FreeCalypso > hg > gsm-codec-lib

annotate doc/FR1-Rx-DTX @ 303:4034c2b06ec8

Find changesets by keywords (author, files, the commit message), revision number or hash, or revset expression.
doc/FR1-Rx-DTX: update for libgsmfr2 and the new landscape
author Mychaela Falconia <falcon@freecalypso.org>
date Mon, 15 Apr 2024 22:07:00 +0000
parents 731c98b67da1
children
Ignore whitespace changes - Everywhere: Within whitespace: At end of lines:
rev   line source
135
22601ae99434 doc/FR1-Rx-DTX article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
1 At the level of provided functionality and architectural structure, ETSI GSM
22601ae99434 doc/FR1-Rx-DTX article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
2 specifications for DTX (discontinuous transmission) are very symmetric between
22601ae99434 doc/FR1-Rx-DTX article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
3 FR and EFR: the same DTX functionality is specified for both codecs, with the
22601ae99434 doc/FR1-Rx-DTX article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
4 same overall architecture. However, there is one important difference: in the
22601ae99434 doc/FR1-Rx-DTX article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
5 case of EFR the complete implementation of all DTX functions (for both Tx and
22601ae99434 doc/FR1-Rx-DTX article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
6 Rx) forms an integral and inseparable part of the reference codec (implemented
22601ae99434 doc/FR1-Rx-DTX article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
7 in C) from the beginning, whereas in the case of FR1 the addition of DTX is
22601ae99434 doc/FR1-Rx-DTX article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
8 somewhat of an afterthought. GSM 06.10 defines a "pure" FR codec without any
22601ae99434 doc/FR1-Rx-DTX article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
9 DTX functions, and this most basic spec can be and has been implemented in this
22601ae99434 doc/FR1-Rx-DTX article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
10 "pure" form - classic Unix libgsm from 1990s is a proper, fully compliant
22601ae99434 doc/FR1-Rx-DTX article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
11 implementation of GSM 06.10, but only this spec, without any DTX. In contrast,
22601ae99434 doc/FR1-Rx-DTX article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
12 there has never existed a "pure" implementation of GSM 06.60 EFR codec without
22601ae99434 doc/FR1-Rx-DTX article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
13 associated Tx and Rx DTX functions. Furthermore, there is an important
22601ae99434 doc/FR1-Rx-DTX article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
14 distinction between Tx and Rx DTX handlers for FR1:
22601ae99434 doc/FR1-Rx-DTX article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
15
22601ae99434 doc/FR1-Rx-DTX article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
16 * Anyone who seeks to implement Tx DTX for FR1 would have to dig into the guts
22601ae99434 doc/FR1-Rx-DTX article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
17 of GSM 06.10 encoder and augment it with VAD and SID encoding functions per
22601ae99434 doc/FR1-Rx-DTX article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
18 GSM 06.32 and 06.12 specs.
22601ae99434 doc/FR1-Rx-DTX article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
19
22601ae99434 doc/FR1-Rx-DTX article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
20 * In contrast, the Rx DTX handler for FR1 is modular: the way it is specified
22601ae99434 doc/FR1-Rx-DTX article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
21 in GSM 06.11, 06.12 and 06.31 is a front-end to unmodified GSM 06.10 decoder.
22601ae99434 doc/FR1-Rx-DTX article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
22 On the Rx side, the interface from the radio subsystem to the Rx DTX handler
22601ae99434 doc/FR1-Rx-DTX article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
23 consists of 260 bits of frame plus BFI and TAF flags (the spec also defines a
22601ae99434 doc/FR1-Rx-DTX article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
24 SID flag, but it is determined from frame payload bits), and then the
22601ae99434 doc/FR1-Rx-DTX article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
25 interface from the Rx DTX handler to the GSM 06.10 decoder is another FR frame
22601ae99434 doc/FR1-Rx-DTX article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
26 of 260 bits.
