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doc/TFO-xform/HRv1: article written
author Mychaela Falconia <falcon@freecalypso.org>
date Mon, 02 Sep 2024 07:32:09 +0000
parents 35d38348c880
children
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35d38348c880 doc/TFO-xform/FRv1: article written
Mychaela Falconia <falcon@freecalypso.org>
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1 Rx DTX handler situation in FRv1
35d38348c880 doc/TFO-xform/FRv1: article written
Mychaela Falconia <falcon@freecalypso.org>
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2 ================================
35d38348c880 doc/TFO-xform/FRv1: article written
Mychaela Falconia <falcon@freecalypso.org>
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3
35d38348c880 doc/TFO-xform/FRv1: article written
Mychaela Falconia <falcon@freecalypso.org>
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4 Before we address the question of how one should implement TFO transform for
35d38348c880 doc/TFO-xform/FRv1: article written
Mychaela Falconia <falcon@freecalypso.org>
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5 FRv1, let's begin with a more basic question: how does the Rx DTX handler (the
35d38348c880 doc/TFO-xform/FRv1: article written
Mychaela Falconia <falcon@freecalypso.org>
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6 "front end" part of the speech decoder in an end-terminal implementation) work
35d38348c880 doc/TFO-xform/FRv1: article written
Mychaela Falconia <falcon@freecalypso.org>
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7 in FRv1? In both HRv1 and EFR, error-free comfort noise generation functions
35d38348c880 doc/TFO-xform/FRv1: article written
Mychaela Falconia <falcon@freecalypso.org>
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8 of this Rx DTX handler are normative per the specs at bit-exact level, while
35d38348c880 doc/TFO-xform/FRv1: article written
Mychaela Falconia <falcon@freecalypso.org>
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9 error handling functions are specified only as a non-normative example - and
35d38348c880 doc/TFO-xform/FRv1: article written
Mychaela Falconia <falcon@freecalypso.org>
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10 the supplied reference C sources implement the full Rx DTX handler (both the
35d38348c880 doc/TFO-xform/FRv1: article written
Mychaela Falconia <falcon@freecalypso.org>
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11 normative part and the "example" part) as an inseparable part of the speech
35d38348c880 doc/TFO-xform/FRv1: article written
Mychaela Falconia <falcon@freecalypso.org>
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12 decoder. But not so for FRv1: there is no reference C source and there are no
35d38348c880 doc/TFO-xform/FRv1: article written
Mychaela Falconia <falcon@freecalypso.org>
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13 bit-exact definitions for any part of Rx DTX handler logic. All Rx DTX handler
35d38348c880 doc/TFO-xform/FRv1: article written
Mychaela Falconia <falcon@freecalypso.org>
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14 functions are defined only in English prose (no code), and even in the most
35d38348c880 doc/TFO-xform/FRv1: article written
Mychaela Falconia <falcon@freecalypso.org>
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15 normative parts the language used in the specs is quite loose.
35d38348c880 doc/TFO-xform/FRv1: article written
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16
35d38348c880 doc/TFO-xform/FRv1: article written
Mychaela Falconia <falcon@freecalypso.org>
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17 Based on what is specified (verbally, loosely) in GSM 06.11 and 06.12, there
35d38348c880 doc/TFO-xform/FRv1: article written
Mychaela Falconia <falcon@freecalypso.org>
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18 are two principal ways in which an Rx-ECU-capable, Rx-DTX-capable FRv1 speech
35d38348c880 doc/TFO-xform/FRv1: article written
Mychaela Falconia <falcon@freecalypso.org>
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19 decoder can be implemented:
35d38348c880 doc/TFO-xform/FRv1: article written
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20
35d38348c880 doc/TFO-xform/FRv1: article written
Mychaela Falconia <falcon@freecalypso.org>
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21 Fully modular approach: the basic GSM 06.10 decoder block (which is bit-exact,
35d38348c880 doc/TFO-xform/FRv1: article written
Mychaela Falconia <falcon@freecalypso.org>
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22 but cannot handle BFIs or SID frames) remains absolutely unmodified, while the
35d38348c880 doc/TFO-xform/FRv1: article written
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23 Rx DTX handler (which includes both error concealment and CN generation) is
35d38348c880 doc/TFO-xform/FRv1: article written
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24 implemented as a modular piece, with an "honest-to-god" 260-bit 06.10 frame
35d38348c880 doc/TFO-xform/FRv1: article written
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25 interface between the two blocks.
