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doc/TFO-xform/EFR: document CN insertion
author Mychaela Falconia <falcon@freecalypso.org>
date Tue, 03 Sep 2024 21:20:47 +0000
parents d9553c7ac6ea
children
Ignore whitespace changes - Everywhere: Within whitespace: At end of lines:
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d9553c7ac6ea doc/TFO-xform/EFR: beginning of article
Mychaela Falconia <falcon@freecalypso.org>
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1 TFO transform for EFR
d9553c7ac6ea doc/TFO-xform/EFR: beginning of article
Mychaela Falconia <falcon@freecalypso.org>
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2 =====================
d9553c7ac6ea doc/TFO-xform/EFR: beginning of article
Mychaela Falconia <falcon@freecalypso.org>
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3
d9553c7ac6ea doc/TFO-xform/EFR: beginning of article
Mychaela Falconia <falcon@freecalypso.org>
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4 Unlike the situation with FRv1 and HRv1, the standard endpoint decoder for EFR
d9553c7ac6ea doc/TFO-xform/EFR: beginning of article
Mychaela Falconia <falcon@freecalypso.org>
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5 provides no help for implementing a TFO transform. The reference EFR decoder
d9553c7ac6ea doc/TFO-xform/EFR: beginning of article
Mychaela Falconia <falcon@freecalypso.org>
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6 source from ETSI includes bad frame handling and Rx DTX functions, but the logic
d9553c7ac6ea doc/TFO-xform/EFR: beginning of article
Mychaela Falconia <falcon@freecalypso.org>
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7 that implements these functions is interwoven throughout the body of the decoder
d9553c7ac6ea doc/TFO-xform/EFR: beginning of article
Mychaela Falconia <falcon@freecalypso.org>
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8 and does not form a separable front-end. Most saliently, this Rx DTX and ECU
d9553c7ac6ea doc/TFO-xform/EFR: beginning of article
Mychaela Falconia <falcon@freecalypso.org>
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9 logic in the reference decoder does not operate on coded parameters as would be
d9553c7ac6ea doc/TFO-xform/EFR: beginning of article
Mychaela Falconia <falcon@freecalypso.org>
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10 needed for a TFO transform, instead it operates on linear values deeper in the
d9553c7ac6ea doc/TFO-xform/EFR: beginning of article
Mychaela Falconia <falcon@freecalypso.org>
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11 decoder after parameter dequantization.
d9553c7ac6ea doc/TFO-xform/EFR: beginning of article
Mychaela Falconia <falcon@freecalypso.org>
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12
d9553c7ac6ea doc/TFO-xform/EFR: beginning of article
Mychaela Falconia <falcon@freecalypso.org>
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13 Given that Abis is a de facto proprietary interface that is not interoperable
d9553c7ac6ea doc/TFO-xform/EFR: beginning of article
Mychaela Falconia <falcon@freecalypso.org>
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14 between different vendors (and the same holds for Ater in those BSS designs
d9553c7ac6ea doc/TFO-xform/EFR: beginning of article
Mychaela Falconia <falcon@freecalypso.org>
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15 that separate the TRAU from the BSC), and given how daunting it seems to
d9553c7ac6ea doc/TFO-xform/EFR: beginning of article
Mychaela Falconia <falcon@freecalypso.org>
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16 implement a true TFO transform for EFR, prior to getting our Nokia TCSM2 lab
d9553c7ac6ea doc/TFO-xform/EFR: beginning of article
Mychaela Falconia <falcon@freecalypso.org>
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17 setup I was wondering if historical TRAU vendors really did implement this
d9553c7ac6ea doc/TFO-xform/EFR: beginning of article
Mychaela Falconia <falcon@freecalypso.org>
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18 TFO transform, or if perhaps they used some kind of "cheating" trick on their
d9553c7ac6ea doc/TFO-xform/EFR: beginning of article
Mychaela Falconia <falcon@freecalypso.org>
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19 Abis similar to what we did in OsmoBTS in mid-2023. However, once I got our
d9553c7ac6ea doc/TFO-xform/EFR: beginning of article
Mychaela Falconia <falcon@freecalypso.org>
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20 Nokia TCSM2 gear working, set up a TFO connection between two active TRAU
d9553c7ac6ea doc/TFO-xform/EFR: beginning of article
Mychaela Falconia <falcon@freecalypso.org>
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21 channels in EFR mode and passed some test sequences through it, it became clear
