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Theory-and-mystery: document written
author Mychaela Falconia <falcon@freecalypso.org>
date Wed, 17 Apr 2024 17:14:41 +0000
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1fd613cec7ab Theory-and-mystery: document written
Mychaela Falconia <falcon@freecalypso.org>
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1 Relation between GSM-EFR and 12k2 mode of AMR
1fd613cec7ab Theory-and-mystery: document written
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2 =============================================
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3
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4 What are the differences between GSM-EFR codec and the highest 12k2 mode of AMR,
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5 or MR122 for short? The most obvious difference is in DTX: the format of SID
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6 frames and even the very paradigm of how DTX works are completely different
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7 between EFR and AMR. But what about non-DTX operation? If a codec session
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8 consists solely of good speech frames, no SIDs and no BFI frame gaps, are EFR
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9 and MR122 strictly identical?
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10
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11 The correct answer is that in the absence of SIDs, EFR and MR122 are directly
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12 interoperable in that the output of an EFR encoder can be fed to the input of
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13 an AMR decoder, and vice-versa. However, the two codecs are NOT identical at
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14 the bit-exact level! The differences are subtle, such that finding them
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15 requires some intense study; here I cover those diffs which I was able to find.
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16
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17 DHF difference and the reason why it occurs
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18 ===========================================
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19
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20 In their official form (non-telco-grade corner-cutting libraries don't count,
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21 no matter how popular among FOSS), both EFR and AMR include codec homing as a
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22 mandatory feature, and the mechanism works on the same principle across all
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23 ETSI/3GPP codecs. The encoder homing frame (EHF) is the same for all codecs:
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24 all 160 samples equal to 0x0008, but each codec has its own decoder homing frame
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25 (DHF). Each codec's respective DHF is the natural output of its encoder when
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26 the input is EHF and the initial state is the reset state - as simple as that.
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27 Note the natural aspect: every spec-defined DHF came about naturally in that
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28 codec, hence the exact set of codec parameters that constitutes a DHF is not a
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29 detail which some standard-setting committee could define arbitrarily.
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30
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Mychaela Falconia <falcon@freecalypso.org>
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31 AMR has 8 different DHFs for its 8 different modes, and the DHF for MR122 is
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32 *not* the same as EFR DHF! Given that this DHF is nothing but the encoder's
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33 natural response to encoding an EHF input, this difference in DHF between EFR
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34 and MR122 indicates the existence of some difference between the two encoders.
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35 A simple experiment, contained in this source tree, reveals what the key
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36 difference is: see src/cod_12k2.c, #ifdef EFR2_VARIANT. When this source is
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37 compiled with -DEFR2_VARIANT in efr2 directory, the resulting encoder produces
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38 DHF (natural response to EHF received in the reset state) that is identical to
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39 the one defined for MR122, proving that this specific change is the reason for
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40 the diff in DHF parameters between EFR and MR122.
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41
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42 The encoder diff that happens here (change from EFR to MR122) is an artificial
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43 delay of 5 ms. In EFR, on each invocation of the encoder, a frame of new 160
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44 speech samples is fed in, and that same frame is subject to encoding. In AMR,
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45 the input is still 160 samples each time, but the frame being encoded consists
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46 of 40 samples from the tail of the previous input and 120 samples from the new
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47 input. The newest 40 samples are used for auto-correlation computation in the
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48 lower modes of AMR (see 3GPP TS 26.090 section 5.2), but in MR122 they do
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49 absolutely nothing until the next invocation of the encoder, effecting an
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50 artificial delay of 5 ms. In true multirate operation this delay is needed to
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51 support seamless mode switching, but in an MR122-only environment it is just
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52 waste.
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53
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54 Other encoder differences
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55 =========================
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56
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57 The 5 ms delay covered above is not the only diff between non-DTX EFR and MR122
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58 encoders. We know that other diffs must exist because the output of the test
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59 encoder built in efr2 directory of this repository does not match that of the
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60 official AMR encoder beyond the initial homing frames; however, those additional
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61 differences have not been studied yet.