22601ae99434 doc/FR1-Rx-DTX article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
27
22601ae99434 doc/FR1-Rx-DTX article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
28 What are the implications of this situation for the GSM published-source
303
4034c2b06ec8 doc/FR1-Rx-DTX: update for libgsmfr2 and the new landscape
Mychaela Falconia <falcon@freecalypso.org>
parents: 250
diff changeset
29 software community? Prior to the present Themyscira offering, there has always
135
22601ae99434 doc/FR1-Rx-DTX article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
30 been libgsm, but no Rx DTX handler. If you are working with a GSM uplink RTP
22601ae99434 doc/FR1-Rx-DTX article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
31 stream from a BTS or a GSM downlink frame stream read out of TI Calypso DSP or
22601ae99434 doc/FR1-Rx-DTX article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
32 some other GSM MS PHY, feeding that stream directly to libgsm (without passing
22601ae99434 doc/FR1-Rx-DTX article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
33 through an Rx DTX handler) is NOT acceptable: a "bare" GSM 06.10 decoder won't
22601ae99434 doc/FR1-Rx-DTX article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
34 recognize SID frames and won't produce the expected comfort noise output, and
22601ae99434 doc/FR1-Rx-DTX article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
35 what are you going to do in those 20 ms windows in which no good traffic frame
22601ae99434 doc/FR1-Rx-DTX article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
36 was received? The situation becomes especially bad (unkind on ears) if you are
22601ae99434 doc/FR1-Rx-DTX article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
37 reading received downlink frames out of TI Calypso DSP: the DSP's buffer will
22601ae99434 doc/FR1-Rx-DTX article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
38 have *some* bit content in every 20 ms window, but naturally this bit content
22601ae99434 doc/FR1-Rx-DTX article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
39 will be garbage during those frame windows when no good frame was received;
22601ae99434 doc/FR1-Rx-DTX article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
40 feeding that garbage to libgsm produces noises that are very unkind on ears.
22601ae99434 doc/FR1-Rx-DTX article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
41
22601ae99434 doc/FR1-Rx-DTX article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
42 The correct solution is to implement an Rx DTX handler, pass the stream of
22601ae99434 doc/FR1-Rx-DTX article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
43 frames and flags from the BTS or the MS PHY to this handler first, and then pass
303
4034c2b06ec8 doc/FR1-Rx-DTX: update for libgsmfr2 and the new landscape
Mychaela Falconia <falcon@freecalypso.org>
parents: 250
diff changeset
44 the output of this handler to the standard GSM 06.10 decoder (classic libgsm or
4034c2b06ec8 doc/FR1-Rx-DTX: update for libgsmfr2 and the new landscape
Mychaela Falconia <falcon@freecalypso.org>
parents: 250
diff changeset
45 some updated port thereof). Themyscira libgsmfrp was our first Free Software
4034c2b06ec8 doc/FR1-Rx-DTX: update for libgsmfr2 and the new landscape
Mychaela Falconia <falcon@freecalypso.org>
parents: 250
diff changeset
46 implementation of Rx DTX handler for GSM-FR, implementing SID classification,
4034c2b06ec8 doc/FR1-Rx-DTX: update for libgsmfr2 and the new landscape
Mychaela Falconia <falcon@freecalypso.org>
parents: 250
diff changeset
47 comfort noise generation and error concealment. Our new libgsmfr2 offering
4034c2b06ec8 doc/FR1-Rx-DTX: update for libgsmfr2 and the new landscape
Mychaela Falconia <falcon@freecalypso.org>
parents: 250
diff changeset
48 takes the harmonization effort (between GSM-FR and other GSM codecs) one step
4034c2b06ec8 doc/FR1-Rx-DTX: update for libgsmfr2 and the new landscape
Mychaela Falconia <falcon@freecalypso.org>
parents: 250
diff changeset
49 further, eliminating the dependency on old libgsm and putting all GSM-FR codec
4034c2b06ec8 doc/FR1-Rx-DTX: update for libgsmfr2 and the new landscape
Mychaela Falconia <falcon@freecalypso.org>
parents: 250
diff changeset
50 functions "under one roof".
