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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 ad032051166a
children
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1 Relation between GSM-EFR and 12k2 mode of AMR
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2 =============================================
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4 What are the differences between GSM-EFR codec and the highest 12k2 mode of AMR,
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5 or MR122 for short? The most obvious difference is in DTX: the format of SID
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6 frames and even the very paradigm of how DTX works are completely different
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7 between EFR and AMR. But what about non-DTX operation? If a codec session
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8 consists solely of good speech frames, no SIDs and no BFI frame gaps, are EFR
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9 and MR122 strictly identical?
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10
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11 The correct answer is that in the absence of SIDs, EFR and MR122 are directly
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12 interoperable in that the output of an EFR encoder can be fed to the input of
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13 an AMR decoder, and vice-versa. However, the two codecs are NOT identical at
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14 the bit-exact level! The differences are subtle, such that finding them
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15 requires some intense study; this article documents some of these study
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16 findings:
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17
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18 https://www.freecalypso.org/hg/efr-experiments/file/tip/Theory-and-mystery
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20 What other DSP/transcoder vendors have done
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21 ===========================================
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23 ETSI had a tradition of defining standard GSM codecs (FR, HR, EFR) in bit-exact
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24 form, and every production implementation was required to match the output of
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25 the official reference bit for bit. However, once AMR came out, the regulation
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26 on EFR was loosened. GSM 06.54 document from 2000-08 (ETSI TS 100 725 V5.2.0)
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27 has an appendix-like chapter (chapter 10) whose first paragraph reads:
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29 The 12.2 kbit/s mode of the Adaptive Multi Rate speech coder described
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30 in TS 26.071 is functionally equivalent to the GSM Enhanced Full Rate
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31 speech coder. An alternative implementation of the Enhanced Full Rate
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32 speech service based on the 12.2 kbit/s mode of the Adaptive Multi Rate
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33 coder is allowed. Alternative implementations shall implement the
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34 functionality specified in TS 26.071 for the 12.2 kbit/s mode, with the
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35 exception that the DTX transmission format (GSM 06.81) and the comfort
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36 noise generation (GSM 06.62) shall be used.
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37
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38 It appears that DSP vendors (for GSM MS or for network transcoders, or perhaps
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39 both) weren't too happy with the prospect of having to include two different
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40 versions of _almost_ the same codec algorithm with a bunch of interspersed
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41 subtle diffs, and so the rules were bent: EFR implementors were given permission
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42 to deviate from the original bit-exact definition of EFR in order to have more
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43 commonality with MR122.
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45 Approach adopted for Themyscira GSM codec libraries suite
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46 =========================================================
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48 I (Mother Mychaela) previously entertained the idea of creating a unified codec
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49 library that supports both AMR and EFR with common code, producing a published-
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50 source, FOSS-culture equivalent of what most proprietary vendors have done.
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51 However, on further reflection, that idea has been rejected. The current
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52 situation as of 2024-05 is as follows:
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54 * Libgsmefr is our production-oriented implementation of GSM-EFR codec. It
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55 implements the original bit-exact definition of EFR, not the AMR-EFR hybrid
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56 version, and it includes full support for DTX encoding and SID decoding with
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57 comfort noise generation per GSM 06.62.
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59 * Libtwamr is our librification of 3GPP AMR reference code. The library is
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60 structured in such a way that libtwamr stateful encoder and decoder functions
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61 can be combined with stateless EFR frame packing and unpacking functions from
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62 libgsmefr, allowing AMR-EFR hybrid encoders and decoders to be built. The
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63 decoder homing function in libtwamr can be told to trigger on EFR DHF instead
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64 of MR122 version, and for the encoder direction there is a simple utility
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65 function that artificially transforms MR122 DHF into EFR DHF post-encoder.
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66 However, there is no support for AMR-EFR hybrid encoding with DTX enabled,
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67 and the low-effort version of AMR-EFR hybrid decoder constructed in this
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68 manner cannot grok EFR SID frames or generate CN per GSM 06.62.
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70 Production implementations of GSM network elements that need to perform EFR
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71 speech transcoding should use libgsmefr, not libtwamr. The limited support
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72 that is provided for AMR-EFR hybrid encoding and decoding with the combination
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73 of libtwamr and libgsmefr is intended for experimentation and reverse
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74 engineering of other people's implementations, for times when it becomes
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75 necessary to model, simulate or replicate bit-exact operation of someone else's
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76 network element.