d9553c7ac6ea doc/TFO-xform/EFR: beginning of article
Mychaela Falconia <falcon@freecalypso.org>
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22 that Nokia did implement a real "honest-to-god" TFO transform for EFR: the
d9553c7ac6ea doc/TFO-xform/EFR: beginning of article
Mychaela Falconia <falcon@freecalypso.org>
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23 TRAU-DL frame stream is 100% valid "speech" frames (no idle frames or other
d9553c7ac6ea doc/TFO-xform/EFR: beginning of article
Mychaela Falconia <falcon@freecalypso.org>
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24 aberrations inserted) even when the TRAU-UL stream fed via TFO contains BFI
d9553c7ac6ea doc/TFO-xform/EFR: beginning of article
Mychaela Falconia <falcon@freecalypso.org>
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25 speech frames and DTXu pauses - the TRAU really does apply bad frame handling
d9553c7ac6ea doc/TFO-xform/EFR: beginning of article
Mychaela Falconia <falcon@freecalypso.org>
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26 and comfort noise insertion on parameter level.
d9553c7ac6ea doc/TFO-xform/EFR: beginning of article
Mychaela Falconia <falcon@freecalypso.org>
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27
d9553c7ac6ea doc/TFO-xform/EFR: beginning of article
Mychaela Falconia <falcon@freecalypso.org>
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28 Seeing that at least one major historical vendor did implement TFO transform
d9553c7ac6ea doc/TFO-xform/EFR: beginning of article
Mychaela Falconia <falcon@freecalypso.org>
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29 for EFR, and seeing the output from that transform, has set up a sportive
d9553c7ac6ea doc/TFO-xform/EFR: beginning of article
Mychaela Falconia <falcon@freecalypso.org>
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30 challenge for me: I no longer have a valid excuse to not do it. I now have a
d9553c7ac6ea doc/TFO-xform/EFR: beginning of article
Mychaela Falconia <falcon@freecalypso.org>
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31 desire to produce a FOSS implementation of TFO transform for EFR in Themyscira
d9553c7ac6ea doc/TFO-xform/EFR: beginning of article
Mychaela Falconia <falcon@freecalypso.org>
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32 libraries (probably in libgsmefr), and make it no worse than Nokia's
d9553c7ac6ea doc/TFO-xform/EFR: beginning of article
Mychaela Falconia <falcon@freecalypso.org>
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33 implementation in TCSM2.
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34
d9553c7ac6ea doc/TFO-xform/EFR: beginning of article
Mychaela Falconia <falcon@freecalypso.org>
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35 Bad frame handling in speech mode
d9553c7ac6ea doc/TFO-xform/EFR: beginning of article
Mychaela Falconia <falcon@freecalypso.org>
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36 =================================
d9553c7ac6ea doc/TFO-xform/EFR: beginning of article
Mychaela Falconia <falcon@freecalypso.org>
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37
d9553c7ac6ea doc/TFO-xform/EFR: beginning of article
Mychaela Falconia <falcon@freecalypso.org>
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38 Looking at the DL speech frames that were synthesized by the TRAU in those
d9553c7ac6ea doc/TFO-xform/EFR: beginning of article
Mychaela Falconia <falcon@freecalypso.org>
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39 frame positions where the incoming UL stream via TFO had BFIs, we can make the
d9553c7ac6ea doc/TFO-xform/EFR: beginning of article
Mychaela Falconia <falcon@freecalypso.org>
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40 following observations:
d9553c7ac6ea doc/TFO-xform/EFR: beginning of article
Mychaela Falconia <falcon@freecalypso.org>
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41
d9553c7ac6ea doc/TFO-xform/EFR: beginning of article
Mychaela Falconia <falcon@freecalypso.org>
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42 * The 5 LPC parameters are different in each generated substitution/muting
d9553c7ac6ea doc/TFO-xform/EFR: beginning of article
Mychaela Falconia <falcon@freecalypso.org>
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43 frame, hence it looks like the TFO transform is running the quantization
d9553c7ac6ea doc/TFO-xform/EFR: beginning of article
Mychaela Falconia <falcon@freecalypso.org>
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44 algorithm for each output frame to produce LPC parameters that aim for the
d9553c7ac6ea doc/TFO-xform/EFR: beginning of article
Mychaela Falconia <falcon@freecalypso.org>
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45 substitution/muting LSFs of the official "example solution".