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62
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63 Decoder diffs between EFR and MR122
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64 ===================================
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65
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66 The two decoders are also different at the bit-exact level: if you take a "pure"
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67 stream of 12k2 speech frames (no DHF, no SIDs and no BFI frame gaps or defects)
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68 and feed it to EFR and AMR decoders, both starting from external reset state,
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69 the resulting outputs will be different.
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70
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71 Two specific differences in the decoder have been identified:
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72
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73 * The AGC module is different: see agc.c vs agc_amr.c in src directory. The
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74 diffs inside AGC have not been studied yet.
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75
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76 * The post-processing step described in 3GPP TS 26.090 section 6.2.2 (high-pass
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77 filtering) is new with AMR.
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78
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79 The code version built in efr2 directory has these two changes applied; it
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80 passes on all available test sequences (amr122_efr.zip described below), but
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81 there may be other diffs that aren't caught by this test sequence set and which
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82 we therefore have not identified yet.
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83
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84 ETSI/3GPP laxness toward EFR implementors
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85 =========================================
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86
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87 ETSI had a tradition of defining standard GSM codecs (FR, HR, EFR) in bit-exact
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88 form, and every production implementation was required to match the output of
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89 the official reference bit for bit. However, once AMR came out, the regulation
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90 on EFR was loosened. GSM 06.54 document from 2000-08 (ETSI TS 100 725 V5.2.0)
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91 has an appendix-like chapter (chapter 10) whose first paragraph reads:
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92
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93 The 12.2 kbit/s mode of the Adaptive Multi Rate speech coder described
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94 in TS 26.071 is functionally equivalent to the GSM Enhanced Full Rate
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95 speech coder. An alternative implementation of the Enhanced Full Rate
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96 speech service based on the 12.2 kbit/s mode of the Adaptive Multi Rate
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97 coder is allowed. Alternative implementations shall implement the
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98 functionality specified in TS 26.071 for the 12.2 kbit/s mode, with the
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99 exception that the DTX transmission format (GSM 06.81) and the comfort
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100 noise generation (GSM 06.62) shall be used.
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101
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102 It appears that DSP vendors (for GSM MS or for network transcoders, or perhaps
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103 both) weren't too happy with the prospect of having to include two different
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104 versions of _almost_ the same codec algorithm with a bunch of interspersed
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105 subtle diffs, and so the rules were bent: EFR implementors were given permission
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106 to deviate from the original bit-exact definition of EFR in order to have more
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107 commonality with MR122.
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108
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109 But the devil is in the details. If I am seeking to implement this "EFR
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110 alternative 2", where is the new bit-exact reference to be followed for this
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111 option? No such reference C code for this AMR-EFR hybrid appears to have been
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112 published anywhere, but this code must have existed once in unpublished form,
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113 as we do have surviving published _output_ from that mystery code.
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114
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115 The digital companion to just-quoted GSM 06.54 is a ZIP archive named
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116 ts_100725v050200p0.zip; inside this ZIP archive there are 9 inner ZIPs: 8 ZIPs
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117 for the 8 original EFR test sequence disks, plus a later addendum named
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118 amr122_efr.zip. The latter ZIP contains *.cod and *.dec test sequence files in
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119 EFR format (*not* AMR), as well as *.out files from the intended decoding of
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120 *.dec. The transformation from *.cod to *.dec in this set is unchanged EFR
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121 ed_iface, but the encoder run that produced *.cod and the decoder run that
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122 produced *.out were quite special:
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123
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124 * t??_efr.cod contain the same codec parameters as the AMR counterpart in 06.74
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125 test sequence set except for the first two frames in each sequence, which are
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126 proper EFR DHFs. It appears that they ran an essentially-unmodified AMR
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127 encoder in MR122 wtth DTX disabled, then artificially patched the DHF after
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128 MR122 encoder output, then packaged the output in EFR *.cod format - but it
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Mychaela Falconia <falcon@freecalypso.org>
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129 must have been more complicated, as this simplistic approach would not support
1fd613cec7ab Theory-and-mystery: document written
Mychaela Falconia <falcon@freecalypso.org>
parents:
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130 DTX.
