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doc/AMR-library-API: describe the decoder
author Mychaela Falconia <falcon@freecalypso.org>
date Sun, 19 May 2024 21:32:31 +0000
parents c84bf526c7eb
children
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1 Themyscira libtwamr general description
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2 =======================================
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3
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4 Libtwamr is a librification of the official AMR reference C code from 3GPP,
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5 produced by Themyscira Wireless and styled to match our libraries for more
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6 classic GSM codecs. This library has been created with the following two goals
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7 in mind:
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8
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9 1) At the present time we (ThemWi) operate our GSM network with only GSM-FR and
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10 GSM-EFR codecs, with the latter being preferred. We do not currently operate
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11 with AMR because the conditions under which AMR becomes advantageous do not
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12 currently exist in our network operation. However, we need to be prepared
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13 for the possibility that the conditions which make AMR desirable may arise
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14 some day, and we may need to start deploying AMR. In order to make AMR
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15 deployment a possibility, many parts will need to be implemented, one of
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16 which is a speech transcoding library that implements the AMR codec in the
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17 same way how libgsmfr2 and libgsmefr implement the more classic codecs which
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18 we use currently.
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19
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20 2) Many other commercial GSM networks have implemented EFR speech service using
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21 a type of AMR-EFR hybrid described in AMR-EFR-philosophy and
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22 AMR-EFR-hybrid-emu articles. As part of certain behavioral reverse
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23 engineering experiments, we sometimes need to model the bit-exact operation
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24 of those other-people-controlled commercial implementations of AMR-EFR, and
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25 our current libtwamr provides one way to do so. Knowing that a proper
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26 implementation of an AMR codec library is likely to be needed some day for
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27 reason 1 above, justification was obtained for expending the effort to
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28 produce the present libtwamr.
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29
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30 Compared to other plausible ways in which someone could reasonably approach the
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31 task of librifying the AMR reference code from 3GPP, the design of libtwamr
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32 includes two somewhat original choices:
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33
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34 * Separation of core and I/O: the stateful encoder and decoder engines in
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35 libtwamr operate on a custom frame structure that includes the array of codec
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36 parameters in their broken-down form (e.g., 57 parameters for MR122), the
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37 frame type as in original RXFrameType and TXFrameType, and the codec mode.
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38 Conversion between this internal canonical form (which is most native to the
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39 guts of the encoder and decoder engines) and external I/O formats (the 3GPP
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40 test sequence format and the more practical RFC 4867 format used in RTP and
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41 in .amr recording files) is relegated to stateless utility functions.
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42
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43 * Both VAD1 and VAD2 included: the reference code from 3GPP includes two
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44 alternative versions of Voice Activity Detection algorithm, VAD1 and VAD2.
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45 Implementors are allowed to use either version and be compliant; 3GPP code
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46 uses conditional compilation to select between the two, and it appears that
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47 no thought was given to the possibility that a real implementation would
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48 incorporate both VAD versions, to be selected at run time. However, given our
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49 (ThemWi) desire for bit-exact testing against other people's implementations,
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50 it made no sense for us to arbitrarily select one VAD version and drop the
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51 other - hence we took the unconventional route of incorporating both VAD1 and
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52 VAD2 into libtwamr, and designing our encoder API so that library users get
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53 to select which VAD they wish to apply.
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54
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55 Like all other Themyscira GSM codec libraries, libtwamr includes the codec
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56 homing feature in both encoder and decoder directions, as required by 3GPP
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57 specs. Furthermore, libtwamr implementation of this codec homing feature
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58 includes the following simple extensions (simple in terms of low implementation
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59 cost) to facilitate construction of an AMR-EFR hybrid encoder and decoder:
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60
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61 * In the decoder direction, the main AMR frame decoder function includes a DHF
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62 detector as required by 3GPP architecture. In libtwamr this function can be
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63 told to trigger on EFR DHF instead of MR122 version, by way of a flag set in
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64 the mode field of the frame structure passed to amr_decode_frame().
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65
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66 * In the encoder direction, the regular call to amr_encode_frame() - standard
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67 for AMR - can be followed with a call to amr_dhf_subst_efr() or
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68 amr_dhf_subst_efr2() before passing the array of encoded parameters to
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69 EFR_params2frame() from libgsmefr. See AMR-EFR-hybrid-emu article for more
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70 information. The AMR-EFR hybrid test sequences in amr122_efr.zip pass on
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71 both amr_dhf_subst_efr() and amr_dhf_subst_efr2() versions, but the latter
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72 additionally matches the observable behavior of T-Mobile USA.
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73
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74 The mechanism that allows libtwamr to be used for AMR-EFR hybrid implementation
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75 (as opposed to the more conventional use case of implementing standard AMR-NB)
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76 is kept out of the main stateful paths: there are no separate AMR-EFR hybrid
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77 encoder or decoder sessions that are distinguishable from regular AMR encoding
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78 and decoding in terms of state. In the decoder direction, the main AMR frame
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79 decoder function needs to know which DHF it should check for, but this
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80 indication is embedded in the mode field in struct amr_param_frame and not in
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81 the state. In the encoder direction, the mechanism is a separate function
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82 (stateless) that needs to be called between amr_encode_frame() and
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83 EFR_params2frame(). This approach dovetails nicely with the core vs I/O
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84 separation: the option of AMR-EFR hybrid can be viewed as a different I/O front
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85 end to the same AMR engine, alongside with 3GPP AMR test sequence and RFC 4867
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86 I/O options.
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87
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88 Please refer to AMR-library-API article for further details.