4034c2b06ec8 doc/FR1-Rx-DTX: update for libgsmfr2 and the new landscape
Mychaela Falconia <falcon@freecalypso.org>
parents: 250
diff changeset
51
4034c2b06ec8 doc/FR1-Rx-DTX: update for libgsmfr2 and the new landscape
Mychaela Falconia <falcon@freecalypso.org>
parents: 250
diff changeset
52 libgsmfrp/libgsmfr2 API documentation
4034c2b06ec8 doc/FR1-Rx-DTX: update for libgsmfr2 and the new landscape
Mychaela Falconia <falcon@freecalypso.org>
parents: 250
diff changeset
53 =====================================
4034c2b06ec8 doc/FR1-Rx-DTX: update for libgsmfr2 and the new landscape
Mychaela Falconia <falcon@freecalypso.org>
parents: 250
diff changeset
54
4034c2b06ec8 doc/FR1-Rx-DTX: update for libgsmfr2 and the new landscape
Mychaela Falconia <falcon@freecalypso.org>
parents: 250
diff changeset
55 The Rx DTX component of libgsmfr2 has the same API as our previous libgsmfrp,
4034c2b06ec8 doc/FR1-Rx-DTX: update for libgsmfr2 and the new landscape
Mychaela Falconia <falcon@freecalypso.org>
parents: 250
diff changeset
56 except for dropping the use of <gsm.h> and its types and needing to include our
4034c2b06ec8 doc/FR1-Rx-DTX: update for libgsmfr2 and the new landscape
Mychaela Falconia <falcon@freecalypso.org>
parents: 250
diff changeset
57 new API header <tw_gsmfr.h>. The present article previously contained the full
4034c2b06ec8 doc/FR1-Rx-DTX: update for libgsmfr2 and the new landscape
Mychaela Falconia <falcon@freecalypso.org>
parents: 250
diff changeset
58 description of this API; that description has now been moved to FR1-library-API
4034c2b06ec8 doc/FR1-Rx-DTX: update for libgsmfr2 and the new landscape
Mychaela Falconia <falcon@freecalypso.org>
parents: 250
diff changeset
59 article, where the whole of libgsmfr2 is documented.
4034c2b06ec8 doc/FR1-Rx-DTX: update for libgsmfr2 and the new landscape
Mychaela Falconia <falcon@freecalypso.org>
parents: 250
diff changeset
60
4034c2b06ec8 doc/FR1-Rx-DTX: update for libgsmfr2 and the new landscape
Mychaela Falconia <falcon@freecalypso.org>
parents: 250
diff changeset
61 Standalone exerciser utility
4034c2b06ec8 doc/FR1-Rx-DTX: update for libgsmfr2 and the new landscape
Mychaela Falconia <falcon@freecalypso.org>
parents: 250
diff changeset
62 ============================
4034c2b06ec8 doc/FR1-Rx-DTX: update for libgsmfr2 and the new landscape
Mychaela Falconia <falcon@freecalypso.org>
parents: 250
diff changeset
63
4034c2b06ec8 doc/FR1-Rx-DTX: update for libgsmfr2 and the new landscape
Mychaela Falconia <falcon@freecalypso.org>
parents: 250
diff changeset
64 The present GSM codec libraries and utilities package includes a standalone
4034c2b06ec8 doc/FR1-Rx-DTX: update for libgsmfr2 and the new landscape
Mychaela Falconia <falcon@freecalypso.org>
parents: 250
diff changeset
65 utility that exercises our Rx DTX handler for GSM-FR. This utility is
4034c2b06ec8 doc/FR1-Rx-DTX: update for libgsmfr2 and the new landscape
Mychaela Falconia <falcon@freecalypso.org>
parents: 250
diff changeset
66 gsmfr-preproc, to be run as follows:
4034c2b06ec8 doc/FR1-Rx-DTX: update for libgsmfr2 and the new landscape
Mychaela Falconia <falcon@freecalypso.org>
parents: 250
diff changeset
67
4034c2b06ec8 doc/FR1-Rx-DTX: update for libgsmfr2 and the new landscape
Mychaela Falconia <falcon@freecalypso.org>
parents: 250
diff changeset
68 gsmfr-preproc input.gsmx output.gsm
4034c2b06ec8 doc/FR1-Rx-DTX: update for libgsmfr2 and the new landscape
Mychaela Falconia <falcon@freecalypso.org>
parents: 250
diff changeset
69
4034c2b06ec8 doc/FR1-Rx-DTX: update for libgsmfr2 and the new landscape
Mychaela Falconia <falcon@freecalypso.org>
parents: 250
diff changeset
70 The input is an extended-libgsm file that can contain SIDs and BFI frame gaps
4034c2b06ec8 doc/FR1-Rx-DTX: update for libgsmfr2 and the new landscape
Mychaela Falconia <falcon@freecalypso.org>
parents: 250
diff changeset
71 in addition to regular GSM 06.10 speech frames (see Binary-file-format article);
4034c2b06ec8 doc/FR1-Rx-DTX: update for libgsmfr2 and the new landscape
Mychaela Falconia <falcon@freecalypso.org>
parents: 250
diff changeset
72 the output is GSM 06.10 speech frames only.
