FreeCalypso > hg > gsm-codec-lib
view doc/Binary-file-format @ 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 | 7e490a8efe8a |
| children | f469bad44c0e |
line wrap: on
line source
We (Themyscira Wireless) define our own binary file format for testing of GSM 06.10 (FR) and EFR codec functions; this format of ours is an extension of classic .gsm format from libgsm/toast. The original libgsm file format is a directly abutted sequence of 33-byte libgsm frames, equivalent to RTP frames for GSM FR, with the upper nibble of the first byte in each frame equal to 0xD, serving as a signature. We simply extend this idea: our version is still a directly abutted sequence of binary records, but each record is now one of 3 possibilities: - a 33-byte GSM FR frame in libgsm/RTP format, 0xD signature - a 31-byte GSM EFR frame in RTP format (ETSI TS 101 318), 0xC signature - a 2-byte Themyscira-extension BFI marker, 0xBF signature, see below File reading functions begin by reading only one byte; this byte, once decoded, tells us how many more bytes need to be read, and frame synchronization is thus maintained. The recommended filename suffix for extended-libgsm binary files in the present format is .gsmx; of course dot-separated filename suffixes hold absolutely no special meaning on Unix systems, but many developers still strongly prefer to have them for psychological comfort. Any gsmx file (FR or EFR) can be dumped in human-readable form with our gsmrec-dump utility. This utility turns every read frame from bytes into codec parameters with gsm_explode() or EFR_frame2params(), and then displays those parameters in a sensible manner, with a per-frame header line followed by 4 lines of subframe parameters. FR and EFR frames are not expected to be mixed in the same stream recording; our low-level binary file reading function and gsmrec-dump will grok such mixing just fine, but each higher-level test program (beyond gsmrec-dump) is expected to be written for only one codec, either FR or EFR. BFI marker format ================= Every 20 ms frame in our gsmx files is either a good FR/EFR frame or a BFI (Bad Frame Indication) marker. The BFI marker format used in our gsmx file format is the same format which we (Themyscira Wireless) previously used in our GSM RAN RTP transport, before switching to our current TRAUlike RTP format. This BFI marker format is quite simple: byte 0: 0xBF signature; byte 1: least-significant bit encoding TAF per GSM 06.31 or GSM 06.81, section 6.1.1 in both documents; other bits are reserved.