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libgsmefr: add RTP-encoded DHF const datum to public API
author Mychaela Falconia <falcon@freecalypso.org>
date Tue, 14 May 2024 07:13:59 +0000
parents 78739fda2856
children
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1 Please see our AMR-EFR-philosophy article for an analysis of differences between
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2 EFR and MR122 (12k2 mode of AMR), and for a discussion of how we handle the
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3 relation between these two codecs. The following article was written in late
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4 2022, before these issues were properly understood:
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5
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6 2022-December description
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7 -------------------------
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8
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9 We have two simple utilities that allow one to experiment with "dumb" bit-
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10 shuffling conversion between AMR 12k2 and EFR codec formats, to explore
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11 capabilities and limitations of this approach.
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12
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13 gsm-amr2efr reads an AMR speech recording in RFC 4867 storage format (the common
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14 .amr format) and converts it to EFR in gsmx format. The AMR input to this
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15 utility must consists of MR122 frames only - no other AMR modes, no SID and no
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16 NO_DATA gaps. The intent is that one can take a starting speech sample in WAV
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17 format, encode it into AMR with amrnb-enc from opencore-amrnb (by default that
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18 utility produces MR122 encoding without DTX), and then convert the AMR output to
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19 EFR with gsm-amr2efr. One can then encode the same starting-point WAV speech
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20 sample with gsmefr-encode (matching official EFR from ETSI) and compare the two
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21 EFR outputs. When you do this experiment, you will see that the two EFR outputs
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22 will be different (you can then analyze encoded speech parameter diffs with
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23 gsmrec-dump), but each version can be fed to an EFR decoder, resulting in
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24 OK-sounding speech.
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25
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26 gsm-efr2amr performs the opposite conversion: it reads an EFR session recording
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27 in gsmx format and converts it to AMR storage format. The input to gsm-efr2amr
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28 is allowed to contain Themyscira BFI markers in addition to EFR frames; these
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29 BFI markers will be turned into AMR NO_DATA frames. The same input can also
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30 contain EFR SID frames - however, gsm-efr2amr will not detect them and won't
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31 give them any special handling, instead they will be bit-reshuffled into MR122
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32 just like EFR speech frames. The result of such "dumb" conversion is invalid
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33 AMR, and when you decode it with amrnb-dec, you will hear some strange noises.
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34
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35 2024-April addendum
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36 -------------------
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37
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38 In addition to the SID issue noted above (if the input to gsm-efr2amr contains
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39 any SID frames, the output will be invalid AMR), these dumb conversion methods
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40 fail to take action on any embedded decoder homing frames. The correct DHF is
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41 different between EFR and MR122, hence a better converter could be made to
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42 recognize EFR DHFs in EFR->AMR direction and convert them to MR122 DHF, and do
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43 the opposite in AMR->EFR direction. However, the implementation of AMR in
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44 libopencore-amrnb has the homing feature stripped out altogether, hence doing
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45 DHF conversion would be pointless as long as amrnb-enc and amrnb-dec utilities
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46 are involved.
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47
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48 Thoughts on more proper conversion
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49 ==================================
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50
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51 Imagine this hypothetical scenario: you operate a GSM network, and you
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52 preferentially use EFR codec. You are then able to obtain TrFO interconnection
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53 with some other mobile network of more "modern" kind, and that "modern" network
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54 uses AMR exclusively, with no ability to use any GSM-only codecs. (The latter
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55 situation holds for UMTS and VoLTE, for example.) Ordinarily, under these
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56 circumstances TrFO won't be possible - instead you have to interconnect in
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57 G.711, have each side transcode its respective codec, and put up with double
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58 transcoding. But what if the AMR side can be told to use MR122 only, without
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59 any of the lower modes? Such arrangement would make no sense in GSM (just use
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60 EFR instead and save the headache of dealing with AMR), but it might be sensible
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61 to ask the UMTS/VoLTE side for that MR122-only config of AMR-NB.
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62
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63 In this hypothetical scenario, would it be possible to pass speech frames
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64 transparently, doing only the necessary bit reshuffling, and only invoke some
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65 slick innovative algorithm during speech pauses to translate between EFR and
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66 AMR SID paradigms?
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67
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68 Right now this idea is fantasy only. I don't know enough about VoLTE to tell
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69 whether or not an MR122-only config of AMR-NB would work there, I have no idea
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70 what codec config VoLTE operators run with currently when the other end of the
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71 call is G.711 PSTN, and there is very little chance that any of the nation-scale
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72 mobile operators would agree to a private peering interconnect with some tiny
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73 community GSM network - while interconnection through fully public, open-to-
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74 everyone IP-PSTN routes allows only G.711 and nothing else, no cellular TrFO.
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75
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76 Nonetheless, the idea of TrFO conversion between EFR and MR122-only AMR remains
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77 interesting as a theoretical exercise, and we currently leave it there, just as
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78 food for thought.