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document gsmfr-tfo-xfrm
author Mychaela Falconia <falcon@freecalypso.org>
date Sat, 05 Oct 2024 06:48:44 +0000
parents doc/FR1-library-API@a3300483ae74
children ebcf414b7d99
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1 TFO transform: general definition and goal
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2 ==========================================
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4 "TFO transform" is the term adopted by Themyscira Wireless for the non-trivial
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5 transform on GSM codec frames called for by the TFO spec, 3GPP TS 28.062
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6 section C.3.2.1.1. We have a goal of implementing TFO transform for all 3
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7 classic GSM codecs (FR, HR and EFR) in our Themyscira codec libraries; in the
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8 present release, only GSM-FR version has been implemented.
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10 The input to this transform is the stream of received uplink frames from call
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11 leg A, possibly containing BFI frame gaps and SID frames if call leg A uses
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12 DTXu. The output from the transform is a "pristine" stream of good codec frames
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13 to be transmitted on the radio downlink for call leg B: good speech frames only
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14 in the non-DTXd case, or a mixture of good speech and valid SID frames with
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15 DTXd. TFO transform is expected to be an identity transform when the input is
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16 100% good speech frames, but it becomes non-trivial when it has to insert
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17 synthetic "speech" frames for comfort noise or as error concealment.
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18
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19 TFO transform for FRv1
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20 ======================
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22 This transform is implemented in libgsmfr2 in both DTXd=0 and DTXd=1
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23 configurations. DTXd=0 version of FRv1 TFO transform is mostly identical with
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24 the Rx DTX handler preprocessor stage of regular speech decoding (the only
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25 difference is in details of the in-band homing function); DTXd=1 version is
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26 specific to this TFO/TrFO application.
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28 In addition to libgsmfr2 functions documented in FR1-library-API article, there
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29 is a command line test program that exercises our implementation of this TFO
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30 transform. Its usage is:
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32 gsmfr-tfo-xfrm [-d] input.hex output.hex
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34 Both input and output files are in TW-TS-005 Annex A hexadecimal format. The
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35 input will typically consist of TW-TS-001 extended RTP format, whereas the
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36 output is always emitted in the basic format, pure GSM-FR codec frames only.
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38 -d option enables DTXd, which is disabled by default.