# HG changeset patch # User Mychaela Falconia # Date 1713206181 0 # Node ID 019eed8b19481081f0514a890ff415f1e6a870d9 # Parent be7ae099cbc36151a578529f7867a114e94303ca doc/FR1-library-history: new article diff -r be7ae099cbc3 -r 019eed8b1948 doc/FR1-library-history --- /dev/null Thu Jan 01 00:00:00 1970 +0000 +++ b/doc/FR1-library-history Mon Apr 15 18:36:21 2024 +0000 @@ -0,0 +1,74 @@ +The first Themyscira library for GSM-FR speech codec was libgsmfrp, an add-on +to legacy libgsm from TU-Berlin. The current offering is libgsmfr2, which +supplants the earlier combination of libgsm+libgsmfrp. This document details +the change history in this continuum of GSM-FR codec libraries. + +Changes from libgsmfrp version 1.0.2 to libgsmfr2 version 2.0.0 +=============================================================== + +* Dependency on defined types abolished, the entire library uses + types instead. + +* The Rx DTX handler component of the new library is unchanged from libgsmfrp + version 1.0.2, aside from the use of new types: uint8_t instead of gsm_byte, + explicit arrays of uint8_t instead of gsm_frame. + +* In addition to this Rx DTX handler component, the new library includes the + GSM 06.10 encoder & decoder component (ported from libgsm pl22), the new + full decoder wrapper with decoder homing, an encoder homing function, and + new stateless frame packing and unpacking functions. + +libgsmfrp change history: version 1.0.1 to version 1.0.2 +======================================================== + +There are only two changes, both involving corner cases with invalid SID frames +being received: + +1) An invalid SID frame was received immediately following a good speech frame. + In this case we start CN generation, but we take the needed LARc and Xmaxc + parameters from the last speech frame, instead of the usual procedure of + extracting them from a valid SID frame. The change from 1.0.1 to 1.0.2 + concerns the Xmaxc parameter in this corner case: in 1.0.1 we took Xmaxc + from the last subframe and used it for ensuing CN generation, but in 1.0.2 + we compute a more proper mean Xmaxc from all 4 subframes, by dequantizing, + summing and requantizing. + +2) An invalid SID frame was received in the speech muting state. The sequence + of inputs would have to be: + + - a good speech frame; + - one or more BFIs, but not too many, so that the cached speech frame + does not decay fully by Xmaxc reduction; + - an invalid SID frame. + + In version 1.0.1 we handled this even more obscure corner case by entering + the CN muting state, i.e., the state that is normally entered upon the + second lost SID. In version 1.0.2 we ignore invalid SID in the speech + muting state and act as if we got BFI, i.e., continue speech muting rather + than switch to CN muting. + +libgsmfrp change history: version 1.0.0 to version 1.0.1 +======================================================== + +Version 1.0.0 exhibited the following defects, which are fixed in 1.0.1: + +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.