FreeCalypso > hg > gsm-codec-lib

annotate doc/AMR-EFR-conversion @ 556:18aca50d68df default tip

Find changesets by keywords (author, files, the commit message), revision number or hash, or revset expression.
doc/Calypso-TCH-downlink: update for FR1 BFI-with-data
author Mychaela Falconia <falcon@freecalypso.org>
date Fri, 11 Oct 2024 01:54:00 +0000
parents 78739fda2856
children
Ignore whitespace changes - Everywhere: Within whitespace: At end of lines:
rev   line source
312
78739fda2856 doc/AMR-EFR-conversion: update for current understanding
Mychaela Falconia <falcon@freecalypso.org>
parents: 136
diff changeset
1 Please see our AMR-EFR-philosophy article for an analysis of differences between
78739fda2856 doc/AMR-EFR-conversion: update for current understanding
Mychaela Falconia <falcon@freecalypso.org>
parents: 136
diff changeset
2 EFR and MR122 (12k2 mode of AMR), and for a discussion of how we handle the
78739fda2856 doc/AMR-EFR-conversion: update for current understanding
Mychaela Falconia <falcon@freecalypso.org>
parents: 136
diff changeset
3 relation between these two codecs. The following article was written in late
78739fda2856 doc/AMR-EFR-conversion: update for current understanding
Mychaela Falconia <falcon@freecalypso.org>
parents: 136
diff changeset
4 2022, before these issues were properly understood:
78739fda2856 doc/AMR-EFR-conversion: update for current understanding
Mychaela Falconia <falcon@freecalypso.org>
parents: 136
diff changeset
5
78739fda2856 doc/AMR-EFR-conversion: update for current understanding
Mychaela Falconia <falcon@freecalypso.org>
parents: 136
diff changeset
6 2022-December description
78739fda2856 doc/AMR-EFR-conversion: update for current understanding
Mychaela Falconia <falcon@freecalypso.org>
parents: 136
diff changeset
7 -------------------------
78739fda2856 doc/AMR-EFR-conversion: update for current understanding
Mychaela Falconia <falcon@freecalypso.org>
parents: 136
diff changeset
8
136
8eb0e7a39409 doc: document command line utilities
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
9 We have two simple utilities that allow one to experiment with "dumb" bit-
8eb0e7a39409 doc: document command line utilities
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
10 shuffling conversion between AMR 12k2 and EFR codec formats, to explore
8eb0e7a39409 doc: document command line utilities
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
11 capabilities and limitations of this approach.
8eb0e7a39409 doc: document command line utilities
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
12
8eb0e7a39409 doc: document command line utilities
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
13 gsm-amr2efr reads an AMR speech recording in RFC 4867 storage format (the common
8eb0e7a39409 doc: document command line utilities
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
14 .amr format) and converts it to EFR in gsmx format. The AMR input to this
8eb0e7a39409 doc: document command line utilities
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
15 utility must consists of MR122 frames only - no other AMR modes, no SID and no
8eb0e7a39409 doc: document command line utilities
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
16 NO_DATA gaps. The intent is that one can take a starting speech sample in WAV
8eb0e7a39409 doc: document command line utilities
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
17 format, encode it into AMR with amrnb-enc from opencore-amrnb (by default that
8eb0e7a39409 doc: document command line utilities
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
18 utility produces MR122 encoding without DTX), and then convert the AMR output to
8eb0e7a39409 doc: document command line utilities
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
19 EFR with gsm-amr2efr. One can then encode the same starting-point WAV speech
8eb0e7a39409 doc: document command line utilities
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
20 sample with gsmefr-encode (matching official EFR from ETSI) and compare the two
8eb0e7a39409 doc: document command line utilities
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
21 EFR outputs. When you do this experiment, you will see that the two EFR outputs
8eb0e7a39409 doc: document command line utilities
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
22 will be different (you can then analyze encoded speech parameter diffs with
8eb0e7a39409 doc: document command line utilities
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
23 gsmrec-dump), but each version can be fed to an EFR decoder, resulting in
8eb0e7a39409 doc: document command line utilities
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
24 OK-sounding speech.
8eb0e7a39409 doc: document command line utilities
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
25
8eb0e7a39409 doc: document command line utilities
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
26 gsm-efr2amr performs the opposite conversion: it reads an EFR session recording
8eb0e7a39409 doc: document command line utilities
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
27 in gsmx format and converts it to AMR storage format. The input to gsm-efr2amr
8eb0e7a39409 doc: document command line utilities
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
28 is allowed to contain Themyscira BFI markers in addition to EFR frames; these
8eb0e7a39409 doc: document command line utilities
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
29 BFI markers will be turned into AMR NO_DATA frames. The same input can also
8eb0e7a39409 doc: document command line utilities
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
30 contain EFR SID frames - however, gsm-efr2amr will not detect them and won't
8eb0e7a39409 doc: document command line utilities
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
31 give them any special handling, instead they will be bit-reshuffled into MR122
