FreeCalypso > hg > gsm-codec-lib
view doc/AMR-study-utils @ 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 | f4f68c652e98 |
| children | e26b974f7ba3 |
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As of this writing (2023-04), the main emphasis here at Themyscira Wireless is on classic FR and EFR codecs, particularly the latter, rather than AMR. However, given the close relation between GSM EFR and the highest MR122 mode of AMR, we are starting to explore AMR a little too, focusing on the reference C implementation and various published test sequences. Working in this direction, we have developed the following AMR study and exploration utilities: amr-cod-parse This utility reads the *.cod binary file format used by 3GPP reference code and test sequences for AMR-encoded speech, groks every frame and dumps all recorded parameters in human-readable form. amr-ietf-parse A similar AMR parse/dump utility to amr-cod-parse, but reading the more common IETF RFC 4867 *.amr binary file format instead. amr-cod2ietf These two utilities convert between 3GPP *.cod and IETF *.amr amr-ietf2cod formats. amr-cod2ietf converts from *.cod to *.amr; amr-ietf2cod converts in the opposite direction. Just like ETSI *.cod and *.dec formats for EFR, 3GPP *.cod format for AMR is endian-dependent. Our amr-cod-parse and amr-cod2ietf utilities expect LE byte order by default, matching the official test sequences in ts_126074v170001p0.zip; they also support BE byte order with -b option. However, our amr-ietf2cod utility (rarely needed) emits its cod-format output in the local machine's native byte order.