FreeCalypso > hg > gsm-codec-lib
view doc/Codec-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 | 04936af99fc8 |
| children | a053cf0bac04 |
line wrap: on
line source
Standalone command line utilities for FR and EFR codecs ======================================================= The pre-existing FOSS opencore-amr package includes amrnb-enc and amrnb-dec test programs: the first reads linear PCM from a WAV file and emits AMR encoder output in a .amr file (RFC 4867 AMR storage format), the second reads this .amr format and emits AMR decoder output as WAV. Inspired by these simple test programs, the present package offers equivalent command line utilities for GSM FR and EFR codecs. Here they are: gsmfr-encode This utility reads linear PCM from a WAV file, runs libgsm 06.10 encoder and writes the output in the classic .gsm format (directly abutted FR codec frames of 33 bytes each). We don't currently have a Tx-side DTX implementation (VAD etc) for GSM FR, hence the output from gsmfr-encode will always consist of good speech frames only. gsmfr-decode This utility reads our gsmx format (see Binary-file-format article), which is a superset of the classic libgsm format. The input to gsmfr-decode may be a pure .gsm recording as produced by gsmfr-encode or toast from libgsm package, or it can also contain SID frames and/or BFI markers. The processing performed by gsmfr-decode begins with our FR1 Rx DTX handler preprocessor, which will be an identity transform for pure .gsm input but becomes important for real-world input containing SIDs and BFIs, and is followed by gsm_decode() from libgsm. The decoded output is written as WAV. gsmefr-encode This utility reads linear PCM from a WAV file, runs our EFR encoder (Themyscira libgsmefr) and writes the output in our gsmx format. There is an option to enable or disable DTX: -d enables DTX, otherwise it is disabled. (This option mirrors amrnb-enc.) gsmefr-decode This utility reads our gsmx format (which must be EFR, not FR1) and feeds all frames and BFIs to our EFR decoder. The decoded output is written as WAV. The above are original programs that read WAV input for encoding and write WAV output from decoding. We now also have raw versions that read and write our "robe" (raw big-endian) format instead: gsmfr-encode-r Just like gsm[e]fr-encode, but reading "robe" instead of WAV. gsmefr-encode-r gsmfr-decode-r Just like gsm[e]fr-decode, but writing "robe" instead of WAV. gsmefr-decode-r Please see PCM-file-formats article for the rationale.