d9553c7ac6ea doc/TFO-xform/EFR: beginning of article
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46
37
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47 If the series of BFI inputs continues for a while, the emitted LPC parameters
4ab7cc414ed2 doc/TFO-xform/EFR: document CN insertion
Mychaela Falconia <falcon@freecalypso.org>
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48 settle into an oscillating pattern that alternates between two sets of
4ab7cc414ed2 doc/TFO-xform/EFR: document CN insertion
Mychaela Falconia <falcon@freecalypso.org>
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49 numbers.
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50
36
d9553c7ac6ea doc/TFO-xform/EFR: beginning of article
Mychaela Falconia <falcon@freecalypso.org>
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51 * LTP lag parameters remain constant for each run of BFIs between good speech
d9553c7ac6ea doc/TFO-xform/EFR: beginning of article
Mychaela Falconia <falcon@freecalypso.org>
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52 frames; the lag value encoded therein matches the LTP lag (integer part only)
d9553c7ac6ea doc/TFO-xform/EFR: beginning of article
Mychaela Falconia <falcon@freecalypso.org>
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53 from the 4th subframe of the last good speech frame, just like in the official
d9553c7ac6ea doc/TFO-xform/EFR: beginning of article
Mychaela Falconia <falcon@freecalypso.org>
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54 endpoint decoder.
d9553c7ac6ea doc/TFO-xform/EFR: beginning of article
Mychaela Falconia <falcon@freecalypso.org>
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55
d9553c7ac6ea doc/TFO-xform/EFR: beginning of article
Mychaela Falconia <falcon@freecalypso.org>
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56 * Surprising bit: the 4 LTP gain values from the last good speech frame are
d9553c7ac6ea doc/TFO-xform/EFR: beginning of article
Mychaela Falconia <falcon@freecalypso.org>
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57 endlessly regurgitated verbatim in each substitution/muting frame, without
d9553c7ac6ea doc/TFO-xform/EFR: beginning of article
Mychaela Falconia <falcon@freecalypso.org>
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58 any signs of the attenuation I expected to see based on the official "example
d9553c7ac6ea doc/TFO-xform/EFR: beginning of article
Mychaela Falconia <falcon@freecalypso.org>
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59 solution".
d9553c7ac6ea doc/TFO-xform/EFR: beginning of article
Mychaela Falconia <falcon@freecalypso.org>
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60
d9553c7ac6ea doc/TFO-xform/EFR: beginning of article
Mychaela Falconia <falcon@freecalypso.org>
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61 * Another surprising bit: the 35-bit fixed codebook sequence in each subframe
d9553c7ac6ea doc/TFO-xform/EFR: beginning of article
Mychaela Falconia <falcon@freecalypso.org>
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62 is taken from the corresponding subframe of the last good speech frame,
d9553c7ac6ea doc/TFO-xform/EFR: beginning of article
Mychaela Falconia <falcon@freecalypso.org>
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63 contrary to the official "example solution" that takes these bits from the
d9553c7ac6ea doc/TFO-xform/EFR: beginning of article
Mychaela Falconia <falcon@freecalypso.org>
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64 errored frames.
d9553c7ac6ea doc/TFO-xform/EFR: beginning of article
Mychaela Falconia <falcon@freecalypso.org>
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65
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Mychaela Falconia <falcon@freecalypso.org>
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66 * The four fixed codebook gain parameters in the emitted substitution/muting
d9553c7ac6ea doc/TFO-xform/EFR: beginning of article
Mychaela Falconia <falcon@freecalypso.org>
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67 frames differ from one frame to the next in the case of multiple BFI frames
d9553c7ac6ea doc/TFO-xform/EFR: beginning of article
Mychaela Falconia <falcon@freecalypso.org>
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68 in a row, and they also differ between subframes in the same frame - hence
d9553c7ac6ea doc/TFO-xform/EFR: beginning of article
Mychaela Falconia <falcon@freecalypso.org>
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69 these parameters are clearly being regenerated as output progresses. However,
d9553c7ac6ea doc/TFO-xform/EFR: beginning of article
Mychaela Falconia <falcon@freecalypso.org>
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70 the quantization algorithm for this parameter is so complex that I haven't
d9553c7ac6ea doc/TFO-xform/EFR: beginning of article
Mychaela Falconia <falcon@freecalypso.org>
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71 been able to make a more intelligent analysis yet.
