valid: while it is
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
277 treated just like unusable frames (CNIBFI) for the purpose of R0 and LPC
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
278 parameters and their interpolation (see next section), for the purpose of SID
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
279 aging and CN muting, invalid SID resets the count of unusable frames, and if
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
280 muting already started previously, it is halted at the current (partially muted)
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
281 R0 value.
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
282
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
283 Comfort noise interpolation
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
284 ===========================
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
285
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
286 When our RxFE is invoked internally by our full speech decoder, the RxFE passes
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
287 some additional flags to the main body of the decoder. One of these flags
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
288 controls interpolation of R0 and LPC parameters for CNI, a function that is
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
289 required by the specs with bit-exact stipulation, but which cannot be
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
290 implemented at the level of speech parameters.
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
291
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
292 The only case in which the behaviour of our libgsmhr1 full speech decoder
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
293 differs from ETSI original is when an invalid SID frame arrives immediately out
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
294 of reset, not preceded by any good speech, valid SID or even unusable frames.
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
295 In this case the original GSM 06.06 code uses initialized all-zero state of
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
296 pswOldFrmKsDec[] array, which cannot happen in any other case. In our
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
297 implementation we use LPC=164,171,cb instead, as already explained.
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
298
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
299 Outside of this corner case, invalid SID frames are handled as follows
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
300 (unchanged between EISI original and our version):
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
301
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
302 * If CN insertion period begins with an invalid SID, R0 and LPC are taken from
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
303 the last good speech frame, the one used by the speech ECU. These R0 and LPC
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
304 params are then fed into the prescribed bit-exact interpolation mechanism as
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
305 if CN insertion started with a valid SID frame with these parameters.
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
306
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
307 * Any invalid SID frames that occur in the middle of a CN insertion period are
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
308 treated just like unusable frames for the purpose of interpolation.
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
309
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
310 Return from CN insertion to speech state
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
311 ========================================
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
312
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
313 Exit from DTX/CNI state happens upon receipt of a good speech frame, i.e., a
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
314 frame that meets this criterion:
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
315
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
316 BFI == 0 && UFI == 0 && SID == 0
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
317
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
318 However, the original implementation in GSM 06.06 reference code exhibits this
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
319 flaw: if the speech ECU is in state 6 (see GSM 06.21 section 6.3) and then an
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
320 accepted SID frame (valid or invalid) puts us into DTX state, the first good
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
321 speech frame after this DTX pause will be dropped and replaced with fully muted
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
322 form of the last good speech frame from before the CN insertion period. This
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
323 effect happens no matter how long that DTX pause was - thus the last good speech
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
324 frame being regurgitated (with R0 reduced to 0) may be indefinitely old and out
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
325 of place. Furthermore, if the CNI-exiting good speech frame that is dropped
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
326 here is followed by BFI unusable frames, the ECU will return to state 6 and the
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
327 parameters (other than muted R0) of the last good speech frame from before the
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
328 DTX pause will continue being reused indefinitely.
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
329
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
330 In our libgsmhr1 version, the state counter for the speech ECU is reset to 7
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
331 (the initial home state) whenever our RxFE passes through DTX/CNI state. Since
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
332 only a good speech frame with BFI=0 and UFI=0 can make exit from CN insertion
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
333 state, this reset of ECU state ensures that this good speech frame will pass
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
334 through, and then the ECU will be in state 0 after this talkspurt-opening good
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
335 speech frame.
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
336
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
337 Fully muted state after unusable frames in input
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
338 ================================================
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
339
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
340 If the input to the speech decoder or TFO transform becomes nothing but BFI
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
341 unusable frames, what is the final fully muted or "decayed" output at the level
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
342 of modified speech parameters? In GSM-FR codec there is a special silence frame
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
343 defined in GSM 06.11 Table 1, and the final decayed state is a continuous output
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
344 of these fixed silence frames - irrespective of whether the Rx DTX handler got
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
345 to this fully decayed state from speech or CN muting.
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
346
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
347 However, no equivalent fully decayed state with fixed output is defined for
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
348 GSM-HR. While this aspect is a non-normative "example" implementation detail,
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
349 in both GSM 06.06 reference code and Themyscira libgsmhr1 the fundamental state
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
350 of speech vs CNI persists indefinitely even when fully muted:
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
351
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
352 * If an indefinitely long string of unusable frames occurs in speech state,
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
353 the speech ECU will be in state 6, and the output from the RxFE (externally
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
354 visible in the case of TFO) will endlessly repeat parameters of the last good
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
355 speech frame, except for R0 reduced to 0.
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
356
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
357 * If an indefinitely long string of unusable frames occurs in DTX/CNI state,
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
358 the output form shown in GSM 06.22 Table 2, complete with bit-exact
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
359 pseudorandom sequence in unvoiced codevector parameters, will likewise
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
360 continue indefinitely. LPC parameters will remain from the most recently
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
361 received valid SID frame (or from the last good speech frame if CNI period
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
362 began with invalid SID and no valid SID was received afterward), but R0 will
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
363 be reduced to 0 by the CN muting logic.