8eb0e7a39409 doc: document command line utilities
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
32 just like EFR speech frames. The result of such "dumb" conversion is invalid
8eb0e7a39409 doc: document command line utilities
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
33 AMR, and when you decode it with amrnb-dec, you will hear some strange noises.
312
78739fda2856 doc/AMR-EFR-conversion: update for current understanding
Mychaela Falconia <falcon@freecalypso.org>
parents: 136
diff changeset
34
78739fda2856 doc/AMR-EFR-conversion: update for current understanding
Mychaela Falconia <falcon@freecalypso.org>
parents: 136
diff changeset
35 2024-April addendum
78739fda2856 doc/AMR-EFR-conversion: update for current understanding
Mychaela Falconia <falcon@freecalypso.org>
parents: 136
diff changeset
36 -------------------
78739fda2856 doc/AMR-EFR-conversion: update for current understanding
Mychaela Falconia <falcon@freecalypso.org>
parents: 136
diff changeset
37
78739fda2856 doc/AMR-EFR-conversion: update for current understanding
Mychaela Falconia <falcon@freecalypso.org>
parents: 136
diff changeset
38 In addition to the SID issue noted above (if the input to gsm-efr2amr contains
78739fda2856 doc/AMR-EFR-conversion: update for current understanding
Mychaela Falconia <falcon@freecalypso.org>
parents: 136
diff changeset
39 any SID frames, the output will be invalid AMR), these dumb conversion methods
78739fda2856 doc/AMR-EFR-conversion: update for current understanding
Mychaela Falconia <falcon@freecalypso.org>
parents: 136
diff changeset
40 fail to take action on any embedded decoder homing frames. The correct DHF is
78739fda2856 doc/AMR-EFR-conversion: update for current understanding
Mychaela Falconia <falcon@freecalypso.org>
parents: 136
diff changeset
41 different between EFR and MR122, hence a better converter could be made to
78739fda2856 doc/AMR-EFR-conversion: update for current understanding
Mychaela Falconia <falcon@freecalypso.org>
parents: 136
diff changeset
42 recognize EFR DHFs in EFR->AMR direction and convert them to MR122 DHF, and do
78739fda2856 doc/AMR-EFR-conversion: update for current understanding
Mychaela Falconia <falcon@freecalypso.org>
parents: 136
diff changeset
43 the opposite in AMR->EFR direction. However, the implementation of AMR in
78739fda2856 doc/AMR-EFR-conversion: update for current understanding
Mychaela Falconia <falcon@freecalypso.org>
parents: 136
diff changeset
44 libopencore-amrnb has the homing feature stripped out altogether, hence doing
78739fda2856 doc/AMR-EFR-conversion: update for current understanding
Mychaela Falconia <falcon@freecalypso.org>
parents: 136
diff changeset
45 DHF conversion would be pointless as long as amrnb-enc and amrnb-dec utilities
78739fda2856 doc/AMR-EFR-conversion: update for current understanding
Mychaela Falconia <falcon@freecalypso.org>
parents: 136
diff changeset
46 are involved.
78739fda2856 doc/AMR-EFR-conversion: update for current understanding
Mychaela Falconia <falcon@freecalypso.org>
parents: 136
diff changeset
47
78739fda2856 doc/AMR-EFR-conversion: update for current understanding
Mychaela Falconia <falcon@freecalypso.org>
parents: 136
diff changeset
48 Thoughts on more proper conversion
78739fda2856 doc/AMR-EFR-conversion: update for current understanding
Mychaela Falconia <falcon@freecalypso.org>
parents: 136
diff changeset
49 ==================================
78739fda2856 doc/AMR-EFR-conversion: update for current understanding
Mychaela Falconia <falcon@freecalypso.org>
parents: 136
diff changeset
50
78739fda2856 doc/AMR-EFR-conversion: update for current understanding
Mychaela Falconia <falcon@freecalypso.org>
parents: 136
diff changeset
51 Imagine this hypothetical scenario: you operate a GSM network, and you
78739fda2856 doc/AMR-EFR-conversion: update for current understanding
Mychaela Falconia <falcon@freecalypso.org>
parents: 136
diff changeset
52 preferentially use EFR codec. You are then able to obtain TrFO interconnection
78739fda2856 doc/AMR-EFR-conversion: update for current understanding
Mychaela Falconia <falcon@freecalypso.org>
parents: 136
diff changeset
53 with some other mobile network of more "modern" kind, and that "modern" network
78739fda2856 doc/AMR-EFR-conversion: update for current understanding
Mychaela Falconia <falcon@freecalypso.org>
parents: 136
diff changeset
54 uses AMR exclusively, with no ability to use any GSM-only codecs. (The latter
78739fda2856 doc/AMR-EFR-conversion: update for current understanding
Mychaela Falconia <falcon@freecalypso.org>
parents: 136
diff changeset
55 situation holds for UMTS and VoLTE, for example.) Ordinarily, under these
78739fda2856 doc/AMR-EFR-conversion: update for current understanding
Mychaela Falconia <falcon@freecalypso.org>
parents: 136
diff changeset
56 circumstances TrFO won't be possible - instead you have to interconnect in
78739fda2856 doc/AMR-EFR-conversion: update for current understanding
Mychaela Falconia <falcon@freecalypso.org>
parents: 136
diff changeset
57 G.711, have each side transcode its respective codec, and put up with double
78739fda2856 doc/AMR-EFR-conversion: update for current understanding
Mychaela Falconia <falcon@freecalypso.org>
parents: 136
diff changeset
58 transcoding. But what if the AMR side can be told to use MR122 only, without
78739fda2856 doc/AMR-EFR-conversion: update for current understanding
Mychaela Falconia <falcon@freecalypso.org>
parents: 136
diff changeset
59 any of the lower modes? Such arrangement would make no sense in GSM (just use
78739fda2856 doc/AMR-EFR-conversion: update for current understanding
Mychaela Falconia <falcon@freecalypso.org>
parents: 136
diff changeset
60 EFR instead and save the headache of dealing with AMR), but it might be sensible
78739fda2856 doc/AMR-EFR-conversion: update for current understanding
Mychaela Falconia <falcon@freecalypso.org>
parents: 136
diff changeset
61 to ask the UMTS/VoLTE side for that MR122-only config of AMR-NB.