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26
35d38348c880 doc/TFO-xform/FRv1: article written
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27 Non-modular approach: the Rx DTX handler and the 06.10-based speech decoder are
35d38348c880 doc/TFO-xform/FRv1: article written
Mychaela Falconia <falcon@freecalypso.org>
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28 integrated more tightly, and there is no possible stream of "pure" 06.10 codec
35d38348c880 doc/TFO-xform/FRv1: article written
Mychaela Falconia <falcon@freecalypso.org>
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29 frames that would produce the same bit-exact PCM output as the actually
35d38348c880 doc/TFO-xform/FRv1: article written
Mychaela Falconia <falcon@freecalypso.org>
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30 implemented "full decoder" with the built-in Rx DTX handler.
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31
35d38348c880 doc/TFO-xform/FRv1: article written
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32 Cursory reading of GSM 06.11 and 06.12 specs strongly suggests that they call
35d38348c880 doc/TFO-xform/FRv1: article written
Mychaela Falconia <falcon@freecalypso.org>
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33 for the fully modular approach as defined above. However, because neither spec
35d38348c880 doc/TFO-xform/FRv1: article written
Mychaela Falconia <falcon@freecalypso.org>
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34 includes any bit-exact definitions, there is no formal stipulation that the
35d38348c880 doc/TFO-xform/FRv1: article written
Mychaela Falconia <falcon@freecalypso.org>
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35 modular approach shall be used - it is entirely conceivable that someone could
35d38348c880 doc/TFO-xform/FRv1: article written
Mychaela Falconia <falcon@freecalypso.org>
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36 implement a non-modular approach, and they would still be spec-compliant.
35d38348c880 doc/TFO-xform/FRv1: article written
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37
35d38348c880 doc/TFO-xform/FRv1: article written
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38 Why would anyone implement the non-modular approach when the fully modular one
35d38348c880 doc/TFO-xform/FRv1: article written
Mychaela Falconia <falcon@freecalypso.org>
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39 seems much simpler? After all, the bit-exact basic 06.10 decoder already
35d38348c880 doc/TFO-xform/FRv1: article written
Mychaela Falconia <falcon@freecalypso.org>
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40 exists - surely it is easier to build a separate front-end to it than dig into
35d38348c880 doc/TFO-xform/FRv1: article written
Mychaela Falconia <falcon@freecalypso.org>
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41 the guts of that pre-existing box? There is, however, one aspect that could
35d38348c880 doc/TFO-xform/FRv1: article written
Mychaela Falconia <falcon@freecalypso.org>
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42 sway implementors toward the non-modular approach: interpolation of CN
35d38348c880 doc/TFO-xform/FRv1: article written
Mychaela Falconia <falcon@freecalypso.org>
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43 parameter updates during prolonged DTX pauses. GSM 06.12 (or rather its latest
35d38348c880 doc/TFO-xform/FRv1: article written
Mychaela Falconia <falcon@freecalypso.org>
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44 incarnation as 3GPP TS 46.012) says, at the very end of section 6.1:
35d38348c880 doc/TFO-xform/FRv1: article written
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45
35d38348c880 doc/TFO-xform/FRv1: article written
Mychaela Falconia <falcon@freecalypso.org>
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46 "When updating the comfort noise, the parameters above should preferably be
35d38348c880 doc/TFO-xform/FRv1: article written
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47 interpolated over a few frames to obtain smooth transitions."
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48
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49 This kind of CN parameter interpolation is mandatory in the newer HRv1 and EFR
35d38348c880 doc/TFO-xform/FRv1: article written
Mychaela Falconia <falcon@freecalypso.org>
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50 codecs where the CN generator function is defined in bit-exact terms, hence it
35d38348c880 doc/TFO-xform/FRv1: article written
Mychaela Falconia <falcon@freecalypso.org>
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51 makes sense that some implementors may have chosen to back-port the same feature
35d38348c880 doc/TFO-xform/FRv1: article written
Mychaela Falconia <falcon@freecalypso.org>
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52 to FRv1.
