FreeCalypso > hg > gsm-codec-lib
view dev/s2u-regen.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 | a5200ad12d58 |
children |
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/* * This program generates a G.711 mu-law encoding table of the same form * as the s2u[] table in the toast_ulaw.c module in libgsm/toast, i.e., * an encoding table that takes only the upper 13 bits of the linear PCM * sample to be encoded, even though canonical mu-law encoding requires * 14-bit input. The 13-bit table constructed by this program is computed * as if the lsb of the "proper" 14-bit input is always zero, like it is * expected to be when the linear PCM samples came from the output of a * GSM speech decoder. * * The "engine" function that does the computation is based on ulaw_compress() * from ITU-T G.191 STL. */ #include <stdio.h> #include <stdlib.h> static unsigned ulaw_compress(input) unsigned input; { short i; /* aux.var. */ short absno; /* absolute value of linear (input) sample */ short segno; /* segment (Table 2/G711, column 1) */ short low_nibble; /* low nibble of log companded sample */ short high_nibble; /* high nibble of log companded sample */ unsigned output; /* -------------------------------------------------------------------- */ /* Input is 14-bit right-justified in this version */ /* Compute absolute value; adjust for easy processing */ /* -------------------------------------------------------------------- */ absno = input >= 0x2000 /* compute 1's complement in case of */ ? (~input & 0x1FFF) + 33 /* negative samples */ : input + 33; /* NB: 33 is the difference value */ /* between the thresholds for */ /* A-law and u-law. */ if (absno > (0x1FFF)) /* limitation to "absno" < 8192 */ absno = (0x1FFF); /* Determination of sample's segment */ i = absno >> 6; segno = 1; while (i != 0) { segno++; i >>= 1; } /* Mounting the high-nibble of the log-PCM sample */ high_nibble = (0x0008) - segno; /* Mounting the low-nibble of the log PCM sample */ low_nibble = (absno >> segno) /* right shift of mantissa and */ &(0x000F); /* masking away leading '1' */ low_nibble = (0x000F) - low_nibble; /* Joining the high-nibble and the low-nibble of the log PCM sample */ output = (high_nibble << 4) | low_nibble; /* Add sign bit */ if (input < 0x2000) output = output | (0x0080); return output; } main(argc, argv) char **argv; { unsigned input, output; for (input = 0; input < 8192; input++) { output = ulaw_compress(input << 1); printf("%04o,", output); if ((input % 15) == 14 || input == 8191) putchar('\n'); } exit(0); }