4034c2b06ec8 doc/FR1-Rx-DTX: update for libgsmfr2 and the new landscape
Mychaela Falconia <falcon@freecalypso.org>
parents: 250
diff changeset
73
4034c2b06ec8 doc/FR1-Rx-DTX: update for libgsmfr2 and the new landscape
Mychaela Falconia <falcon@freecalypso.org>
parents: 250
diff changeset
74 False SID detection
4034c2b06ec8 doc/FR1-Rx-DTX: update for libgsmfr2 and the new landscape
Mychaela Falconia <falcon@freecalypso.org>
parents: 250
diff changeset
75 ===================
4034c2b06ec8 doc/FR1-Rx-DTX: update for libgsmfr2 and the new landscape
Mychaela Falconia <falcon@freecalypso.org>
parents: 250
diff changeset
76
4034c2b06ec8 doc/FR1-Rx-DTX: update for libgsmfr2 and the new landscape
Mychaela Falconia <falcon@freecalypso.org>
parents: 250
diff changeset
77 The intent of GSM-FR spec authors was that the sets of possible speech frames
4034c2b06ec8 doc/FR1-Rx-DTX: update for libgsmfr2 and the new landscape
Mychaela Falconia <falcon@freecalypso.org>
parents: 250
diff changeset
78 and possible SID frames be disjoint. Prior to introduction of DTX, there were
4034c2b06ec8 doc/FR1-Rx-DTX: update for libgsmfr2 and the new landscape
Mychaela Falconia <falcon@freecalypso.org>
parents: 250
diff changeset
79 only regular speech frames per GSM 06.10, no SID, and a receiver had to deal
4034c2b06ec8 doc/FR1-Rx-DTX: update for libgsmfr2 and the new landscape
Mychaela Falconia <falcon@freecalypso.org>
parents: 250
diff changeset
80 with only two possibilities: either a good speech frame was received, or the
4034c2b06ec8 doc/FR1-Rx-DTX: update for libgsmfr2 and the new landscape
Mychaela Falconia <falcon@freecalypso.org>
parents: 250
diff changeset
81 frame was lost to radio errors or FACCH stealing (unusable frame). When SID
4034c2b06ec8 doc/FR1-Rx-DTX: update for libgsmfr2 and the new landscape
Mychaela Falconia <falcon@freecalypso.org>
parents: 250
diff changeset
82 frames were introduced for the purpose of intentional DTX as distinct from
4034c2b06ec8 doc/FR1-Rx-DTX: update for libgsmfr2 and the new landscape
Mychaela Falconia <falcon@freecalypso.org>
parents: 250
diff changeset
83 radio errors, the intent was that SID was to be a "new animal" not seen before,
4034c2b06ec8 doc/FR1-Rx-DTX: update for libgsmfr2 and the new landscape
Mychaela Falconia <falcon@freecalypso.org>
parents: 250
diff changeset
84 distinct from regular speech frames. There is, however, a small blemish in the
4034c2b06ec8 doc/FR1-Rx-DTX: update for libgsmfr2 and the new landscape
Mychaela Falconia <falcon@freecalypso.org>
parents: 250
diff changeset
85 actual system as realized: if the SID frame detector and the Rx DTX handler
4034c2b06ec8 doc/FR1-Rx-DTX: update for libgsmfr2 and the new landscape
Mychaela Falconia <falcon@freecalypso.org>
parents: 250
diff changeset
86 that follows it in the Rx chain follow the rules of GSM 06.31 sections 6.1.1
4034c2b06ec8 doc/FR1-Rx-DTX: update for libgsmfr2 and the new landscape
Mychaela Falconia <falcon@freecalypso.org>
parents: 250
diff changeset
