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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.