FreeCalypso > hg > gsm-codec-lib
annotate doc/EFR-rationale @ 155:9814041e8096
gsmfr-encode-r utility put together
author | Mychaela Falconia <falcon@freecalypso.org> |
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date | Thu, 15 Dec 2022 01:30:59 +0000 |
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1 Problem in need of solving |
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2 ========================== |
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3 |
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4 At the time of the undertaking of Themyscira libgsmefr project (late 2022), |
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5 there did not exist any readily available library solution for GSM EFR codec. |
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6 The community of FOSS offers classic libgsm from 1990s for FR1 codec (it's an |
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7 implementation of GSM 06.10, on top of which we had to implement our own Rx DTX |
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8 handler) and opencore-amrnb for AMR (based on Android OpenCORE framework) - but |
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9 nothing for EFR. This situation creates a problem for anyone seeking to deploy |
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10 their own GSM network with a voice interface to PSTN or other networks: such |
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11 voice interface generally requires implementing a transcoder, and doing the |
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12 latter in turn requires a library that implements the codec to be supported. |
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13 In the present situation, anyone who wishes to implement a speech transcoder |
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14 for GSM networks can easily support FR1 and AMR codecs, but not EFR. |
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15 |
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16 EFR is more than just 12k2 mode of AMR! |
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17 ======================================= |
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18 |
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19 There is a common misconception in the GSM hacker community that EFR is nothing |
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20 but the highest 12k2 mode of AMR, and that any library that implements AMR, |
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21 such as opencore-amrnb, is thus sufficient to support EFR as well. However, |
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22 the reality is more complex: |
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23 |
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24 * If an AMR encoder operates with DTX disabled, such that the output contains |
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25 only speech frames and no SID, and the mode is forced to 12k2, then indeed a |
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26 simple reshuffling of bits will produce speech frames that can be fed to an |
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27 EFR decoder on the other end. Note that the two encoders (EFR and AMR 12k2) |
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28 will produce *different* encoded speech parameters from the same input, and |
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29 the decoded speech output on the other end will also be different, but the |
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30 two versions are expected to be equally good for human ears. |
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31 |
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32 * In the other direction, if an EFR input stream contains only good speech |
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33 frames (no SID and no lost, FACCH-stolen or DTX-suppressed frames), one can |
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34 likewise do a simple bit reordering and feed these frames to an AMR decoder. |
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35 The output of this AMR decoder will once again be different from a proper |
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36 (bit-exact) EFR decoder for the same speech parameter inputs. but as long as |
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37 the EFR input stream is all good speech frames, the output will be good enough |
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38 for human ears. |
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39 |
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40 * The real problem occurs when the EFR input stream contains SID frames and BFI |
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41 frame gaps, as will always happen in reality if this stream is an uplink from |
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42 a GSM call. AMR SID mechanism is different from that of EFR, and an AMR |
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43 decoder will NOT recognize EFR SID frames. A quick experiment confirms that |
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44 when a real GSM EFR uplink RTP capture is converted to AMR by non-SID-aware |
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45 bit reshuffling and then fed to amrnb-dec from opencore-amrnb, unpleasant |
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46 sounds appear in the output whenever GSM uplink goes into SID. |
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47 |
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48 EFR reference code from ETSI |
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49 ============================ |
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50 |
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51 A published-source bit-exact implementation of GSM EFR encoder and decoder, |
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52 complete with all beyond-speech functions of DTX, VAD, comfort noise generation, |
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53 error concealment etc does exist in the form of reference code from ETSI. |
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54 However, this code has never been turned into a usable codec library by anyone |
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55 prior to us (at least not by anyone who freely published their work), and doing |
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56 such librification (producing an EFR analogue to what Android OpenCORE people |
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57 did with AMR) is no easy feat! The original EFR code from ETSI exhibits two |
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58 problems which need to be remedied in the librification project: |
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59 |
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60 1) The original code maintains all codec state in global variables (lots of |
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61 them) that are scattered throughout. 3GPP reference code for AMR (naturally |
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62 later than EFR in chronological order) is better in this regard (in the AMR |
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63 version they gathered their global vars into structs and pass pointers to |
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64 these structs, although still many separately-malloc'ed structs instead of |
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65 single unified encoder state and decoder state), but we need the EFR version |
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66 for correct handling of all beyond-speech aspects, and this version is all |
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67 global vars. |
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68 |
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69 2) These reference codes from ETSI/3GPP (both EFR and AMR versions, it seems) |
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70 were intended to serve as simulations, not as production code, and the code |
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71 is very inefficient. |
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72 |
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73 Themyscira libgsmefr |
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74 ==================== |
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75 |
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76 Libgsmefr presented in this code repository is our current solution for EFR. |
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77 It is a library styled after classic libgsm for FR1, but its guts consist of a |
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78 librified derivative of ETSI EFR code. The problem of global vars has been |
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79 solved in this library version - they've been gathered into one unified struct |
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80 for encoder state and another unified struct for decoder state - but the problem |
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81 of poor performance (significantly worse than opencore-amrnb) still remains for |
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82 now. |
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83 |
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84 Future roadmap |
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85 ============== |
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86 |
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87 If someone is implementing a DSP vocoder block for a GSM MS or a network-side |
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88 speech transcoder that needs to support all standard GSM codecs, at some point |
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89 they will need to implement both EFR and AMR. Given the close relation between |
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90 these two codecs (they are not perfectly compatible as we started out saying, |
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91 but they are still very closely related), keeping two entirely separate library |
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92 implementations for AMR and EFR will be very inefficient in the long run, and a |
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93 nightmare to get them to perform equally well. It seems to me (Mother Mychaela) |
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94 that the correct solution will be to produce a single codec library that |
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95 implements both AMR and EFR, probably by starting with an AMR library and |
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96 extending it with special modes to handle those aspects where EFR differs. It |
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97 is my forecast that we are going to end up doing something along these lines in |
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98 Themyscira - but it will be much later down the road; for the time being, our |
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99 initial version of ThemWi will only support FR and EFR, but not AMR. |