87 and 6.1.2, respectively (like our implementation does), then some speech frames
4034c2b06ec8 doc/FR1-Rx-DTX: update for libgsmfr2 and the new landscape
Mychaela Falconia <falcon@freecalypso.org>
parents: 250
diff changeset
88 may be mistaken for invalid SID, or perhaps even for valid SID, producing a
4034c2b06ec8 doc/FR1-Rx-DTX: update for libgsmfr2 and the new landscape
Mychaela Falconia <falcon@freecalypso.org>
parents: 250
diff changeset
89 nonzero failure rate in this mechanism.
4034c2b06ec8 doc/FR1-Rx-DTX: update for libgsmfr2 and the new landscape
Mychaela Falconia <falcon@freecalypso.org>
parents: 250
diff changeset
90
4034c2b06ec8 doc/FR1-Rx-DTX: update for libgsmfr2 and the new landscape
Mychaela Falconia <falcon@freecalypso.org>
parents: 250
diff changeset
91 Official test sequence 02 in the set of 5 provided by ETSI exhibits this effect:
4034c2b06ec8 doc/FR1-Rx-DTX: update for libgsmfr2 and the new landscape
Mychaela Falconia <falcon@freecalypso.org>
parents: 250
diff changeset
92 Seq02.inp is a legitimate 13-bit linear PCM input to the speech encoder, and the
4034c2b06ec8 doc/FR1-Rx-DTX: update for libgsmfr2 and the new landscape
Mychaela Falconia <falcon@freecalypso.org>
parents: 250
diff changeset
93 corresponding output of GSM 06.10 encoder is contained in Seq02.cod. However,
4034c2b06ec8 doc/FR1-Rx-DTX: update for libgsmfr2 and the new landscape
Mychaela Falconia <falcon@freecalypso.org>
parents: 250
diff changeset
94 that output contains some frames that are mistakenly classified as SID=1
4034c2b06ec8 doc/FR1-Rx-DTX: update for libgsmfr2 and the new landscape
Mychaela Falconia <falcon@freecalypso.org>
parents: 250
diff changeset
95 (invalid SID) by the rules of GSM 06.31 section 6.1.1! It is true that these
4034c2b06ec8 doc/FR1-Rx-DTX: update for libgsmfr2 and the new landscape
Mychaela Falconia <falcon@freecalypso.org>
parents: 250
diff changeset
96 ancient test sequences chronologically predate the invention of DTX and
4034c2b06ec8 doc/FR1-Rx-DTX: update for libgsmfr2 and the new landscape
Mychaela Falconia <falcon@freecalypso.org>
parents: 250
diff changeset
97 GSM 06.31, but we still need to bear in mind that this problematic Seq02.cod is
4034c2b06ec8 doc/FR1-Rx-DTX: update for libgsmfr2 and the new landscape
Mychaela Falconia <falcon@freecalypso.org>
parents: 250
diff changeset
98 not an artificially constructed sequence of 06.10 codec parameters: it is the
4034c2b06ec8 doc/FR1-Rx-DTX: update for libgsmfr2 and the new landscape
Mychaela Falconia <falcon@freecalypso.org>
parents: 250
diff changeset
99 required output of the prescribed bit-exact encoder given a legitimate PCM
4034c2b06ec8 doc/FR1-Rx-DTX: update for libgsmfr2 and the new landscape
Mychaela Falconia <falcon@freecalypso.org>
parents: 250
diff changeset
100 input! There does not exist a perfect solution to this problem: as usual,
4034c2b06ec8 doc/FR1-Rx-DTX: update for libgsmfr2 and the new landscape
Mychaela Falconia <falcon@freecalypso.org>
parents: 250
diff changeset
101 real-world engineering is all about trade-offs and compromises, and occasionally