d9553c7ac6ea doc/TFO-xform/EFR: beginning of article
Mychaela Falconia <falcon@freecalypso.org>
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72
37
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73 If the series of BFI inputs continues for a while, the emitted fixed codebook
4ab7cc414ed2 doc/TFO-xform/EFR: document CN insertion
Mychaela Falconia <falcon@freecalypso.org>
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74 gain parameters slowly go down and eventually become all zeros - although the
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Mychaela Falconia <falcon@freecalypso.org>
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75 exact meaning is still unclear given the highly non-intuitive quantization
4ab7cc414ed2 doc/TFO-xform/EFR: document CN insertion
Mychaela Falconia <falcon@freecalypso.org>
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76 algorithm.
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Mychaela Falconia <falcon@freecalypso.org>
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77
36
d9553c7ac6ea doc/TFO-xform/EFR: beginning of article
Mychaela Falconia <falcon@freecalypso.org>
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78 Looking at the first good speech frame that follows each BFI substitution/muting
d9553c7ac6ea doc/TFO-xform/EFR: beginning of article
Mychaela Falconia <falcon@freecalypso.org>
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79 insert, we see that it is mostly unaltered: no alterations were seen to LPC or
d9553c7ac6ea doc/TFO-xform/EFR: beginning of article
Mychaela Falconia <falcon@freecalypso.org>
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80 LTP parameters, in particular. However, in the case of the fixed codebook gain
d9553c7ac6ea doc/TFO-xform/EFR: beginning of article
Mychaela Falconia <falcon@freecalypso.org>
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81 parameter we see a different behavioral pattern: most of the time it is also
d9553c7ac6ea doc/TFO-xform/EFR: beginning of article
Mychaela Falconia <falcon@freecalypso.org>
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82 unaltered, but sometimes we see reduction in this parameter, and even then it
d9553c7ac6ea doc/TFO-xform/EFR: beginning of article
Mychaela Falconia <falcon@freecalypso.org>
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83 is only in certain subframes. Are we perhaps seeing a capping of the fixed
d9553c7ac6ea doc/TFO-xform/EFR: beginning of article
Mychaela Falconia <falcon@freecalypso.org>
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84 codebook gain in the first good frame following BFI, similar to that implemented
d9553c7ac6ea doc/TFO-xform/EFR: beginning of article
Mychaela Falconia <falcon@freecalypso.org>
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85 in the reference endpoint decoder? A better understanding of the quantization
d9553c7ac6ea doc/TFO-xform/EFR: beginning of article
Mychaela Falconia <falcon@freecalypso.org>
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86 mechanism for this parameter will be needed.
37
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87
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88 CN insertion by TFO transform
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89 =============================
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90
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91 Looking at the DL speech frames that were synthesized by the TRAU in those
4ab7cc414ed2 doc/TFO-xform/EFR: document CN insertion
Mychaela Falconia <falcon@freecalypso.org>
parents: 36
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92 frame positions where the incoming UL stream via TFO had DTXu pauses (valid SID
4ab7cc414ed2 doc/TFO-xform/EFR: document CN insertion
Mychaela Falconia <falcon@freecalypso.org>
parents: 36
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93 frames followed by BFIs), we can make the following observations:
4ab7cc414ed2 doc/TFO-xform/EFR: document CN insertion
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94
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95 * The 5 LPC parameters appear to be generated anew on each output frame just
4ab7cc414ed2 doc/TFO-xform/EFR: document CN insertion
Mychaela Falconia <falcon@freecalypso.org>
parents: 36
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96 like in the substitution/muting case, and it likewise appears that the TFO
4ab7cc414ed2 doc/TFO-xform/EFR: document CN insertion
Mychaela Falconia <falcon@freecalypso.org>
parents: 36
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97 transform is running the regular LSF quantization algorithm taken from the
4ab7cc414ed2 doc/TFO-xform/EFR: document CN insertion
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98 encoder.