78739fda2856 doc/AMR-EFR-conversion: update for current understanding
Mychaela Falconia <falcon@freecalypso.org>
parents: 136
diff changeset
62
78739fda2856 doc/AMR-EFR-conversion: update for current understanding
Mychaela Falconia <falcon@freecalypso.org>
parents: 136
diff changeset
63 In this hypothetical scenario, would it be possible to pass speech frames
78739fda2856 doc/AMR-EFR-conversion: update for current understanding
Mychaela Falconia <falcon@freecalypso.org>
parents: 136
diff changeset
64 transparently, doing only the necessary bit reshuffling, and only invoke some
78739fda2856 doc/AMR-EFR-conversion: update for current understanding
Mychaela Falconia <falcon@freecalypso.org>
parents: 136
diff changeset
65 slick innovative algorithm during speech pauses to translate between EFR and
78739fda2856 doc/AMR-EFR-conversion: update for current understanding
Mychaela Falconia <falcon@freecalypso.org>
parents: 136
diff changeset
66 AMR SID paradigms?
78739fda2856 doc/AMR-EFR-conversion: update for current understanding
Mychaela Falconia <falcon@freecalypso.org>
parents: 136
diff changeset
67
78739fda2856 doc/AMR-EFR-conversion: update for current understanding
Mychaela Falconia <falcon@freecalypso.org>
parents: 136
diff changeset
68 Right now this idea is fantasy only. I don't know enough about VoLTE to tell
78739fda2856 doc/AMR-EFR-conversion: update for current understanding
Mychaela Falconia <falcon@freecalypso.org>
parents: 136
diff changeset
69 whether or not an MR122-only config of AMR-NB would work there, I have no idea
78739fda2856 doc/AMR-EFR-conversion: update for current understanding
Mychaela Falconia <falcon@freecalypso.org>
parents: 136
diff changeset
70 what codec config VoLTE operators run with currently when the other end of the
78739fda2856 doc/AMR-EFR-conversion: update for current understanding
Mychaela Falconia <falcon@freecalypso.org>
parents: 136
diff changeset
71 call is G.711 PSTN, and there is very little chance that any of the nation-scale
78739fda2856 doc/AMR-EFR-conversion: update for current understanding
Mychaela Falconia <falcon@freecalypso.org>
parents: 136
diff changeset
72 mobile operators would agree to a private peering interconnect with some tiny
78739fda2856 doc/AMR-EFR-conversion: update for current understanding
Mychaela Falconia <falcon@freecalypso.org>
parents: 136
diff changeset
73 community GSM network - while interconnection through fully public, open-to-
78739fda2856 doc/AMR-EFR-conversion: update for current understanding
Mychaela Falconia <falcon@freecalypso.org>
parents: 136
diff changeset
74 everyone IP-PSTN routes allows only G.711 and nothing else, no cellular TrFO.
78739fda2856 doc/AMR-EFR-conversion: update for current understanding
Mychaela Falconia <falcon@freecalypso.org>
parents: 136
diff changeset
75
78739fda2856 doc/AMR-EFR-conversion: update for current understanding
Mychaela Falconia <falcon@freecalypso.org>
parents: 136
diff changeset
76 Nonetheless, the idea of TrFO conversion between EFR and MR122-only AMR remains
78739fda2856 doc/AMR-EFR-conversion: update for current understanding
Mychaela Falconia <falcon@freecalypso.org>
parents: 136
diff changeset
77 interesting as a theoretical exercise, and we currently leave it there, just as
78739fda2856 doc/AMR-EFR-conversion: update for current understanding
Mychaela Falconia <falcon@freecalypso.org>
parents: 136
diff changeset
78 food for thought.