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doc/AMR-library-API: document AMR-EFR hybrid decoder
author Mychaela Falconia <falcon@freecalypso.org>
date Sun, 19 May 2024 22:22:40 +0000
parents ad032051166a
children
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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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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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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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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.