FreeCalypso > hg > gsm-codec-lib
changeset 551:8f44d7064c56
document gsmfr-tfo-xfrm
| author | Mychaela Falconia <falcon@freecalypso.org> |
|---|---|
| date | Sat, 05 Oct 2024 06:48:44 +0000 |
| parents | de333989a12b |
| children | 6ab066180ec2 |
| files | doc/TFO-transform doc/Utils-overview |
| diffstat | 2 files changed, 40 insertions(+), 0 deletions(-) [+] |
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--- /dev/null Thu Jan 01 00:00:00 1970 +0000 +++ b/doc/TFO-transform Sat Oct 05 06:48:44 2024 +0000 @@ -0,0 +1,38 @@ +TFO transform: general definition and goal +========================================== + +"TFO transform" is the term adopted by Themyscira Wireless for the non-trivial +transform on GSM codec frames called for by the TFO spec, 3GPP TS 28.062 +section C.3.2.1.1. We have a goal of implementing TFO transform for all 3 +classic GSM codecs (FR, HR and EFR) in our Themyscira codec libraries; in the +present release, only GSM-FR version has been implemented. + +The input to this transform is the stream of received uplink frames from call +leg A, possibly containing BFI frame gaps and SID frames if call leg A uses +DTXu. The output from the transform is a "pristine" stream of good codec frames +to be transmitted on the radio downlink for call leg B: good speech frames only +in the non-DTXd case, or a mixture of good speech and valid SID frames with +DTXd. TFO transform is expected to be an identity transform when the input is +100% good speech frames, but it becomes non-trivial when it has to insert +synthetic "speech" frames for comfort noise or as error concealment. + +TFO transform for FRv1 +====================== + +This transform is implemented in libgsmfr2 in both DTXd=0 and DTXd=1 +configurations. DTXd=0 version of FRv1 TFO transform is mostly identical with +the Rx DTX handler preprocessor stage of regular speech decoding (the only +difference is in details of the in-band homing function); DTXd=1 version is +specific to this TFO/TrFO application. + +In addition to libgsmfr2 functions documented in FR1-library-API article, there +is a command line test program that exercises our implementation of this TFO +transform. Its usage is: + +gsmfr-tfo-xfrm [-d] input.hex output.hex + +Both input and output files are in TW-TS-005 Annex A hexadecimal format. The +input will typically consist of TW-TS-001 extended RTP format, whereas the +output is always emitted in the basic format, pure GSM-FR codec frames only. + +-d option enables DTXd, which is disabled by default.