gsm-net-reveng: doc/TFO-xform/FRv1 annotate

annotate doc/TFO-xform/FRv1 @ 56:b32b644b7d96

d144/nokia-tcsm2-atrau.bin: captured A-TRAU output from Nokia TCSM2, fed with ul-input from Ater

author	Mychaela Falconia <falcon@freecalypso.org>
date	Wed, 25 Sep 2024 07:42:04 +0000
parents	35d38348c880
children

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

FreeCalypso > hg > gsm-net-reveng

annotate doc/TFO-xform/FRv1 @ 56:b32b644b7d96