FreeCalypso > hg > gsm-codec-lib
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libgsmfrp: new refined implementation
The previous implementation exhibited the following defects,
which are now fixed:
1) The last received valid SID was cached forever for the purpose of
handling future invalid SIDs - we could have received some valid
SID ages ago, then lots of speech or NO_DATA, and if we then get
an invalid SID, we would resurrect the last valid SID from ancient
history - a bad design. In our new design, we handle invalid SID
based on the current state, much like BFI.
2) GSM 06.11 spec says clearly that after the second lost SID
(received BFI=1 && TAF=1 in CN state) we need to gradually decrease
the output level, rather than jump directly to emitting silence
frames - we previously failed to implement such logic.
3) Per GSM 06.12 section 5.2, Xmaxc should be the same in all 4 subframes
in a SID frame. What should we do if we receive an otherwise valid
SID frame with different Xmaxc? Our previous approach would
replicate this Xmaxc oddity in every subsequent generated CN frame,
which is rather bad. In our new design, the very first CN frame
(which can be seen as a transformation of the SID frame itself)
retains the original 4 distinct Xmaxc, but all subsequent CN frames
are based on the Xmaxc from the last subframe of the most recent SID.
author | Mychaela Falconia <falcon@freecalypso.org> |
---|---|
date | Tue, 09 May 2023 05:16:31 +0000 |
parents | 1b8cb3490d48 |
children | 614ae8dc6807 |
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Themyscira Wireless GSM codec libraries & utilities package =========================================================== Themyscira Wireless, a technical initiative of the Women's Republic of Themyscira, is proud to present this package of GSM codec libraries and associated command line test and development utilities. The present package provides two linkable C libraries, intended to be usable by any application that needs GSM codec functionality: libgsmefr An implementation of EFR codec in the same spirit as classic libgsm, i.e., like libgsm, but for EFR instead of 06.10. It is based on the reference code from ETSI. libgsmfrp An Rx DTX handler preprocessor to be invoked prior to gsm_decode() from classic libgsm, implementing GSM 06.11, 06.12 and 06.31 functionality in the Rx direction. These two libraries are intended primarily for use as part of GSM network deployment, i.e., as part of the necessary speech transcoder implementation in whichever network-side server process acts as the voice gateway to PSTN or other networks. However, they can also be used as part of development and testing on the mobile side of GSM: for example, to decode TCH downlink bits and voice memo recordings read out of FreeCalypso GSM MS devices, or to implement your own lab-oriented GSM MS on top of some other PHY. Each of these two Themyscira libraries (libgsmefr and libgsmfrp) is free- standing and independent of the other. libgsmfrp makes use of <gsm.h> header file from libgsm (for gsm_byte and gsm_frame defined types) and requires this header file in order to compile; libgsmefr has zero dependencies nominally but will often be used together with libgsm and libgsmfrp. Included command line utilities =============================== In addition to the two main libraries, the present package includes a number of command line utilities, falling into the following groups: * Some utilities specifically exercise and test the two core libraries; * Some utilities are not specifically for library testing but make use of our libraries; * Some utilities are included because they implement some closely related and relevant debug and development functionality, even though they don't link with either libgsmefr or libgsmfrp. Please see the included documentation in the doc subdirectory.