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53
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54 CN parameter interpolation: deeper analysis of the problem
35d38348c880 doc/TFO-xform/FRv1: article written
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55 ==========================================================
35d38348c880 doc/TFO-xform/FRv1: article written
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56
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57 How does this interpolation feature affect the choice of modular or non-modular
35d38348c880 doc/TFO-xform/FRv1: article written
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58 design? As a non-expert on the subject of codec design, I am not able to say
35d38348c880 doc/TFO-xform/FRv1: article written
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59 authoritatively if it is possible to implement the feature of CN parameter
35d38348c880 doc/TFO-xform/FRv1: article written
Mychaela Falconia <falcon@freecalypso.org>
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60 interpolation (and do it well) while staying with the fully modular design in
35d38348c880 doc/TFO-xform/FRv1: article written
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61 which the basic 06.10 decoder block remains absolutely unchanged, or if high-
35d38348c880 doc/TFO-xform/FRv1: article written
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62 quality implementation of this feature would require foregoing the modularity
35d38348c880 doc/TFO-xform/FRv1: article written
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63 and moving the CN-specific interpolation function somewhere inside that block,
35d38348c880 doc/TFO-xform/FRv1: article written
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64 e.g., between the output of GSM 06.10 section 4.2.8 and the input to section
35d38348c880 doc/TFO-xform/FRv1: article written
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65 4.2.9, as referenced from section 4.3.3 for the decoder.
35d38348c880 doc/TFO-xform/FRv1: article written
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66
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67 We can, however, look at how ETSI handled this problem in other codecs for
35d38348c880 doc/TFO-xform/FRv1: article written
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68 which they did mandate CN parameter interpolation in bit-exact form. HRv1 is
35d38348c880 doc/TFO-xform/FRv1: article written
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69 the best point of comparison in this regard because of this detail: the Rx DTX
35d38348c880 doc/TFO-xform/FRv1: article written
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70 handler front-end part of the official bit-exact HRv1 decoder (delivered as C
35d38348c880 doc/TFO-xform/FRv1: article written
Mychaela Falconia <falcon@freecalypso.org>
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71 source this time, not just verbiage) is _almost_ modular, i.e., one could
35d38348c880 doc/TFO-xform/FRv1: article written
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72 _almost_ detach it into a modular piece whose output could be fed to the
35d38348c880 doc/TFO-xform/FRv1: article written
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73 decoder as a new "cleaned up" stream of HRv1 codec frames. Where is the
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74 "almost" part? Answer: interpolation of CN parameters! When HRv1 decoder is
35d38348c880 doc/TFO-xform/FRv1: article written
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75 in CN insertion state, it dequantizes R0 and LPC parameters from SID frames
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Mychaela Falconia <falcon@freecalypso.org>
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76 only when initial and update frames come in - but when it generates the actual
35d38348c880 doc/TFO-xform/FRv1: article written
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77 CN between those updates, it performs smooth linear interpolation on the decoded
35d38348c880 doc/TFO-xform/FRv1: article written
Mychaela Falconia <falcon@freecalypso.org>
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78 parameters, *without* requantizing them into something that can be retransmitted
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79 as new HRv1 codec frames representing the CN.
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80
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81 Once again, as a non-expert on the subject of codec design, I am not able to say
35d38348c880 doc/TFO-xform/FRv1: article written
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82 authoritatively if the same approach that was prescribed by ETSI for HRv1 would
35d38348c880 doc/TFO-xform/FRv1: article written
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83 also work for FRv1, or if CN parameter interpolation for FRv1 can be done well
35d38348c880 doc/TFO-xform/FRv1: article written
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84 by requantizing the interpolated parameters for each individual CN output frame
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85 and feeding them to a strictly unmodified 06.10 decoder block. It is the case,
35d38348c880 doc/TFO-xform/FRv1: article written
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86 however, that there is no pre-existing implementation available to us which we
35d38348c880 doc/TFO-xform/FRv1: article written
Mychaela Falconia <falcon@freecalypso.org>
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87 can look at that does CN parameter interpolation for FRv1 - the TFO transform
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88 in Nokia TCSM2 does _not_ interpolate - hence without a reference to look at,
35d38348c880 doc/TFO-xform/FRv1: article written
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89 this optional feature is a can of worms which we should stay away from.
