FreeCalypso > hg > gsm-codec-lib
view libgsmefr/dec_lag6.c @ 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 | f387ee919f2c |
children |
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/************************************************************************* * FUNCTION: Dec_lag6 * * PURPOSE: Decoding of fractional pitch lag with 1/6 resolution. * Extract the integer and fraction parts of the pitch lag from * the received adaptive codebook index. * * See "Enc_lag6.c" for more details about the encoding procedure. * * The fractional lag in 1st and 3rd subframes is encoded with 9 bits * while that in 2nd and 4th subframes is relatively encoded with 6 bits. * Note that in relative encoding only 61 values are used. If the * decoder receives 61, 62, or 63 as the relative pitch index, it means * that a transmission error occurred. In this case, the pitch lag from * previous subframe is used. * *************************************************************************/ #include "gsm_efr.h" #include "typedef.h" #include "namespace.h" #include "basic_op.h" #include "no_count.h" #include "codec.h" #include "cnst.h" #include "dec_state.h" Word16 Dec_lag6 ( /* output: return integer pitch lag */ struct EFR_decoder_state *st, Word16 index, /* input : received pitch index */ Word16 pit_min, /* input : minimum pitch lag */ Word16 pit_max, /* input : maximum pitch lag */ Word16 i_subfr, /* input : subframe flag */ Word16 L_frame_by2,/* input : speech frame size divided by 2 */ Word16 *T0_frac, /* output: fractional part of pitch lag */ Word16 bfi, /* input : bad frame indicator */ Word16 *T0_min, Word16 *T0_max ) { Word16 pit_flag; Word16 T0, i; pit_flag = i_subfr; move16 (); /* flag for 1st or 3rd subframe */ test (); if (i_subfr == L_frame_by2) { pit_flag = 0; move16 (); } test (); if (pit_flag == 0) /* if 1st or 3rd subframe */ { test (); if (bfi == 0) { /* if bfi == 0 decode pitch */ test (); if (index < 463) { /* T0 = (index+5)/6 + 17 */ T0 = add (mult (add (index, 5), 5462), 17); i = add (add (T0, T0), T0); /* *T0_frac = index - T0*6 + 105 */ *T0_frac = add (sub (index, add (i, i)), 105); move16 (); } else { T0 = index - 368; *T0_frac = 0; move16 (); } } else /* bfi == 1 */ { T0 = st->old_T0; *T0_frac = 0; } /* find T0_min and T0_max for 2nd (or 4th) subframe */ *T0_min = sub (T0, 5); if (*T0_min < pit_min) { *T0_min = pit_min; } *T0_max = add (*T0_min, 9); if (*T0_max > pit_max) { *T0_max = pit_max; *T0_min = sub (*T0_max, 9); } } else /* second or fourth subframe */ { test (); test (); /* if bfi == 0 decode pitch */ if ((bfi == 0) && (index < 61)) { /* i = (index+5)/6 - 1 */ i = sub (mult (add (index, 5), 5462), 1); T0 = add (i, *T0_min); i = add (add (i, i), i); *T0_frac = sub (sub (index, 3), add (i, i)); } else /* bfi == 1 OR index >= 61 */ { T0 = st->old_T0; *T0_frac = 0; } } st->old_T0 = T0; return T0; }