FreeCalypso > hg > gsm-codec-lib
annotate doc/AMR-EFR-philosophy @ 458:e26b974f7ba3
doc/AMR-study-utils: update for the current situation
| author | Mychaela Falconia <falcon@freecalypso.org> |
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| date | Fri, 10 May 2024 18:45:36 +0000 |
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1 Relation between GSM-EFR and 12k2 mode of AMR |
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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; this article documents some of these study |
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16 findings: |
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17 |
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18 https://www.freecalypso.org/hg/efr-experiments/file/tip/Theory-and-mystery |
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19 |
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20 What other DSP/transcoder vendors have done |
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21 =========================================== |
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23 ETSI had a tradition of defining standard GSM codecs (FR, HR, EFR) in bit-exact |
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24 form, and every production implementation was required to match the output of |
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25 the official reference bit for bit. However, once AMR came out, the regulation |
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26 on EFR was loosened. GSM 06.54 document from 2000-08 (ETSI TS 100 725 V5.2.0) |
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27 has an appendix-like chapter (chapter 10) whose first paragraph reads: |
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28 |
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29 The 12.2 kbit/s mode of the Adaptive Multi Rate speech coder described |
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30 in TS 26.071 is functionally equivalent to the GSM Enhanced Full Rate |
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31 speech coder. An alternative implementation of the Enhanced Full Rate |
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32 speech service based on the 12.2 kbit/s mode of the Adaptive Multi Rate |
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33 coder is allowed. Alternative implementations shall implement the |
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34 functionality specified in TS 26.071 for the 12.2 kbit/s mode, with the |
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35 exception that the DTX transmission format (GSM 06.81) and the comfort |
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36 noise generation (GSM 06.62) shall be used. |
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37 |
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38 It appears that DSP vendors (for GSM MS or for network transcoders, or perhaps |
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39 both) weren't too happy with the prospect of having to include two different |
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40 versions of _almost_ the same codec algorithm with a bunch of interspersed |
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41 subtle diffs, and so the rules were bent: EFR implementors were given permission |
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42 to deviate from the original bit-exact definition of EFR in order to have more |
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43 commonality with MR122. |
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44 |
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45 Approach adopted for Themyscira GSM codec libraries suite |
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46 ========================================================= |
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47 |
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48 I (Mother Mychaela) previously entertained the idea of creating a unified codec |
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49 library that supports both AMR and EFR with common code, producing a published- |
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50 source, FOSS-culture equivalent of what most proprietary vendors have done. |
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51 However, on further reflection, that idea has been rejected. The current |
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52 situation as of 2024-05 is as follows: |
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53 |
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54 * Libgsmefr is our production-oriented implementation of GSM-EFR codec. It |
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55 implements the original bit-exact definition of EFR, not the AMR-EFR hybrid |
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56 version, and it includes full support for DTX encoding and SID decoding with |
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57 comfort noise generation per GSM 06.62. |
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58 |
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59 * Libtwamr is our librification of 3GPP AMR reference code. The library is |
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60 structured in such a way that libtwamr stateful encoder and decoder functions |
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61 can be combined with stateless EFR frame packing and unpacking functions from |
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62 libgsmefr, allowing AMR-EFR hybrid encoders and decoders to be built. The |
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63 decoder homing function in libtwamr can be told to trigger on EFR DHF instead |
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64 of MR122 version, and for the encoder direction there is a simple utility |
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65 function that artificially transforms MR122 DHF into EFR DHF post-encoder. |
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66 However, there is no support for AMR-EFR hybrid encoding with DTX enabled, |
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67 and the low-effort version of AMR-EFR hybrid decoder constructed in this |
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68 manner cannot grok EFR SID frames or generate CN per GSM 06.62. |
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69 |
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70 Production implementations of GSM network elements that need to perform EFR |
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71 speech transcoding should use libgsmefr, not libtwamr. The limited support |
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72 that is provided for AMR-EFR hybrid encoding and decoding with the combination |
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73 of libtwamr and libgsmefr is intended for experimentation and reverse |
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74 engineering of other people's implementations, for times when it becomes |
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75 necessary to model, simulate or replicate bit-exact operation of someone else's |
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76 network element. |
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77 |
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78 Command line utilities for AMR-EFR hybrid |
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79 ========================================= |
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80 |
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81 The present package includes a small set of command line utilities that work |
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82 with the AMR-EFR hybrid described above: |
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83 |
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84 amrefr-encode-r |
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85 amrefr-decode-r |
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86 |
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87 These two utilities function just like gsmefr-encode-r and |
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88 gsmefr-decode-r described in Codec-utils article, but implement the |
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89 AMR-EFR hybrid version of the codec instead of original EFR. The |
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90 no-DTX limitation applies: amrefr-encode-r lacks -d option, and the |
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91 input to amrefr-decode-r must not contain any SID frames. |
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92 |
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93 amrefr-tseq-enc |
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94 amrefr-tseq-dec |
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95 |
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96 These two utilities are AMR-EFR counterparts to gsmefr-etsi-enc and |
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97 gsmefr-etsi-dec test programs described in EFR-testing article. They |
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98 pass all tests on the non-DTX t??_efr.* sequences in ETSI's |
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99 amr122_efr.zip, but not on any of the DTX sequences included in the |
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doc/AMR-EFR-philosophy: document implemented solution
Mychaela Falconia <falcon@freecalypso.org>
parents:
311
diff
changeset
|
100 same ZIP. Just like amrefr-encode-r, amrefr-tseq-enc lacks -d option, |
|
9bcf65088006
doc/AMR-EFR-philosophy: document implemented solution
Mychaela Falconia <falcon@freecalypso.org>
parents:
311
diff
changeset
|
101 and amrefr-tseq-dec rejects input containing SID frames. |