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90
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91 Front-end part of the speech decoder and TFO transform
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92 ======================================================
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93
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94 If the party who implemented the regular end-decoder for FRv1 chose the fully
35d38348c880 doc/TFO-xform/FRv1: article written
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95 modular approach, either by disregarding the call for interpolation of CN
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96 parameters (the spec language is "should preferably", rather than "shall") or
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97 by requantizing the interpolated parameters on each CN output frame, then a
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98 corresponding implementation of TFO transform for non-DTXd operation becomes
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99 trivial: the modularized Rx DTX handler front-end can also serve unchanged as
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100 the TFO transform!
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101
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102 This just-described situation holds for the current Themyscira Wireless
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103 implementation of FRv1 codec, named libgsmfr2. (The 2 in the library name
35d38348c880 doc/TFO-xform/FRv1: article written
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104 refers to the major version of library API and dependency structure; the codec
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105 it implements is still FRv1.) Specifically:
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106
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107 * The full decoder implementation in libgsmfr2 follows the modular approach:
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108 the front-end Rx DTX handler preprocessor feeds "cleaned up" FRv1 codec frames
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109 to an unmodified GSM 06.10 decoder.
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110
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111 * No interpolation is done on CN parameters: as soon as each SID update comes
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112 in, the new parameters are used immediately for all generated CN frames.
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113
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114 The preprocessor part of libgsmfr2 is thus already suitable to serve as a TFO
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115 transform for FRv1. However, before formally adopting it as such, I have had a
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116 long-standing desire to see how this function was implemented by other vendors;
35d38348c880 doc/TFO-xform/FRv1: article written
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117 particularly, how it's been implemented in real historical TRAUs.
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118
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119 Nokia TCSM2 TRAU implementation
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120 ===============================
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121
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122 As of 2024-08, we finally have a working bank-of-TRAUs apparatus in our lab:
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123 Nokia TCSM2. This TRAU implements TFO for FRv1, HRv1 and EFR, hence we finally
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124 got the ability to see how this vendor (Nokia) implemented the elusive TFO
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125 transform.
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126
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127 Here are our findings:
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128
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129 Error concealment function
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130 --------------------------
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131
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132 Themyscira implementation is based on the "example solution" of TS 46.011
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133 chapter 6; Nokia's implementation appears to be very similar, with only a few
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134 visible differences:
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135
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136 * When the ECU enters the state of "speech muting" (after the first speech-state
35d38348c880 doc/TFO-xform/FRv1: article written
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137 BFI for which the last good speech frame is simply repeated), instead of
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138 decrementing each of the 4 Xmaxcr numbers by 4, it decrements them by 11,
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139 thereby producing noticeably faster muting than what the spec calls for.
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140
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141 * The state of emitting fixed silence frames is entered not after the
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142 algorithmically-muted frame in which the lowest Xmaxcr reached 0 (my reading
35d38348c880 doc/TFO-xform/FRv1: article written
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143 of the "example solution" in the spec), but after the state of algorithmic
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144 muting (decrementing Xmaxcr's by 11 each time) persisted for exactly 5 frames.
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145 If the original speech frame had its highest Xmaxcr equal to 63, the last
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146 algorithmically muted frame before fixed silence frames will have 8 in that
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147 Xmaxcr; if all starting Xmaxcr numbers were low, there will be 5 frames with
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148 all zeros in Xmaxcr, random Mcr and other parameters unchanged before the
35d38348c880 doc/TFO-xform/FRv1: article written
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149 switch to fixed silence frames.
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150
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151 Nokia's TFO transform exhibits additional logic whereby the first good speech
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152 frame after prolonged BFIs has its highest Xmaxcr reduced (but not messed with
35d38348c880 doc/TFO-xform/FRv1: article written
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153 otherwise); if that good speech frame is again followed by BFIs, the ECU goes
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154 back to silence frame output right away - or at least that's what we saw in one
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155 experiment. This aspect has not been studied in detail.
