FreeCalypso > hg > gsm-codec-lib
view libgsmfrp/internal.h @ 242:f081a6850fb5
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 | 3b64f255689a |
children | 6ac547f0b903 |
line wrap: on
line source
/* * This header file is internal to libgsmfrp; * here we define our state structure. */ enum rx_state { NO_DATA = 0, SPEECH, SPEECH_MUTING, COMFORT_NOISE, LOST_SID, CN_MUTING, }; struct gsmfr_preproc_state { enum rx_state rx_state; gsm_frame speech_frame; gsm_byte sid_prefix[5]; uint8_t sid_xmaxc; uint32_t cn_random_lfsr; unsigned cn_random_6fold; }; /* we use the same LFSR PRNG for CN as ETSI EFR implementation */ #define PN_INITIAL_SEED 0x70816958L /* Pseudo noise generator seed value */ /* internal functions */ extern void gsmfr_preproc_gen_cn(struct gsmfr_preproc_state *state, gsm_byte *frame); extern void gsmfr_preproc_sid2cn(struct gsmfr_preproc_state *state, gsm_byte *frame); extern void gsmfr_preproc_invalid_sid(struct gsmfr_preproc_state *state, gsm_byte *frame); extern uint16_t gsmfr_preproc_prng(struct gsmfr_preproc_state *state, uint16_t no_bits);