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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 516e84085a15
children
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1 The first Themyscira library for GSM-FR speech codec was libgsmfrp, an add-on
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2 to legacy libgsm from TU-Berlin. The current offering is libgsmfr2, which
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3 supplants the earlier combination of libgsm+libgsmfrp. This document details
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4 the change history in this continuum of GSM-FR codec libraries.
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5
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6 libgsmfr2 change history: version 2.0.0 to version 2.1.0
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7 ========================================================
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8
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9 * New functions gsmfr_{preproc,fulldec}_bfi_bits() handle the case of received
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10 BFI with payload bits included. GSM 06.31 requires that the combination of
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11 BFI with "accepted SID frame" bit pattern be treated as an invalid SID frame,
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12 not as an unusable frame; this requirement could not be fulfilled with the
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13 previous gsmfr_{preproc,fulldec}_bfi() API that only does unusable frame
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14 handling.
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15
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16 * Several new API functions were added that facilitate the use of our Rx DTX
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17 preprocessor block as a TFO transform, including the possibility of DTXd.
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19 * Convenience wrapper functions were added for both the full decoder and the
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20 TFO transform that take RTP input (payload bytes and length), encapsulating
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21 the logic of TW-TS-001 for both basic and extended RTP formats.
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22
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23 * Sizes of internal state structures are exported as public const "variables"
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24 (const memory words really), allowing alternative methods of allocating memory
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25 for these structs besides the standard malloc-calling gsmfr_*_create()
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26 functions.
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28 Changes from libgsmfrp version 1.0.2 to libgsmfr2 version 2.0.0
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29 ===============================================================
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31 * Dependency on <gsm.h> defined types abolished, the entire library uses
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32 <stdint.h> types instead.
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34 * The Rx DTX handler component of the new library is unchanged from libgsmfrp
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35 version 1.0.2, aside from the use of new types: uint8_t instead of gsm_byte,
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36 explicit arrays of uint8_t instead of gsm_frame.
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38 * In addition to this Rx DTX handler component, the new library includes the
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39 GSM 06.10 encoder & decoder component (ported from libgsm pl22), the new
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40 full decoder wrapper with decoder homing, an encoder homing function, and
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41 new stateless frame packing and unpacking functions.
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43 libgsmfrp change history: version 1.0.1 to version 1.0.2
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44 ========================================================
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46 There are only two changes, both involving corner cases with invalid SID frames
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47 being received:
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49 1) An invalid SID frame was received immediately following a good speech frame.
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50 In this case we start CN generation, but we take the needed LARc and Xmaxc
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51 parameters from the last speech frame, instead of the usual procedure of
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52 extracting them from a valid SID frame. The change from 1.0.1 to 1.0.2
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53 concerns the Xmaxc parameter in this corner case: in 1.0.1 we took Xmaxc
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54 from the last subframe and used it for ensuing CN generation, but in 1.0.2
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55 we compute a more proper mean Xmaxc from all 4 subframes, by dequantizing,
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56 summing and requantizing.
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58 2) An invalid SID frame was received in the speech muting state. The sequence
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59 of inputs would have to be:
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61 - a good speech frame;
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62 - one or more BFIs, but not too many, so that the cached speech frame
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63 does not decay fully by Xmaxc reduction;
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64 - an invalid SID frame.
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66 In version 1.0.1 we handled this even more obscure corner case by entering
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67 the CN muting state, i.e., the state that is normally entered upon the
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68 second lost SID. In version 1.0.2 we ignore invalid SID in the speech
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69 muting state and act as if we got BFI, i.e., continue speech muting rather
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70 than switch to CN muting.
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72 libgsmfrp change history: version 1.0.0 to version 1.0.1
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73 ========================================================
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75 Version 1.0.0 exhibited the following defects, which are fixed in 1.0.1:
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77 1) The last received valid SID was cached forever for the purpose of
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78 handling future invalid SIDs - we could have received some valid
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79 SID ages ago, then lots of speech or NO_DATA, and if we then get
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80 an invalid SID, we would resurrect the last valid SID from ancient
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81 history - a bad design. In our new design, we handle invalid SID
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82 based on the current state, much like BFI.
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84 2) GSM 06.11 spec says clearly that after the second lost SID
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85 (received BFI=1 && TAF=1 in CN state) we need to gradually decrease
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86 the output level, rather than jump directly to emitting silence
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87 frames - we previously failed to implement such logic.
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89 3) Per GSM 06.12 section 5.2, Xmaxc should be the same in all 4 subframes
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90 in a SID frame. What should we do if we receive an otherwise valid
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91 SID frame with different Xmaxc? Our previous approach would
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92 replicate this Xmaxc oddity in every subsequent generated CN frame,
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93 which is rather bad. In our new design, the very first CN frame
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94 (which can be seen as a transformation of the SID frame itself)
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95 retains the original 4 distinct Xmaxc, but all subsequent CN frames
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96 are based on the Xmaxc from the last subframe of the most recent SID.