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156
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157 Comfort noise generation (DTXd=0)
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158 ---------------------------------
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159
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160 The comfort noise output from Nokia's TFO transform generally agrees with my
35d38348c880 doc/TFO-xform/FRv1: article written
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161 reading of GSM 06.12 spec section 6.1, the section that describes CN generation.
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162 However, the following parts were surprising/unexpected:
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163
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164 1) The TRAU reacts to SID updates with a delay of 24 frames. Suppose that frame
35d38348c880 doc/TFO-xform/FRv1: article written
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165 #20 in the input is the initial SID, frame #24 (TAF position) is the first
35d38348c880 doc/TFO-xform/FRv1: article written
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166 SID update, frame #48 is the next SID update and so forth. In the output
35d38348c880 doc/TFO-xform/FRv1: article written
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167 from Nokia's TFO transform, the updated parameters from input frame #24 will
35d38348c880 doc/TFO-xform/FRv1: article written
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168 appear in output frame #48, those from input frame #48 will appear in output
35d38348c880 doc/TFO-xform/FRv1: article written
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169 frame #72 and so forth. There is no sensible explanation for this extraneous
35d38348c880 doc/TFO-xform/FRv1: article written
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170 buffering delay; at first I thought it was an artifact of the CN parameter
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171 interpolation mechanism, but:
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172
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173 2) No interpolation is done! I deliberately constructed input sequences in
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174 which each subsequent SID update has wildly different parameters from the
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175 previous, and when the changeover does happen in the DL output after the
35d38348c880 doc/TFO-xform/FRv1: article written
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176 strange delay of 24 frames, the change is immediate and abrupt.
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177
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178 CN muting after two missed SID updates (BFI received instead of SID in the TAF
35d38348c880 doc/TFO-xform/FRv1: article written
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179 position twice in a row) is done the same way as speech muting: the TRAU emits
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180 exactly 5 frames with decreasing Xmaxcr (same decrement by 11), then switches
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181 to emitting fixed silence frames.
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182
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183 SID forwarding (DTXd=1)
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184 -----------------------
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185
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186 When DTXd is enabled on the destination call leg and the input frame stream to
35d38348c880 doc/TFO-xform/FRv1: article written
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187 the TFO transform includes SID frames (considering only valid SID for now), the
35d38348c880 doc/TFO-xform/FRv1: article written
Mychaela Falconia <falcon@freecalypso.org>
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188 transform does not generate comfort noise - instead received SID frames are
35d38348c880 doc/TFO-xform/FRv1: article written
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189 passed through to call leg B DL, unless they are invalid SID or the muting
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190 mechanism has to kick in because of lost SID updates.
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191
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192 Nokia's implementation does pass valid SID frames through (I haven't tested
35d38348c880 doc/TFO-xform/FRv1: article written
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193 invalid SID yet), but it applies the same weird delay of 24 frames to the
35d38348c880 doc/TFO-xform/FRv1: article written
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194 switchover point for each update as it does when generating CN for DTXd=0.
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195
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196 However, the part where Nokia's TFO transform (at least for FRv1) is plain
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197 broken is CN muting in the case of lost SID updates. Here is what it does: it
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198 decrements Xmaxcr by 4 (yes, by 4, not by 11) once every 24 frames (probably in
35d38348c880 doc/TFO-xform/FRv1: article written
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diff changeset
199 each TAF position), such that if the level of CN was very high before channel
35d38348c880 doc/TFO-xform/FRv1: article written
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diff changeset
200 breakdown, it will take up to 7.68 s before this CN is fully muted at the end
35d38348c880 doc/TFO-xform/FRv1: article written
Mychaela Falconia <falcon@freecalypso.org>
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diff changeset
201 receiver.