1fd613cec7ab Theory-and-mystery: document written
Mychaela Falconia <falcon@freecalypso.org>
parents:
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131
1fd613cec7ab Theory-and-mystery: document written
Mychaela Falconia <falcon@freecalypso.org>
parents:
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132 * dtx?_efr.cod and dtx?_efr2.cod are more intriguing: they are said to
1fd613cec7ab Theory-and-mystery: document written
Mychaela Falconia <falcon@freecalypso.org>
parents:
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133 correspond to VAD1 and VAD2 in the AMR reference source, yet these sequences
1fd613cec7ab Theory-and-mystery: document written
Mychaela Falconia <falcon@freecalypso.org>
parents:
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134 have EFR SID frames in their silence parts, not AMR DTX. Thus someone must
1fd613cec7ab Theory-and-mystery: document written
Mychaela Falconia <falcon@freecalypso.org>
parents:
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135 have constructed an encoder that combines most of AMR code (including AMR VAD
1fd613cec7ab Theory-and-mystery: document written
Mychaela Falconia <falcon@freecalypso.org>
parents:
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136 and the AMR version of 12k2 speech encoding) with EFR Tx DTX logic and EFR SID
1fd613cec7ab Theory-and-mystery: document written
Mychaela Falconia <falcon@freecalypso.org>
parents:
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137 generation - quite a feat!
1fd613cec7ab Theory-and-mystery: document written
Mychaela Falconia <falcon@freecalypso.org>
parents:
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138
1fd613cec7ab Theory-and-mystery: document written
Mychaela Falconia <falcon@freecalypso.org>
parents:
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139 * In the decoder direction, the hack presented in efr2 directory of this code
1fd613cec7ab Theory-and-mystery: document written
Mychaela Falconia <falcon@freecalypso.org>
parents:
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140 repository is sufficient to produce a matching *.out for every *.dec in the
1fd613cec7ab Theory-and-mystery: document written
Mychaela Falconia <falcon@freecalypso.org>
parents:
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141 amr122_efr.zip mystery collection, including dtx?_efr.dec and dtx?_efr2.dec.
1fd613cec7ab Theory-and-mystery: document written
Mychaela Falconia <falcon@freecalypso.org>
parents:
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142 However, we made our hack by starting with EFR reference source and making
1fd613cec7ab Theory-and-mystery: document written
Mychaela Falconia <falcon@freecalypso.org>
parents:
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143 small surgical changes to it; I wonder if whoever did the original feat at
1fd613cec7ab Theory-and-mystery: document written
Mychaela Falconia <falcon@freecalypso.org>
parents:
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144 ETSI/3GPP started with AMR source instead and outfitted it with ability to
1fd613cec7ab Theory-and-mystery: document written
Mychaela Falconia <falcon@freecalypso.org>
parents:
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145 understand EFR SID frames and do comfort noise generation per GSM 06.62 -
1fd613cec7ab Theory-and-mystery: document written
Mychaela Falconia <falcon@freecalypso.org>
parents:
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146 that approach would be a big feat, just like with the encoder.
1fd613cec7ab Theory-and-mystery: document written
Mychaela Falconia <falcon@freecalypso.org>
parents:
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147
1fd613cec7ab Theory-and-mystery: document written
Mychaela Falconia <falcon@freecalypso.org>
parents:
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148 The present author considers it a shame that whatever AMR-EFR hybrid programs
1fd613cec7ab Theory-and-mystery: document written
Mychaela Falconia <falcon@freecalypso.org>
parents:
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149 were used to generate the sequences in amr122_efr.zip were never published. In
1fd613cec7ab Theory-and-mystery: document written
Mychaela Falconia <falcon@freecalypso.org>
parents:
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150 the absence of such published code, the details of exactly what was done by
1fd613cec7ab Theory-and-mystery: document written
Mychaela Falconia <falcon@freecalypso.org>
parents:
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151 those commercial DSP/transcoder vendors who combined AMR with EFR will remain
1fd613cec7ab Theory-and-mystery: document written
Mychaela Falconia <falcon@freecalypso.org>
parents:
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152 elusive.