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TW-TS-005 reader: fix maximum line length bug TW-TS-005 section 4.1 states: The maximum allowed length of each line is 80 characters, not including the OS-specific newline encoding. The implementation of this line length limit in the TW-TS-005 hex file reader function in the present suite was wrong, such that lines of the full maximum length could not be read. Fix it. Note that this bug affects comment lines too, not just actual RTP payloads. Neither Annex A nor Annex B features an RTP payload format that goes to the maximum of 40 bytes, but if a comment line goes to the maximum allowed length of 80 characters not including the terminating newline, the bug will be triggered, necessitating the present fix.
author Mychaela Falconia <falcon@freecalypso.org>
date Tue, 25 Feb 2025 07:49:28 +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.