4034c2b06ec8 doc/FR1-Rx-DTX: update for libgsmfr2 and the new landscape
Mychaela Falconia <falcon@freecalypso.org>
parents: 250
diff changeset
102 a gear will slip. The best we can do is to model the probability of such
4034c2b06ec8 doc/FR1-Rx-DTX: update for libgsmfr2 and the new landscape
Mychaela Falconia <falcon@freecalypso.org>
parents: 250
diff changeset
103 gear-slip or wrong detection events, and engineer our systems to reduce this
4034c2b06ec8 doc/FR1-Rx-DTX: update for libgsmfr2 and the new landscape
Mychaela Falconia <falcon@freecalypso.org>
parents: 250
diff changeset
104 probability to a level that is deemed acceptable - which is exactly what GSM
4034c2b06ec8 doc/FR1-Rx-DTX: update for libgsmfr2 and the new landscape
Mychaela Falconia <falcon@freecalypso.org>
parents: 250
diff changeset
105 spec designers did here.
4034c2b06ec8 doc/FR1-Rx-DTX: update for libgsmfr2 and the new landscape
Mychaela Falconia <falcon@freecalypso.org>
parents: 250
diff changeset
106
4034c2b06ec8 doc/FR1-Rx-DTX: update for libgsmfr2 and the new landscape
Mychaela Falconia <falcon@freecalypso.org>
parents: 250
diff changeset
107 As of gsm-codec-lib-r3, gsmrec-dump utility shows the SID classification result
4034c2b06ec8 doc/FR1-Rx-DTX: update for libgsmfr2 and the new landscape
Mychaela Falconia <falcon@freecalypso.org>
parents: 250
diff changeset
108 (GSM 06.31 section 6.1.1) in addition to parsed 06.10 codec parameters for each
4034c2b06ec8 doc/FR1-Rx-DTX: update for libgsmfr2 and the new landscape
Mychaela Falconia <falcon@freecalypso.org>
parents: 250
diff changeset
109 frame, thus one can inspect FR-encoded streams and check for this blemish.
135
22601ae99434 doc/FR1-Rx-DTX article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
110
22601ae99434 doc/FR1-Rx-DTX article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
111 Effect of extra preprocessing
22601ae99434 doc/FR1-Rx-DTX article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
112 =============================
22601ae99434 doc/FR1-Rx-DTX article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
113
303
4034c2b06ec8 doc/FR1-Rx-DTX: update for libgsmfr2 and the new landscape
Mychaela Falconia <falcon@freecalypso.org>
parents: 250
diff changeset
114 What will happen if the output of our Rx DTX preprocessor (e.g., the output of
4034c2b06ec8 doc/FR1-Rx-DTX: update for libgsmfr2 and the new landscape
Mychaela Falconia <falcon@freecalypso.org>
parents: 250
diff changeset
115 gsmfr-preproc utility) is fed to another utility such as gsmfr-decode that also
4034c2b06ec8 doc/FR1-Rx-DTX: update for libgsmfr2 and the new landscape
Mychaela Falconia <falcon@freecalypso.org>
parents: 250
diff changeset
116 applies the same preprocessor to its input? In other words, what is the effect
4034c2b06ec8 doc/FR1-Rx-DTX: update for libgsmfr2 and the new landscape
Mychaela Falconia <falcon@freecalypso.org>
parents: 250
diff changeset
117 of a secondary preprocessor application to previous preprocessor output?