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
364
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
365 Because R0 is reduced to 0 in both cases, the above details are generally
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
366 invisible with full endpoint speech decoding. However, they become fully
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
367 visible in the case of TFO transform with DTXd=0.
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
368
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
369 TFO transform with DTXd=1
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
370 =========================
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
371
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
372 The internal RxFE block that emits CN parameters during DTX/CNI state is correct
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
373 for the full endpoint speech decoder application and for TFO transform with
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
374 DTXd=0. The case of TFO transform with DTXd=1 is implemented by calling the
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
375 same RxFE block, then applying this simple modification to its output: if the
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
376 current frame was processed in DTX/CNI mode, the frame of CN parameters is
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
377 transformed into a downlink SID frame by replacing all speech parameters beyond
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
378 R0 and LPC with all-ones SID codeword.
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
379
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
380 The internal RxFE block tells the TFO wrapper when this just-described
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
381 modification should be applied by way of an internal flag. This flag is set
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
382 in two cases:
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
383
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
384 1) When the current frame was processed in DTX/CNI mode, or
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
385
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
386 2) When the speech ECU applied substitution/muting handling to the current
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
387 frame, and the ECU state was 6 or 7 at the beginning of current frame
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
388 processing.
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
389
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
390 The effects of this logic are as follows:
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
391
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
392 1) DTX pauses in UL pass through into DTX pauses in DL, with unusable frames
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
393 and invalid SID replaced with the most recent valid SID, or with R0+LPC from
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
394 the last good speech frame in the case of initial invalid SID. The
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
395 spec-compliant Rx DTX handler in the destination MS can then produce the
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
396 most correct form of comfort noise, including interpolation of R0 and LPC
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
397 parameters.
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
398
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
399 2) When the input to TFO transform is nothing but unusable frames, the downlink
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
400 radio leg should go into DTXd state in order to produce the desired reduction
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
401 in radio interference and BTS power consumption. This effect should happen
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
402 irrespective of whether the "fully decayed" state of RxFE is DTX/CNI muting
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
403 or speech ECU, as covered in the previous section. Our logic of turning
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
404 "fully decayed" ECU state into DTXd SID achieves the desired effect.
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
405
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
406 Finally, there is one more modification applied only in the case of TFO
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
407 transform with DTXd=1 and not in other cases: muting of comfort noise. In the
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
408 case of full endpoint speech decoding or TFO transform with DTXd=0, when the
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
409 criterion for CN muting is first reached, the muting proceeds by decrementing
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
410 R0 by 2 on every frame, i.e., gradually. (See GSM 06.21 section 6.4.) However,
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
411 in the case of TFO transform with DTXd=1, CN muting is effected by reducing R0
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
412 to 0 immediately as soon as CN muting criterion is reached. The rationale is
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
413 as follows:
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
414
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
415 * A TRAU (or TRAU-emulating MGW that feeds Abis to a BTS) has no way of knowing
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
416 exactly which of its continuously emitted DL SID frames will actually get
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
417 transmitted on the air and seen by the MS. Therefore, a muting process that
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
418 gradually decrements R0 with every emitted SID frame would make no sense.
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
419
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
420 * If the destination MS receives a SID update with R0=0 subsequent to whatever
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
421 previous SID it received with non-zero R0, the spec-required CN interpolation
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
422 logic in that MS will produce the desired effect of gradual muting over 240 ms
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
423 - not too far from the 320 ms muting time called for in GSM 06.21 section
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
424 5.2.4.
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
425
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
426 TFO transform homing
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
427 ====================
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
428
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
429 ThemWi implementation of TFO transform includes the feature of in-band homing:
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
430 if the input to the transform is the spec-defined decoder homing frame (DHF),
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
431 this DHF is passed through to the output just like any other good speech frame,
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
432 but the internal state is reset to the initial "home" state.
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
433
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
434 The check for DHF (all bits must match, plus (BFI == 0 && SID == 0) criterion)
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
435 and the resulting state reset happen at the end of frame processing, after the
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
436 output for the current frame has been generated. In the case of ThemWi TFO
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
437 transform for GSM-HR, there are two corner cases in which an incoming DHF may
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
438 be acted upon (produce state reset), but not appear in the output:
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
439
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
440 1) The overall state of RxFE was speech (as opposed to DTX/CNI) and the speech
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
441 ECU state was 6 - the state in which the first received good speech frame
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
442 gets dropped.
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
443
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
444 2) The overall state of RxFE was DTX/CNI and the incoming DHF is marked with
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
445 UFI=1. UFI is not a criterion for DHF detection, only BFI is, but UFI in
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
446 DTX/CNI state will cause current frame processing to treat the frame as
7fc57e2a6784 beginning of GSM-HR documentation
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
447 unusable.