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99
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100 * The 4 LTP lag parameters are set to {135, 33, 135, 33} in each generated CN
4ab7cc414ed2 doc/TFO-xform/EFR: document CN insertion
Mychaela Falconia <falcon@freecalypso.org>
parents: 36
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101 frame, in agreement with how the official endpoint decoder sets the pitch
4ab7cc414ed2 doc/TFO-xform/EFR: document CN insertion
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102 delay to constant value 40.
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103
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104 * The 4 LTP gain parameters are all set to 0, also in agreement with CN
4ab7cc414ed2 doc/TFO-xform/EFR: document CN insertion
Mychaela Falconia <falcon@freecalypso.org>
parents: 36
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105 generation in the official endpoint decoder.
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106
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107 * The 35-bit fixed codebook part of each subframe appears to be set to a
4ab7cc414ed2 doc/TFO-xform/EFR: document CN insertion
Mychaela Falconia <falcon@freecalypso.org>
parents: 36
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108 pseudorandom sequence, different in each emitted frame and subframe. My
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parents: 36
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109 analysis tells me it should be possible to construct fixed codebook sequences
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Mychaela Falconia <falcon@freecalypso.org>
parents: 36
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110 in "speech" output frames that would produce the same excitation as the
4ab7cc414ed2 doc/TFO-xform/EFR: document CN insertion
Mychaela Falconia <falcon@freecalypso.org>
parents: 36
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111 official bit-exact CN - although the final PCM output probably won't match
4ab7cc414ed2 doc/TFO-xform/EFR: document CN insertion
Mychaela Falconia <falcon@freecalypso.org>
parents: 36
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112 the official bit-exact CN because of LSF and fixed codebook gain
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Mychaela Falconia <falcon@freecalypso.org>
parents: 36
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113 requantization. However, we won't know whether or not the output from
4ab7cc414ed2 doc/TFO-xform/EFR: document CN insertion
Mychaela Falconia <falcon@freecalypso.org>
parents: 36
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114 Nokia's TFO transform matches our idea of official-CN-matching fixed codebook
4ab7cc414ed2 doc/TFO-xform/EFR: document CN insertion
Mychaela Falconia <falcon@freecalypso.org>
parents: 36
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115 excitation until we have our own implementation of this idea and compare
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116 the two.
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117
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118 * The four fixed codebook gain parameters in the emitted CN frames are once
4ab7cc414ed2 doc/TFO-xform/EFR: document CN insertion
Mychaela Falconia <falcon@freecalypso.org>
parents: 36
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119 again too difficult to understand for now - but they are definitely being
4ab7cc414ed2 doc/TFO-xform/EFR: document CN insertion
Mychaela Falconia <falcon@freecalypso.org>
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120 recomputed anew for each emitted CN frame and subframe.
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121
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122 If CN muting kicks in on the second lost SID (BFI instead of SID received in
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123 TAF position), we see the following additional behaviour:
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124
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125 * On the TAF-position frame that initiates CN muting, the emitted LPC parameters
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126 break out of the alternating pattern they previously settled into. They go
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127 through a few unique number sets, then settle into a two-state oscillating
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128 pattern once again. Is the TFO transform perhaps making a switch from
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129 last-SID LSF numbers to the static "mean" ones when it goes into CN muting?
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130
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131 * The emitted fixed codebook gain parameters start going down and eventually
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132 become all zeros.
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parents: 36
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133
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134 Looking at the first good speech frame that follows each CN insertion period,
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parents: 36
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135 we see only two alterations made by the TFO transform: the 5 LPC parameters and
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parents: 36
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136 the first subframe fixed codebook gain parameter are modified, presumably to
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137 compensate for the lack of quantizer state reset that happens when the end
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138 decoder has seen a CN insert. No more speech parameter alterations are seen
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139 past the first subframe of the first frame following the DTXu pause.