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doc/FR1-Rx-DTX: update for libgsmfr2 and the new landscape
author | Mychaela Falconia <falcon@freecalypso.org> |
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date | Mon, 15 Apr 2024 22:07:00 +0000 |
parents | 019eed8b1948 |
children | 516e84085a15 |
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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 <gsm.h> defined types abolished, the entire library uses <stdint.h> 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.