gsm-codec-lib: doc/FR1-Rx-DTX annotate

annotate doc/FR1-Rx-DTX @ 585:3c6bf0d26ee7 default tip

TW-TS-005 reader: fix maximum line length bug TW-TS-005 section 4.1 states: The maximum allowed length of each line is 80 characters, not including the OS-specific newline encoding. The implementation of this line length limit in the TW-TS-005 hex file reader function in the present suite was wrong, such that lines of the full maximum length could not be read. Fix it. Note that this bug affects comment lines too, not just actual RTP payloads. Neither Annex A nor Annex B features an RTP payload format that goes to the maximum of 40 bytes, but if a comment line goes to the maximum allowed length of 80 characters not including the terminating newline, the bug will be triggered, necessitating the present fix.

author	Mychaela Falconia <falcon@freecalypso.org>
date	Tue, 25 Feb 2025 07:49:28 +0000
parents	4034c2b06ec8
children

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.

FreeCalypso > hg > gsm-codec-lib

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