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doc/Calypso-TCH-downlink: update for FR1 BFI-with-data
author Mychaela Falconia <falcon@freecalypso.org>
date Fri, 11 Oct 2024 01:54:00 +0000
parents 6b479cfb06a4
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1 Themyscira libgsmfr2 is our new (2024) library offering for GSM FRv1 codec,
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2 replacing the previous combination of libgsm (classic 1990s free sw offering)
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3 and libgsmfrp (our add-on). That combination appeared satisfactory at first
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4 because of how the decoder processing chain is defined for FRv1 (the Rx DTX
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5 handler forms a modular piece, passing a frame of 260 bits to unmodified "pure"
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6 GSM 06.10 decoder), but use of legacy libgsm presents some difficulties:
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7
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8 * Inconvenience and inconsistency: for all other supported GSM speech codecs,
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9 Themyscira libraries provide the complete solution, not depending on anyone
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10 else's software, but for FRv1 we depended on a library that was created in a
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11 different era for non-GSM applications but just happens, almost by accident,
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12 to be a bit-exact implementation of GSM 06.10 spec.
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13
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14 * Poor design in frame packing and unpacking functions: the operations of
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15 bit-shuffling a GSM-FR codec frame between the array of parameters form
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16 (76 words) and the packed RTP format used in IP-based GSM RAN (33 bytes) are
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17 stateless pure functions, but their implementations in libgsm (gsm_explode()
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18 and gsm_implode()) require a state structure. (Libgsm supports WAV49 format
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19 in addition to the RTP-adopted one, and WAV49 packing is stateful - but this
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20 WAV49 feature is of no use and no relevance in real GSM applications.)
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21
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22 * No ability to implement homing: the internal state structure used by libgsm
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23 is set to the home state when it is allocated with gsm_create(), but there is
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24 no reset function, and such function cannot be implemented externally when
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25 the state structure is private to the library and not exposed. Therefore,
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26 the optional codec homing feature defined in later versions of GSM 06.10 spec
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27 cannot be implemented in a wrapper around legacy libgsm, causing the resulting
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28 FOSS implementation to be inferior to existing commercial implementations
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29 (deployed in practice by incumbent nation-scale networks) which do implement
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30 this feature.
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31
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32 In response to the above issues, we now have a new library named libgsmfr2 that
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33 provides all needed functions for GSM-FR codec "under one roof", harmonized
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34 with our support for other GSM speech codecs. However, the modularity that is
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35 inherent in the way this codec is defined in the specs (contrast with EFR) is
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36 still retained in the design of our library, which exhibits the following 4
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37 functional divisions:
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38
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39 1) Libgsmfr2 includes a "pure" GSM 06.10 encoder and decoder, directly
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40 corresponding to old libgsm. This code is in fact lifted from libgsm,
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41 ported into Themyscira gsm-codec-lib style in terms of C dialect, defined
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42 types and naming conventions. The reset function that is missing in libgsm
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43 is now provided, however, fixing that defect.
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44
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45 2) The Rx DTX handler component is unchanged from our previous libgsmfrp. Per
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46 the relevant specs (GSM 06.11, 06.12 and 06.31) this component is a modular
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47 piece: it emits a standard 260-bit frame of GSM 06.10 parameters that can be
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48 fed to anyone else's implementation of the latter standard.
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49
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50 3) Full decoder: this component is a wrapper around an 06.10 decoder instance
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51 and an Rx DTX instance, providing functionality equivalent to the standard
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52 decoder function in other GSM speech codecs.
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53
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54 4) Stateless utility functions for frame format conversions (packing and
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55 unpacking) and for incoming SID classification. An application can freely
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56 use just these functions, without pulling in any encoder or decoder or
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57 stateful preprocessor functionality, making the present library very
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58 convenient for debug utilities.
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59
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60 The homing feature is available in both encoder and decoder directions, but it
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61 is implemented differently between the two:
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62
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63 * In the encoder direction, if the application wishes to enable the possibility
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64 of in-band homing, it needs to call gsmfr_0610_encoder_homing() after the
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65 regular call to gsmfr_0610_encode_frame() or gsmfr_0610_encode_params().
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66
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67 * In the decoder direction, the homing feature is always included if one uses
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68 the "fulldec" (full decoder) wrapper (it is implemented in that layer), and
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69 never included if one uses the "basic" GSM 06.10 decoder by itself, or the
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70 Rx DTX handler block by itself.
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71
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72 The only major feature of GSM-FR codec that is currently absent in libgsmfr2 is
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73 application of DTX in the encoder direction: GSM 06.32 VAD followed by DTX
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74 hangover logic and SID output. This omission is currently acceptable for
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75 Themyscira Wireless: DTXd (DTX in the radio downlink direction) is not allowed
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76 when each BTS operates at only one carrier frequency, which makes it pointless
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77 to enable DTX for speech encoding at the network edge transcoder, and we are not
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78 trying to replace TI Calypso DSP on the MS side of GSM. However, if this
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79 situation changes and some need arises for a FOSS implementation of DTX-enabled
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80 GSM-FR encoder, the architecture of Themyscira libgsmfr2 should make it possible
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81 to integrate such addition.