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annotate doc/AMR-EFR-philosophy @ 436:e80632ccb745

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author Mychaela Falconia <falcon@freecalypso.org>
date Wed, 08 May 2024 05:32:27 +0000
parents 83408f67a96c
children 9bcf65088006
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1 Relation between GSM-EFR and 12k2 mode of AMR
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2 =============================================
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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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20 What other DSP/transcoder vendors have done
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21 ===========================================
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22
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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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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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45 Approach adopted for Themyscira GSM codec libraries suite
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46 =========================================================
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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 vision
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52 (as of 2024-04) is that libgsmefr (stable since early 2023) and libtwamr
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53 (currently a work in progress) shall remain separate and independent libraries,
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54 the former implementing GSM-EFR (the original bit-exact definition) and the
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55 latter implementing AMR. My reasons for this decision are:
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56
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57 * Libgsmefr already exists, and it is already a bit of a jewel compared to the
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58 sorry state of true GSM codec support in the world of FOSS outside Themyscira.
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59 Giving up on this library and moving to some nebulous new one does not sound
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60 appealing.
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62 * There does not exist any formal, bit-exact definition for what we informally
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63 call "EFR version 2": the realization of EFR as implemented by post-AMR-era
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64 proprietary vendors, some sort of AMR-EFR hybrid. As I see it, it is not my
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65 place to try to innovate in speech codec design, instead it is my job to
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66 provide 100% correct, bit-exact implementations of existing solid standards -
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67 and there is no bit-exact standard to follow for "EFR version 2".
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68
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69 * Libtwamr project: the task of turning the original AMR code from 3GPP into a
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70 proper library, style-consistent with Themyscira libgsmfr2 and libgsmefr,
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71 without the ugliness of opencore-amr, is already a lot of work as it is.
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72 There is no need to make it harder by adding the task of supporting AMR-based
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73 EFR, especially when the latter lacks formal definition.
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74
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75 Performance issues
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76 ==================
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77
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78 Right now the only significant downside of libgsmefr compared to
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79 libopencore-amrnb is that our library is significantly slower: almost 7 times
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80 slower on non-DTX encode and a little over 3 times slower on SID-free decode.
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81 However, this performance problem will need to be solved by profiling the code
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82 to find the slowest spots, comparing the code of individual blocks between ours
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83 and theirs, and porting over whatever performance-optimizing strategies were
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84 implemented in OpenCORE code base. The latter code base is a derivative work
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85 based on 3GPP AMR source, hence the guts of the codec are largely the same
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86 between 3GPP AMR and libopencore-amrnb; the latter has been significantly
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87 performance-optimized, but also heavily uglified. But there is no reason why
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88 the same performance fixes can't be applied to EFR code base - it will simply
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89 take work. This work is currently part of our future roadmap.