35d38348c880 doc/TFO-xform/FRv1: article written
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diff changeset
202
35d38348c880 doc/TFO-xform/FRv1: article written
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diff changeset
203 GSM 06.12 section 5.4 says: "For the second lost SID frame, a muting technique
35d38348c880 doc/TFO-xform/FRv1: article written
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204 shall be used on the comfort noise that will gradually decrease the output
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diff changeset
205 level, resulting in silencing of the output after a maximum of 320 ms." The
35d38348c880 doc/TFO-xform/FRv1: article written
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206 spec gives a maximum of 320 ms for total muting of CN, but with Nokia's TFO
35d38348c880 doc/TFO-xform/FRv1: article written
Mychaela Falconia <falcon@freecalypso.org>
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diff changeset
207 transform in DTXd=1 case, that maximum time is 7.68 s - spec requirement
35d38348c880 doc/TFO-xform/FRv1: article written
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diff changeset
208 violated.
35d38348c880 doc/TFO-xform/FRv1: article written
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diff changeset
209
35d38348c880 doc/TFO-xform/FRv1: article written
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210 Only TFO, or regular FRv1 decoder too?
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211 --------------------------------------
35d38348c880 doc/TFO-xform/FRv1: article written
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212
35d38348c880 doc/TFO-xform/FRv1: article written
Mychaela Falconia <falcon@freecalypso.org>
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diff changeset
213 How does the regular FRv1 speech decoder (the one that ultimately emits G.711)
35d38348c880 doc/TFO-xform/FRv1: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
214 in Nokia TCSM2 TRAU implementation compare to what we've observed with their
35d38348c880 doc/TFO-xform/FRv1: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
215 TFO transform? Do they use a modular design where the regular decoder is a copy
35d38348c880 doc/TFO-xform/FRv1: article written
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216 of the same TFO transform followed by a standard GSM 06.10 decoder block, or do
35d38348c880 doc/TFO-xform/FRv1: article written
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diff changeset
217 they do something fancier?
35d38348c880 doc/TFO-xform/FRv1: article written
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diff changeset
218
35d38348c880 doc/TFO-xform/FRv1: article written
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diff changeset
219 Unfortunately we have no realistic way to answer this question: Nokia chose to
35d38348c880 doc/TFO-xform/FRv1: article written
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220 not implement the optional in-band homing mechanism for FRv1, thus we have no
35d38348c880 doc/TFO-xform/FRv1: article written
Mychaela Falconia <falcon@freecalypso.org>
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diff changeset
221 way to pass test sequences through the TRAU in the decoder direction and see if
35d38348c880 doc/TFO-xform/FRv1: article written
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diff changeset
222 the output matches our hypothesis as to decoder logic. Hence the TFO transform
35d38348c880 doc/TFO-xform/FRv1: article written
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parents:
diff changeset
223 is the only part whose detailed behaviour we can realistically study in this
35d38348c880 doc/TFO-xform/FRv1: article written
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diff changeset
224 TRAU.
35d38348c880 doc/TFO-xform/FRv1: article written
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parents:
diff changeset
225
35d38348c880 doc/TFO-xform/FRv1: article written
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diff changeset
226 Take-away for Themyscira implementation
35d38348c880 doc/TFO-xform/FRv1: article written
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diff changeset
227 =======================================
35d38348c880 doc/TFO-xform/FRv1: article written
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228
35d38348c880 doc/TFO-xform/FRv1: article written
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229 My take-away points from the preceding examination of FRv1 TFO transform in
35d38348c880 doc/TFO-xform/FRv1: article written
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diff changeset
230 Nokia TCSM2 are:
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diff changeset
231
35d38348c880 doc/TFO-xform/FRv1: article written
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232 * Our current Rx DTX handler front-end in libgsmfr2 is fine - Nokia's
35d38348c880 doc/TFO-xform/FRv1: article written
Mychaela Falconia <falcon@freecalypso.org>
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diff changeset
233 implementation is not any fancier at least in the case of TFO.
35d38348c880 doc/TFO-xform/FRv1: article written
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diff changeset
234
35d38348c880 doc/TFO-xform/FRv1: article written
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235 * Modularity is a good thing, and so is consistency. There is nothing wrong
35d38348c880 doc/TFO-xform/FRv1: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
236 with using the same Rx DTX handler block both as our TFO transform and as the
35d38348c880 doc/TFO-xform/FRv1: article written
Mychaela Falconia <falcon@freecalypso.org>
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diff changeset
237 front-end portion of the full decoder in end terminal operation.