135
22601ae99434 doc/FR1-Rx-DTX article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
118
303
4034c2b06ec8 doc/FR1-Rx-DTX: update for libgsmfr2 and the new landscape
Mychaela Falconia <falcon@freecalypso.org>
parents: 250
diff changeset
119 Most of the time, the second preprocessor pass will be an identity transform
4034c2b06ec8 doc/FR1-Rx-DTX: update for libgsmfr2 and the new landscape
Mychaela Falconia <falcon@freecalypso.org>
parents: 250
diff changeset
120 under these conditions, as the input to that second pass will consist entirely
4034c2b06ec8 doc/FR1-Rx-DTX: update for libgsmfr2 and the new landscape
Mychaela Falconia <falcon@freecalypso.org>
parents: 250
diff changeset
121 of good speech frames, no SIDs and no BFIs. Any speech frames in the original
4034c2b06ec8 doc/FR1-Rx-DTX: update for libgsmfr2 and the new landscape
Mychaela Falconia <falcon@freecalypso.org>
parents: 250
diff changeset
122 input that were mistakenly classified as SID (valid or invalid) have already
4034c2b06ec8 doc/FR1-Rx-DTX: update for libgsmfr2 and the new landscape
Mychaela Falconia <falcon@freecalypso.org>
parents: 250
diff changeset
123 been converted to comfort noise (or to the silence frame in one corner case of
4034c2b06ec8 doc/FR1-Rx-DTX: update for libgsmfr2 and the new landscape
Mychaela Falconia <falcon@freecalypso.org>
parents: 250
diff changeset
124 invalid SID), hence they are no longer present in the output to trigger this
4034c2b06ec8 doc/FR1-Rx-DTX: update for libgsmfr2 and the new landscape
Mychaela Falconia <falcon@freecalypso.org>
parents: 250
diff changeset
125 effect a second time. However, there is still a small possibility that a
4034c2b06ec8 doc/FR1-Rx-DTX: update for libgsmfr2 and the new landscape
Mychaela Falconia <falcon@freecalypso.org>
parents: 250
diff changeset
126 second pass will be a non-identity transform: pseudorandom RPE pulse parameters
4034c2b06ec8 doc/FR1-Rx-DTX: update for libgsmfr2 and the new landscape
Mychaela Falconia <falcon@freecalypso.org>
parents: 250
diff changeset
127 in our comfort noise output are uniformly distributed between 1 and 6 (GSM 06.12
4034c2b06ec8 doc/FR1-Rx-DTX: update for libgsmfr2 and the new landscape
Mychaela Falconia <falcon@freecalypso.org>
parents: 250
diff changeset
128 section 6.1), and if PRNG dice roll such that at least 80 out of 95 SID codeword
4034c2b06ec8 doc/FR1-Rx-DTX: update for libgsmfr2 and the new landscape
Mychaela Falconia <falcon@freecalypso.org>
parents: 250
diff changeset
129 bit positions (all in the xMc part of the frame) are all zeros, the resulting
4034c2b06ec8 doc/FR1-Rx-DTX: update for libgsmfr2 and the new landscape
Mychaela Falconia <falcon@freecalypso.org>
parents: 250
diff changeset
130 CN frame will be liable to misinterpretation as SID (invalid SID most of the
4034c2b06ec8 doc/FR1-Rx-DTX: update for libgsmfr2 and the new landscape
Mychaela Falconia <falcon@freecalypso.org>
parents: 250
diff changeset
131 time, or even more rarely valid SID if at least 94 out of 95 SID codeword bit
4034c2b06ec8 doc/FR1-Rx-DTX: update for libgsmfr2 and the new landscape
Mychaela Falconia <falcon@freecalypso.org>
parents: 250
diff changeset
132 positions are all zeros) if fed to the preprocessor a second time. That second
4034c2b06ec8 doc/FR1-Rx-DTX: update for libgsmfr2 and the new landscape
Mychaela Falconia <falcon@freecalypso.org>
parents: 250
diff changeset
133 pass would then further alter those affected frames, but no others.